linux-kernel - [tip:x86/asm] lib/syscall: Pin the task stack in collect

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Message-ID: <tip-aa1f1a639621672b68f654dc815a7d8298ff396f@git.kernel.org>
Date:   Fri, 16 Sep 2016 02:18:38 -0700
From:   tip-bot for Andy Lutomirski <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     hpa@...or.com, bp@...en8.de, brgerst@...il.com,
        peterz@...radead.org, jann@...jh.net, dvlasenk@...hat.com,
        torvalds@...ux-foundation.org, jpoimboe@...hat.com,
        luto@...nel.org, mingo@...nel.org, tglx@...utronix.de,
        linux-kernel@...r.kernel.org
Subject: [tip:x86/asm] lib/syscall: Pin the task stack in collect_syscall()

Commit-ID:  aa1f1a639621672b68f654dc815a7d8298ff396f
Gitweb:     http://git.kernel.org/tip/aa1f1a639621672b68f654dc815a7d8298ff396f
Author:     Andy Lutomirski <luto@...nel.org>
AuthorDate: Thu, 15 Sep 2016 22:45:47 -0700
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Fri, 16 Sep 2016 09:18:53 +0200

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>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Brian Gerst <brgerst@...il.com>
Cc: Denys Vlasenko <dvlasenk@...hat.com>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Josh Poimboeuf <jpoimboe@...hat.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Link: http://lkml.kernel.org/r/0bfd8e6d4729c97745d3781a29610a33d0a8091d.1474003868.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 lib/syscall.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/lib/syscall.c b/lib/syscall.c
index e30e039..63239e0 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;
 }