11 files changed, 538 insertions, 6 deletions

diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
index 8da78595d69d..1f9607ed087c 100644
--- a/arch/x86/entry/syscalls/syscall_32.tbl
+++ b/arch/x86/entry/syscalls/syscall_32.tbl
@@ -429,6 +429,7 @@
 421     i386    rt_sigtimedwait_time64  sys_rt_sigtimedwait             __ia32_compat_sys_rt_sigtimedwait_time64
 422     i386    futex_time64            sys_futex                       __ia32_sys_futex
 423     i386    sched_rr_get_interval_time64    sys_sched_rr_get_interval       __ia32_sys_sched_rr_get_interval
+424     i386    pidfd_send_signal       sys_pidfd_send_signal           __ia32_sys_pidfd_send_signal
 425     i386    io_uring_setup          sys_io_uring_setup              __ia32_sys_io_uring_setup
 426     i386    io_uring_enter          sys_io_uring_enter              __ia32_sys_io_uring_enter
 427     i386    io_uring_register       sys_io_uring_register           __ia32_sys_io_uring_register
diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
index c768447f97ec..92ee0b4378d4 100644
--- a/arch/x86/entry/syscalls/syscall_64.tbl
+++ b/arch/x86/entry/syscalls/syscall_64.tbl
@@ -345,6 +345,7 @@
 334     common  rseq                    __x64_sys_rseq
 # don't use numbers 387 through 423, add new calls after the last
 # 'common' entry
+424     common  pidfd_send_signal       __x64_sys_pidfd_send_signal
 425     common  io_uring_setup          __x64_sys_io_uring_setup
 426     common  io_uring_enter          __x64_sys_io_uring_enter
 427     common  io_uring_register       __x64_sys_io_uring_register
diff --git a/fs/proc/base.c b/fs/proc/base.c
index f5ebdd87afb2..ddef482f1334 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -3074,6 +3074,15 @@ static const struct file_operations proc_tgid_base_operations = {
         .llseek         = generic_file_llseek,
 };
 
+struct pid *tgid_pidfd_to_pid(const struct file *file)
+{
+        if (!d_is_dir(file->f_path.dentry) ||
+            (file->f_op != &proc_tgid_base_operations))
+                return ERR_PTR(-EBADF);
+
+        return proc_pid(file_inode(file));
+}
+
 static struct dentry *proc_tgid_base_lookup(struct inode *dir, struct dentry *dentry, unsigned int flags)
 {
         return proc_pident_lookup(dir, dentry,
diff --git a/include/linux/proc_fs.h b/include/linux/proc_fs.h
index d0e1f1522a78..52a283ba0465 100644
--- a/include/linux/proc_fs.h
+++ b/include/linux/proc_fs.h
@@ -73,6 +73,7 @@ struct proc_dir_entry *proc_create_net_single_write(const char *name, umode_t mo
                                                     int (*show)(struct seq_file *, void *),
                                                     proc_write_t write,
                                                     void *data);
+extern struct pid *tgid_pidfd_to_pid(const struct file *file);
 
 #else /* CONFIG_PROC_FS */
 
@@ -114,6 +115,11 @@ static inline int remove_proc_subtree(const char *name, struct proc_dir_entry *p
 #define proc_create_net(name, mode, parent, state_size, ops) ({NULL;})
 #define proc_create_net_single(name, mode, parent, show, data) ({NULL;})
 
+static inline struct pid *tgid_pidfd_to_pid(const struct file *file)
+{
+        return ERR_PTR(-EBADF);
+}
+
 #endif /* CONFIG_PROC_FS */
 
 struct net;
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index c2962953bf11..e446806a561f 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -985,6 +985,9 @@ asmlinkage long sys_statx(int dfd, const char __user *path, unsigned flags,
                           unsigned mask, struct statx __user *buffer);
 asmlinkage long sys_rseq(struct rseq __user *rseq, uint32_t rseq_len,
                          int flags, uint32_t sig);
+asmlinkage long sys_pidfd_send_signal(int pidfd, int sig,
+                                       siginfo_t __user *info,
+                                       unsigned int flags);
 
