seccomp: Make syscall skipping and nr changes more consistent

This fixes two issues that could cause incompatibility between
kernel versions:

 - If a tracer uses SECCOMP_RET_TRACE to select a syscall number
   higher than the largest known syscall, emulate the unknown
   vsyscall by returning -ENOSYS.  (This is unlikely to make a
   noticeable difference on x86-64 due to the way the system call
   entry works.)

 - On x86-64 with vsyscall=emulate, skipped vsyscalls were buggy.

This updates the documentation accordingly.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Acked-by: Will Drewry <wad@chromium.org>
Signed-off-by: James Morris <james.l.morris@oracle.com>

1 files changed, 59 insertions, 51 deletions

diff --git a/arch/x86/kernel/vsyscall_64.c b/arch/x86/kernel/vsyscall_64.c
index 8d141b309046..b2e58a248b3b 100644
--- a/arch/x86/kernel/vsyscall_64.c
+++ b/arch/x86/kernel/vsyscall_64.c
@@ -136,19 +136,6 @@ static int addr_to_vsyscall_nr(unsigned long addr)
         return nr;
 }
 
-#ifdef CONFIG_SECCOMP
-static int vsyscall_seccomp(struct task_struct *tsk, int syscall_nr)
-{
-        if (!seccomp_mode(&tsk->seccomp))
-                return 0;
-        task_pt_regs(tsk)->orig_ax = syscall_nr;
-        task_pt_regs(tsk)->ax = syscall_nr;
-        return __secure_computing(syscall_nr);
-}
-#else
-#define vsyscall_seccomp(_tsk, _nr) 0
-#endif
-
 static bool write_ok_or_segv(unsigned long ptr, size_t size)
 {
         /*
@@ -181,10 +168,9 @@ bool emulate_vsyscall(struct pt_regs *regs, unsigned long address)
 {
         struct task_struct *tsk;
         unsigned long caller;
-        int vsyscall_nr;
+        int vsyscall_nr, syscall_nr, tmp;
         int prev_sig_on_uaccess_error;
         long ret;
-        int skip;
 
         /*
          * No point in checking CS -- the only way to get here is a user mode
@@ -216,56 +202,84 @@ bool emulate_vsyscall(struct pt_regs *regs, unsigned long address)
         }
 
         tsk = current;
-        /*
-         * With a real vsyscall, page faults cause SIGSEGV.  We want to
-         * preserve that behavior to make writing exploits harder.
-         */
-        prev_sig_on_uaccess_error = current_thread_info()->sig_on_uaccess_error;
-        current_thread_info()->sig_on_uaccess_error = 1;
 
         /*
+         * Check for access_ok violations and find the syscall nr.
+         *
          * NULL is a valid user pointer (in the access_ok sense) on 32-bit and
          * 64-bit, so we don't need to special-case it here.  For all the
          * vsyscalls, NULL means "don't write anything" not "write it at
          * address 0".
          */
-        ret = -EFAULT;
-        skip = 0;
         switch (vsyscall_nr) {
         case 0:
-                skip = vsyscall_seccomp(tsk, __NR_gettimeofday);
-                if (skip)
-                        break;
-
                 if (!write_ok_or_segv(regs->di, sizeof(struct timeval)) ||
-                    !write_ok_or_segv(regs->si, sizeof(struct timezone)))
-                        break;
+                    !write_ok_or_segv(regs->si, sizeof(struct timezone))) {
+                        ret = -EFAULT;
+                        goto check_fault;
+                }
+
+                syscall_nr = __NR_gettimeofday;
+                break;
+
+        case 1:
+                if (!write_ok_or_segv(regs->di, sizeof(time_t))) {
+                        ret = -EFAULT;
+                        goto check_fault;
+                }
+
+                syscall_nr = __NR_time;
+                break;
+
+        case 2:
+                if (!write_ok_or_segv(regs->di, sizeof(unsigned)) ||
+                    !write_ok_or_segv(regs->si, sizeof(unsigned))) {
+                        ret = -EFAULT;
+                        goto check_fault;
+                }
+
+                syscall_nr = __NR_getcpu;
+                break;
+        }
+
+        /*
+         * Handle seccomp.  regs->ip must be the original value.
+         * See seccomp_send_sigsys and Documentation/prctl/seccomp_filter.txt.
+         *
+         * We could optimize the seccomp disabled case, but performance
+         * here doesn't matter.
+         */
+        regs->orig_ax = syscall_nr;
+        regs->ax = -ENOSYS;
+        tmp = secure_computing(syscall_nr);
+        if ((!tmp && regs->orig_ax != syscall_nr) || regs->ip != address) {
+                warn_bad_vsyscall(KERN_DEBUG, regs,
+                                  "seccomp tried to change syscall nr or ip");
+                do_exit(SIGSYS);
+        }
+        if (tmp)
+                goto do_ret;  /* skip requested */
 
+        /*
+         * With a real vsyscall, page faults cause SIGSEGV.  We want to
+         * preserve that behavior to make writing exploits harder.
+         */
+        prev_sig_on_uaccess_error = current_thread_info()->sig_on_uaccess_error;
+        current_thread_info()->sig_on_uaccess_error = 1;
+
+        ret = -EFAULT;
+        switch (vsyscall_nr) {
+        case 0:
                 ret = sys_gettimeofday(
                         (struct timeval __user *)regs->di,
                         (struct timezone __user *)regs->si);
                 break;
 
         case 1:
-                skip = vsyscall_seccomp(tsk, __NR_time);
-                if (skip)
-                        break;
-
-                if (!write_ok_or_segv(regs->di, sizeof(time_t)))
-                        break;
-
                 ret = sys_time((time_t __user *)regs->di);
                 break;
 
         case 2:
-                skip = vsyscall_seccomp(tsk, __NR_getcpu);
-                if (skip)
-                        break;
-
-                if (!write_ok_or_segv(regs->di, sizeof(unsigned)) ||
-                    !write_ok_or_segv(regs->si, sizeof(unsigned)))
-                        break;
-
                 ret = sys_getcpu((unsigned __user *)regs->di,
                                  (unsigned __user *)regs->si,
                                  NULL);
@@ -274,12 +288,7 @@ bool emulate_vsyscall(struct pt_regs *regs, unsigned long address)
 
         current_thread_info()->sig_on_uaccess_error = prev_sig_on_uaccess_error;
 
-        if (skip) {
-                if ((long)regs->ax <= 0L) /* seccomp errno emulation */
-                        goto do_ret;
-                goto done; /* seccomp trace/trap */
-        }
-
+check_fault:
         if (ret == -EFAULT) {
                 /* Bad news -- userspace fed a bad pointer to a vsyscall. */
                 warn_bad_vsyscall(KERN_INFO, regs,
@@ -302,7 +311,6 @@ do_ret:
         /* Emulate a ret instruction. */
         regs->ip = caller;
         regs->sp += 8;
-done:
         return true;
 
 sigsegv: