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>

1 files changed, 13 insertions, 2 deletions

diff --git a/lib/syscall.c b/lib/syscall.c
index e30e03932480..63239e097b13 100644
--- 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;
 }