 /*
  * Architecture-specific system calls
diff --git a/include/uapi/asm-generic/unistd.h b/include/uapi/asm-generic/unistd.h
index bf4624efe5e6..dee7292e1df6 100644
--- a/include/uapi/asm-generic/unistd.h
+++ b/include/uapi/asm-generic/unistd.h
@@ -824,6 +824,8 @@ __SYSCALL(__NR_futex_time64, sys_futex)
 __SYSCALL(__NR_sched_rr_get_interval_time64, sys_sched_rr_get_interval)
 #endif
 
+#define __NR_pidfd_send_signal 424
+__SYSCALL(__NR_pidfd_send_signal, sys_pidfd_send_signal)
 #define __NR_io_uring_setup 425
 __SYSCALL(__NR_io_uring_setup, sys_io_uring_setup)
 #define __NR_io_uring_enter 426
diff --git a/kernel/signal.c b/kernel/signal.c
index 5d53183e2705..b7953934aa99 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -19,7 +19,9 @@
 #include <linux/sched/task.h>
 #include <linux/sched/task_stack.h>
 #include <linux/sched/cputime.h>
+#include <linux/file.h>
 #include <linux/fs.h>
+#include <linux/proc_fs.h>
 #include <linux/tty.h>
 #include <linux/binfmts.h>
 #include <linux/coredump.h>
@@ -3487,6 +3489,16 @@ COMPAT_SYSCALL_DEFINE4(rt_sigtimedwait_time32, compat_sigset_t __user *, uthese,
 #endif
 #endif
 
+static inline void prepare_kill_siginfo(int sig, struct kernel_siginfo *info)
+{
+        clear_siginfo(info);
+        info->si_signo = sig;
+        info->si_errno = 0;
+        info->si_code = SI_USER;
+        info->si_pid = task_tgid_vnr(current);
+        info->si_uid = from_kuid_munged(current_user_ns(), current_uid());
+}
+
 /**
  *  sys_kill - send a signal to a process
  *  @pid: the PID of the process
@@ -3496,16 +3508,125 @@ SYSCALL_DEFINE2(kill, pid_t, pid, int, sig)
 {
         struct kernel_siginfo info;
 
-        clear_siginfo(&info);
-        info.si_signo = sig;
-        info.si_errno = 0;
-        info.si_code = SI_USER;
-        info.si_pid = task_tgid_vnr(current);
-        info.si_uid = from_kuid_munged(current_user_ns(), current_uid());
+        prepare_kill_siginfo(sig, &info);
 
