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