lib/syscall: Pin the task stack in collect_syscall()

This will avoid a potential read-after-free if collect_syscall()
(e.g. /proc/PID/syscall) is called on an exiting task.

Reported-by: Jann Horn <jann@thejh.net>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/0bfd8e6d4729c97745d3781a29610a33d0a8091d.1474003868.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/lib/syscall.c b/lib/syscall.c
index e30e039..63239e0 100644 (file)
--- a/lib/syscall.c
+++ b/lib/syscall.c
@@ -7,9 +7,19 @@ static int collect_syscall(struct task_struct *target, long *callno,
                           unsigned long args[6], unsigned int maxargs,
                           unsigned long *sp, unsigned long *pc)
 {
-       struct pt_regs *regs = task_pt_regs(target);
-       if (unlikely(!regs))
+       struct pt_regs *regs;
+
+       if (!try_get_task_stack(target)) {
+               /* Task has no stack, so the task isn't in a syscall. */
+               *callno = -1;
+               return 0;
+       }
+
+       regs = task_pt_regs(target);
+       if (unlikely(!regs)) {
+               put_task_stack(target);
                return -EAGAIN;
+       }
 
        *sp = user_stack_pointer(regs);
        *pc = instruction_pointer(regs);
@@ -18,6 +28,7 @@ static int collect_syscall(struct task_struct *target, long *callno,
        if (*callno != -1L && maxargs > 0)
                syscall_get_arguments(target, regs, 0, maxargs, args);
 
+       put_task_stack(target);
        return 0;
 }