         return kill_something_info(sig, &info, pid);
 }
 
+#ifdef CONFIG_PROC_FS
+/*
+ * Verify that the signaler and signalee either are in the same pid namespace
+ * or that the signaler's pid namespace is an ancestor of the signalee's pid
+ * namespace.
+ */
+static bool access_pidfd_pidns(struct pid *pid)
+{
+        struct pid_namespace *active = task_active_pid_ns(current);
+        struct pid_namespace *p = ns_of_pid(pid);
+
+        for (;;) {
+                if (!p)
+                        return false;
+                if (p == active)
+                        break;
+                p = p->parent;
+        }
+
+        return true;
+}
+
+static int copy_siginfo_from_user_any(kernel_siginfo_t *kinfo, siginfo_t *info)
+{
+#ifdef CONFIG_COMPAT
+        /*
+         * Avoid hooking up compat syscalls and instead handle necessary
+         * conversions here. Note, this is a stop-gap measure and should not be
+         * considered a generic solution.
+         */
+        if (in_compat_syscall())
+                return copy_siginfo_from_user32(
+                        kinfo, (struct compat_siginfo __user *)info);
+#endif
+        return copy_siginfo_from_user(kinfo, info);
+}
+
+/**
+ * sys_pidfd_send_signal - send a signal to a process through a task file
+ *                          descriptor
+ * @pidfd:  the file descriptor of the process
+ * @sig:    signal to be sent
+ * @info:   the signal info
+ * @flags:  future flags to be passed
+ *
+ * The syscall currently only signals via PIDTYPE_PID which covers
+ * kill(<positive-pid>, <signal>. It does not signal threads or process
+ * groups.
+ * In order to extend the syscall to threads and process groups the @flags
+ * argument should be used. In essence, the @flags argument will determine
+ * what is signaled and not the file descriptor itself. Put in other words,
+ * grouping is a property of the flags argument not a property of the file
+ * descriptor.
+ *
+ * Return: 0 on success, negative errno on failure
+ */
+SYSCALL_DEFINE4(pidfd_send_signal, int, pidfd, int, sig,
+                siginfo_t __user *, info, unsigned int, flags)
+{
+        int ret;
+        struct fd f;
+        struct pid *pid;
+        kernel_siginfo_t kinfo;
+
+        /* Enforce flags be set to 0 until we add an extension. */
+        if (flags)
+                return -EINVAL;
+
+        f = fdget_raw(pidfd);
+        if (!f.file)
+                return -EBADF;
+
+        /* Is this a pidfd? */
+        pid = tgid_pidfd_to_pid(f.file);
+        if (IS_ERR(pid)) {
+                ret = PTR_ERR(pid);
+                goto err;
+        }
+
+        ret = -EINVAL;
+        if (!access_pidfd_pidns(pid))
+                goto err;
+
+        if (info) {
+                ret = copy_siginfo_from_user_any(&kinfo, info);
+                if (unlikely(ret))
+                        goto err;
+
+                ret = -EINVAL;
+                if (unlikely(sig != kinfo.si_signo))
+                        goto err;
+
+                if ((task_pid(current) != pid) &&
+                    (kinfo.si_code >= 0 || kinfo.si_code == SI_TKILL)) {
+                        /* Only allow sending arbitrary signals to yourself. */
+                        ret = -EPERM;
+                        if (kinfo.si_code != SI_USER)
+                                goto err;
+
+                        /* Turn this into a regular kill signal. */
+                        prepare_kill_siginfo(sig, &kinfo);
+                }
+        } else {
+                prepare_kill_siginfo(sig, &kinfo);
+        }
+
+        ret = kill_pid_info(sig, &kinfo, pid);
+
+err:
+        fdput(f);
+        return ret;
+}
+#endif /* CONFIG_PROC_FS */
+
 static int
 do_send_specific(pid_t tgid, pid_t pid, int sig, struct kernel_siginfo *info)
 {
diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
index 51d7c6794bf1..d21f4befaea4 100644
--- a/kernel/sys_ni.c
+++ b/kernel/sys_ni.c
@@ -168,6 +168,7 @@ COND_SYSCALL(syslog);
 /* kernel/sched/core.c */
 
 /* kernel/signal.c */
+COND_SYSCALL(pidfd_send_signal);
 
