[Devel] [PATCH rh7 5/8] x86/process: Add proper bound checks in 64bit get_wchan()

[Andrey Ryabinin]

From: Thomas Gleixner <tglx at linutronix.de>

Dmitry Vyukov reported the following using trinity and the memory
error detector AddressSanitizer
(https://code.google.com/p/address-sanitizer/wiki/AddressSanitizerForKernel).

[ 124.575597] ERROR: AddressSanitizer: heap-buffer-overflow on
address ffff88002e280000
[ 124.576801] ffff88002e280000 is located 131938492886538 bytes to
the left of 28857600-byte region [ffffffff81282e0a, ffffffff82e0830a)
[ 124.578633] Accessed by thread T10915:
[ 124.579295] inlined in describe_heap_address
./arch/x86/mm/asan/report.c:164
[ 124.579295] #0 ffffffff810dd277 in asan_report_error
./arch/x86/mm/asan/report.c:278
[ 124.580137] #1 ffffffff810dc6a0 in asan_check_region
./arch/x86/mm/asan/asan.c:37
[ 124.581050] #2 ffffffff810dd423 in __tsan_read8 ??:0
[ 124.581893] #3 ffffffff8107c093 in get_wchan
./arch/x86/kernel/process_64.c:444

The address checks in the 64bit implementation of get_wchan() are
wrong in several ways:

 - The lower bound of the stack is not the start of the stack
   page. It's the start of the stack page plus sizeof (struct
   thread_info)

 - The upper bound must be:

       top_of_stack - TOP_OF_KERNEL_STACK_PADDING - 2 * sizeof(unsigned long).

   The 2 * sizeof(unsigned long) is required because the stack pointer
   points at the frame pointer. The layout on the stack is: ... IP FP
   ... IP FP. So we need to make sure that both IP and FP are in the
   bounds.

Fix the bound checks and get rid of the mix of numeric constants, u64
and unsigned long. Making all unsigned long allows us to use the same
function for 32bit as well.

Use READ_ONCE() when accessing the stack. This does not prevent a
concurrent wakeup of the task and the stack changing, but at least it
avoids TOCTOU.

Also check task state at the end of the loop. Again that does not
prevent concurrent changes, but it avoids walking for nothing.

Add proper comments while at it.

Reported-by: Dmitry Vyukov <dvyukov at google.com>
Reported-by: Sasha Levin <sasha.levin at oracle.com>
Based-on-patch-from: Wolfram Gloger <wmglo at dent.med.uni-muenchen.de>
Signed-off-by: Thomas Gleixner <tglx at linutronix.de>
Reviewed-by: Borislav Petkov <bp at alien8.de>
Reviewed-by: Dmitry Vyukov <dvyukov at google.com>
Cc: Andrey Ryabinin <ryabinin.a.a at gmail.com>
Cc: Andy Lutomirski <luto at amacapital.net>
Cc: Andrey Konovalov <andreyknvl at google.com>
Cc: Kostya Serebryany <kcc at google.com>
Cc: Alexander Potapenko <glider at google.com>
Cc: kasan-dev <kasan-dev at googlegroups.com>
Cc: Denys Vlasenko <dvlasenk at redhat.com>
Cc: Andi Kleen <ak at linux.intel.com>
Cc: Wolfram Gloger <wmglo at dent.med.uni-muenchen.de>
Cc: stable at vger.kernel.org
Link: http://lkml.kernel.org/r/20150930083302.694788319@linutronix.de
Signed-off-by: Thomas Gleixner <tglx at linutronix.de>
(cherry picked from commit eddd3826a1a0190e5235703d1e666affa4d13b96)
Signed-off-by: Andrey Ryabinin <aryabinin at virtuozzo.com>
---
 arch/x86/kernel/process_64.c | 52 +++++++++++++++++++++++++++++++++++---------
 1 file changed, 42 insertions(+), 10 deletions(-)

diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 2cfcea0..6970509 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -422,27 +422,59 @@ void set_personality_ia32(bool x32)
 }
 EXPORT_SYMBOL_GPL(set_personality_ia32);
 
+/*
+ * Called from fs/proc with a reference on @p to find the function
+ * which called into schedule(). This needs to be done carefully
+ * because the task might wake up and we might look at a stack
+ * changing under us.
+ */
 unsigned long get_wchan(struct task_struct *p)
 {
-	unsigned long stack;
-	u64 fp, ip;
+	unsigned long start, bottom, top, sp, fp, ip;
 	int count = 0;
 
 	if (!p || p == current || p->state == TASK_RUNNING)
 		return 0;
-	stack = (unsigned long)task_stack_page(p);
-	if (p->thread.sp < stack || p->thread.sp >= stack+THREAD_SIZE)
+
+	start = (unsigned long)task_stack_page(p);
+	if (!start)
+		return 0;
+
+	/*
+	 * Layout of the stack page:
+	 *
+	 * ----------- topmax = start + THREAD_SIZE - sizeof(unsigned long)
+	 * PADDING
+	 * ----------- top = topmax - TOP_OF_KERNEL_STACK_PADDING
+	 * stack
+	 * ----------- bottom = start + sizeof(thread_info)
+	 * thread_info
+	 * ----------- start
+	 *
+	 * The tasks stack pointer points at the location where the
+	 * framepointer is stored. The data on the stack is:
+	 * ... IP FP ... IP FP
+	 *
+	 * We need to read FP and IP, so we need to adjust the upper
+	 * bound by another unsigned long.
+	 */
+	top = start + THREAD_SIZE - TOP_OF_KERNEL_STACK_PADDING;
+	top -= 2 * sizeof(unsigned long);
+	bottom = start + sizeof(struct thread_info);
+
+	sp = READ_ONCE(p->thread.sp);
+	if (sp < bottom || sp > top)
 		return 0;
-	fp = *(u64 *)(p->thread.sp);
+
+	fp = READ_ONCE(*(unsigned long *)sp);
 	do {
-		if (fp < (unsigned long)stack ||
-		    fp >= (unsigned long)stack+THREAD_SIZE)
+		if (fp < bottom || fp > top)
 			return 0;
-		ip = *(u64 *)(fp+8);
+		ip = READ_ONCE(*(unsigned long *)(fp + sizeof(unsigned long)));
 		if (!in_sched_functions(ip))
 			return ip;
-		fp = *(u64 *)fp;
-	} while (count++ < 16);
+		fp = READ_ONCE(*(unsigned long *)fp);
+	} while (count++ < 16 && p->state != TASK_RUNNING);
 	return 0;
 }
 
-- 
2.4.10