linux-kernel - [PATCH] lib/syscall: Pin the task stack in collect

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Message-Id: <e37e5f6fa33a0da4bb1959fb030a46e4de1da7e3.1473313784.git.luto@kernel.org>
Date:   Wed,  7 Sep 2016 22:51:31 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     x86@...nel.org
Cc:     Borislav Petkov <bp@...en8.de>, linux-kernel@...r.kernel.org,
        Brian Gerst <brgerst@...il.com>, Jann Horn <jann@...jh.net>,
        Andy Lutomirski <luto@...nel.org>
Subject: [PATCH] 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@...jh.net>
Signed-off-by: Andy Lutomirski <luto@...nel.org>
---

Ingo-

This applies on top of the patches I sent out earlier.  If you want
to avoid any possible bisection issue, you could also apply it in
the middle of the series: put it right after "x86/process: Pin the
target stack in get_wchan".

 lib/syscall.c | 15 +++++++++++++--
 1 file 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;
 }
 
-- 
2.7.4