 /* kernel/sys.c */
 COND_SYSCALL(setregid);
diff --git a/tools/testing/selftests/Makefile b/tools/testing/selftests/Makefile
index fb5758ac469e..971fc8428117 100644
--- a/tools/testing/selftests/Makefile
+++ b/tools/testing/selftests/Makefile
@@ -32,6 +32,7 @@ TARGETS += net
 TARGETS += netfilter
 TARGETS += networking/timestamping
 TARGETS += nsfs
+TARGETS += pidfd
 TARGETS += powerpc
 TARGETS += proc
 TARGETS += pstore
diff --git a/tools/testing/selftests/pidfd/Makefile b/tools/testing/selftests/pidfd/Makefile
new file mode 100644
index 000000000000..deaf8073bc06
--- /dev/null
+++ b/tools/testing/selftests/pidfd/Makefile
@@ -0,0 +1,6 @@
+CFLAGS += -g -I../../../../usr/include/
+
+TEST_GEN_PROGS := pidfd_test
+
+include ../lib.mk
+
diff --git a/tools/testing/selftests/pidfd/pidfd_test.c b/tools/testing/selftests/pidfd/pidfd_test.c
new file mode 100644
index 000000000000..d59378a93782
--- /dev/null
+++ b/tools/testing/selftests/pidfd/pidfd_test.c
@@ -0,0 +1,381 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+
+#define _GNU_SOURCE
+#include <errno.h>
+#include <fcntl.h>
+#include <linux/types.h>
+#include <sched.h>
+#include <signal.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <syscall.h>
+#include <sys/mount.h>
+#include <sys/wait.h>
+#include <unistd.h>
+
+#include "../kselftest.h"
+
+static inline int sys_pidfd_send_signal(int pidfd, int sig, siginfo_t *info,
+                                        unsigned int flags)
+{
+        return syscall(__NR_pidfd_send_signal, pidfd, sig, info, flags);
+}
+
+static int signal_received;
+
+static void set_signal_received_on_sigusr1(int sig)
+{
+        if (sig == SIGUSR1)
+                signal_received = 1;
+}
+
+/*
+ * Straightforward test to see whether pidfd_send_signal() works is to send
+ * a signal to ourself.
+ */
+static int test_pidfd_send_signal_simple_success(void)
+{
+        int pidfd, ret;
+        const char *test_name = "pidfd_send_signal send SIGUSR1";
+
+        pidfd = open("/proc/self", O_DIRECTORY | O_CLOEXEC);
+        if (pidfd < 0)
+                ksft_exit_fail_msg(
+                        "%s test: Failed to open process file descriptor\n",
+                        test_name);
+
+        signal(SIGUSR1, set_signal_received_on_sigusr1);
+
+        ret = sys_pidfd_send_signal(pidfd, SIGUSR1, NULL, 0);
+        close(pidfd);
+        if (ret < 0)
+                ksft_exit_fail_msg("%s test: Failed to send signal\n",
+                                   test_name);
+
+        if (signal_received != 1)
+                ksft_exit_fail_msg("%s test: Failed to receive signal\n",
+                                   test_name);
+
+        signal_received = 0;
+        ksft_test_result_pass("%s test: Sent signal\n", test_name);
+        return 0;
+}
+
+static int wait_for_pid(pid_t pid)
+{
+        int status, ret;
+
+again:
+        ret = waitpid(pid, &status, 0);
+        if (ret == -1) {
+                if (errno == EINTR)
+                        goto again;
+
+                return -1;
+        }
+
+        if (ret != pid)
+                goto again;
+
+        if (!WIFEXITED(status))
+                return -1;
+
+        return WEXITSTATUS(status);
+}
+
+static int test_pidfd_send_signal_exited_fail(void)
+{
+        int pidfd, ret, saved_errno;
+        char buf[256];
+        pid_t pid;
+        const char *test_name = "pidfd_send_signal signal exited process";
+
+        pid = fork();
+        if (pid < 0)
+                ksft_exit_fail_msg("%s test: Failed to create new process\n",
+                                   test_name);
+
+        if (pid == 0)
+                _exit(EXIT_SUCCESS);
+
+        snprintf(buf, sizeof(buf), "/proc/%d", pid);
+
+        pidfd = open(buf, O_DIRECTORY | O_CLOEXEC);
+
+        (void)wait_for_pid(pid);
+
+        if (pidfd < 0)
+                ksft_exit_fail_msg(
+                        "%s test: Failed to open process file descriptor\n",
+                        test_name);
+
+        ret = sys_pidfd_send_signal(pidfd, 0, NULL, 0);
+        saved_errno = errno;
+        close(pidfd);
+        if (ret == 0)
+                ksft_exit_fail_msg(
+                        "%s test: Managed to send signal to process even though it should have failed\n",
+                        test_name);
+
+        if (saved_errno != ESRCH)
+                ksft_exit_fail_msg(
+                        "%s test: Expected to receive ESRCH as errno value but received %d instead\n",
+                        test_name, saved_errno);
+
+        ksft_test_result_pass("%s test: Failed to send signal as expected\n",
+                              test_name);
+        return 0;
+}
+
+/*
+ * The kernel reserves 300 pids via RESERVED_PIDS in kernel/pid.c
+ * That means, when it wraps around any pid < 300 will be skipped.
+ * So we need to use a pid > 300 in order to test recycling.
+ */
+#define PID_RECYCLE 1000
+
+/*
+ * Maximum number of cycles we allow. This is equivalent to PID_MAX_DEFAULT.
+ * If users set a higher limit or we have cycled PIDFD_MAX_DEFAULT number of
+ * times then we skip the test to not go into an infinite loop or block for a
+ * long time.
+ */
+#define PIDFD_MAX_DEFAULT 0x8000
+
+/*
+ * Define a few custom error codes for the child process to clearly indicate
+ * what is happening. This way we can tell the difference between a system
+ * error, a test error, etc.
+ */
+#define PIDFD_PASS 0
+#define PIDFD_FAIL 1
+#define PIDFD_ERROR 2
+#define PIDFD_SKIP 3
+#define PIDFD_XFAIL 4
+
+static int test_pidfd_send_signal_recycled_pid_fail(void)
+{
+        int i, ret;
+        pid_t pid1;
+        const char *test_name = "pidfd_send_signal signal recycled pid";
+
+        ret = unshare(CLONE_NEWPID);
+        if (ret < 0)
+                ksft_exit_fail_msg("%s test: Failed to unshare pid namespace\n",
+                                   test_name);
+
+        ret = unshare(CLONE_NEWNS);
+        if (ret < 0)
+                ksft_exit_fail_msg(
+                        "%s test: Failed to unshare mount namespace\n",
+                        test_name);
+
+        ret = mount(NULL, "/", NULL, MS_REC | MS_PRIVATE, 0);
+        if (ret < 0)
+                ksft_exit_fail_msg("%s test: Failed to remount / private\n",
+                                   test_name);
+
+        /* pid 1 in new pid namespace */
+        pid1 = fork();
+        if (pid1 < 0)
+                ksft_exit_fail_msg("%s test: Failed to create new process\n",
+                                   test_name);
+
+        if (pid1 == 0) {
+                char buf[256];
+                pid_t pid2;
+                int pidfd = -1;
+
+                (void)umount2("/proc", MNT_DETACH);
+                ret = mount("proc", "/proc", "proc", 0, NULL);
+                if (ret < 0)
+                        _exit(PIDFD_ERROR);
+
+                /* grab pid PID_RECYCLE */
+                for (i = 0; i <= PIDFD_MAX_DEFAULT; i++) {
+                        pid2 = fork();
+                        if (pid2 < 0)
+                                _exit(PIDFD_ERROR);
+
+                        if (pid2 == 0)
+                                _exit(PIDFD_PASS);
+
+                        if (pid2 == PID_RECYCLE) {
+                                snprintf(buf, sizeof(buf), "/proc/%d", pid2);
+                                ksft_print_msg("pid to recycle is %d\n", pid2);
+                                pidfd = open(buf, O_DIRECTORY | O_CLOEXEC);
+                        }
+
+                        if (wait_for_pid(pid2))
+                                _exit(PIDFD_ERROR);
+
+                        if (pid2 >= PID_RECYCLE)
+                                break;
+                }
+
+                /*
+                 * We want to be as predictable as we can so if we haven't been
+                 * able to grab pid PID_RECYCLE skip the test.
+                 */
+                if (pid2 != PID_RECYCLE) {
+                        /* skip test */
+                        close(pidfd);
+                        _exit(PIDFD_SKIP);
+                }
+
+                if (pidfd < 0)
+                        _exit(PIDFD_ERROR);
+
+                for (i = 0; i <= PIDFD_MAX_DEFAULT; i++) {
+                        char c;
+                        int pipe_fds[2];
+                        pid_t recycled_pid;
+                        int child_ret = PIDFD_PASS;
+
+                        ret = pipe2(pipe_fds, O_CLOEXEC);
+                        if (ret < 0)
+                                _exit(PIDFD_ERROR);
+
+                        recycled_pid = fork();
+                        if (recycled_pid < 0)
+                                _exit(PIDFD_ERROR);
+
+                        if (recycled_pid == 0) {
+                                close(pipe_fds[1]);
+                                (void)read(pipe_fds[0], &c, 1);
+                                close(pipe_fds[0]);
+
+                                _exit(PIDFD_PASS);
+                        }
+
+                        /*
+                         * Stop the child so we can inspect whether we have
+                         * recycled pid PID_RECYCLE.
+                         */
+                        close(pipe_fds[0]);
+                        ret = kill(recycled_pid, SIGSTOP);
+                        close(pipe_fds[1]);
+                        if (ret) {
+                                (void)wait_for_pid(recycled_pid);
+                                _exit(PIDFD_ERROR);
+                        }
+
+                        /*
+                         * We have recycled the pid. Try to signal it. This
+                         * needs to fail since this is a different process than
+                         * the one the pidfd refers to.
+                         */
+                        if (recycled_pid == PID_RECYCLE) {
+                                ret = sys_pidfd_send_signal(pidfd, SIGCONT,
+                                                            NULL, 0);
+                                if (ret && errno == ESRCH)
+                                        child_ret = PIDFD_XFAIL;
+                                else
+                                        child_ret = PIDFD_FAIL;
+                        }
+
+                        /* let the process move on */
+                        ret = kill(recycled_pid, SIGCONT);
+                        if (ret)
+                                (void)kill(recycled_pid, SIGKILL);
+
+                        if (wait_for_pid(recycled_pid))
+                                _exit(PIDFD_ERROR);
+
+                        switch (child_ret) {
+                        case PIDFD_FAIL:
+                                /* fallthrough */
+                        case PIDFD_XFAIL:
+                                _exit(child_ret);
+                        case PIDFD_PASS:
+                                break;
+                        default:
+                                /* not reached */
+                                _exit(PIDFD_ERROR);
+                        }
+
+                        /*
+                         * If the user set a custom pid_max limit we could be
+                         * in the millions.
+                         * Skip the test in this case.
+                         */
+                        if (recycled_pid > PIDFD_MAX_DEFAULT)
+                                _exit(PIDFD_SKIP);
+                }
+
+                /* failed to recycle pid */
+                _exit(PIDFD_SKIP);
+        }
+
+        ret = wait_for_pid(pid1);
+        switch (ret) {
+        case PIDFD_FAIL:
+                ksft_exit_fail_msg(
+                        "%s test: Managed to signal recycled pid %d\n",
+                        test_name, PID_RECYCLE);
+        case PIDFD_PASS:
+                ksft_exit_fail_msg("%s test: Failed to recycle pid %d\n",
+                                   test_name, PID_RECYCLE);
+        case PIDFD_SKIP:
+                ksft_print_msg("%s test: Skipping test\n", test_name);
+                ret = 0;
+                break;
+        case PIDFD_XFAIL:
+                ksft_test_result_pass(
+                        "%s test: Failed to signal recycled pid as expected\n",
+                        test_name);
+                ret = 0;
+                break;
+        default /* PIDFD_ERROR */:
+                ksft_exit_fail_msg("%s test: Error while running tests\n",
+                                   test_name);
+        }
+
+        return ret;
+}
+
+static int test_pidfd_send_signal_syscall_support(void)
+{
+        int pidfd, ret;
+        const char *test_name = "pidfd_send_signal check for support";
+
+        pidfd = open("/proc/self", O_DIRECTORY | O_CLOEXEC);
+        if (pidfd < 0)
+                ksft_exit_fail_msg(
+                        "%s test: Failed to open process file descriptor\n",
+                        test_name);
+
+        ret = sys_pidfd_send_signal(pidfd, 0, NULL, 0);
+        if (ret < 0) {
+                /*
+                 * pidfd_send_signal() will currently return ENOSYS when
+                 * CONFIG_PROC_FS is not set.
+                 */
+                if (errno == ENOSYS)
+                        ksft_exit_skip(
+                                "%s test: pidfd_send_signal() syscall not supported (Ensure that CONFIG_PROC_FS=y is set)\n",
+                                test_name);
+
+                ksft_exit_fail_msg("%s test: Failed to send signal\n",
+                                   test_name);
+        }
+
+        close(pidfd);
+        ksft_test_result_pass(
+                "%s test: pidfd_send_signal() syscall is supported. Tests can be executed\n",
+                test_name);
+        return 0;
+}
+
+int main(int argc, char **argv)
+{
+        ksft_print_header();
+
+        test_pidfd_send_signal_syscall_support();
+        test_pidfd_send_signal_simple_success();
+        test_pidfd_send_signal_exited_fail();
+        test_pidfd_send_signal_recycled_pid_fail();
+
+        return ksft_exit_pass();
+}