Re: [PATCH] mm: don't allow fault_around_bytes to be 0

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Mon, Jul 28, 2014 at 11:43:20AM +0400, Andrey Ryabinin wrote:
> Sasha Levin triggered use-after-free when fuzzing using trinity and the KASAN
> patchset:
> 
> 	AddressSanitizer: use after free in do_read_fault.isra.40+0x3c2/0x510 at addr ffff88048a733110
> 	page:ffffea001229ccc0 count:0 mapcount:0 mapping:          (null) index:0x0
> 	page flags: 0xafffff80008000(tail)
> 	page dumped because: kasan error
> 	CPU: 6 PID: 9262 Comm: trinity-c104 Not tainted 3.16.0-rc6-next-20140723-sasha-00047-g289342b-dirty #929
> 	 00000000000000fb 0000000000000000 ffffea001229ccc0 ffff88038ac0fb78
> 	 ffffffffa5e40903 ffff88038ac0fc48 ffff88038ac0fc38 ffffffffa142acfc
> 	 0000000000000001 ffff880509ff5aa8 ffff88038ac10038 ffff88038ac0fbb0
> 	Call Trace:
> 	dump_stack (lib/dump_stack.c:52)
> 	kasan_report_error (mm/kasan/report.c:98 mm/kasan/report.c:166)
> 	? debug_smp_processor_id (lib/smp_processor_id.c:57)
> 	? preempt_count_sub (kernel/sched/core.c:2606)
> 	? put_lock_stats.isra.13 (./arch/x86/include/asm/preempt.h:98 kernel/locking/lockdep.c:254)
> 	? do_read_fault.isra.40 (mm/memory.c:2784 mm/memory.c:2849 mm/memory.c:2898)
> 	__asan_load8 (mm/kasan/kasan.c:364)
> 	? do_read_fault.isra.40 (mm/memory.c:2864 mm/memory.c:2898)
> 	do_read_fault.isra.40 (mm/memory.c:2864 mm/memory.c:2898)
> 	? _raw_spin_unlock (./arch/x86/include/asm/preempt.h:98 include/linux/spinlock_api_smp.h:152 kernel/locking/spinlock.c:183)
> 	? __pte_alloc (mm/memory.c:598)
> 	handle_mm_fault (mm/memory.c:3092 mm/memory.c:3225 mm/memory.c:3345 mm/memory.c:3374)
> 	? pud_huge (./arch/x86/include/asm/paravirt.h:611 arch/x86/mm/hugetlbpage.c:76)
> 	__get_user_pages (mm/gup.c:286 mm/gup.c:478)
> 	__mlock_vma_pages_range (mm/mlock.c:262)
> 	__mm_populate (mm/mlock.c:710)
> 	SyS_remap_file_pages (mm/mmap.c:2653 mm/mmap.c:2593)
> 	tracesys (arch/x86/kernel/entry_64.S:541)
> 	Read of size 8 by thread T9262:
> 	Memory state around the buggy address:
> 	 ffff88048a732e80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> 	 ffff88048a732f00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> 	 ffff88048a732f80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> 	 ffff88048a733000: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> 	 ffff88048a733080: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> 	>ffff88048a733100: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> 	                         ^
> 	 ffff88048a733180: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> 	 ffff88048a733200: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> 	 ffff88048a733280: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> 	 ffff88048a733300: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> 	 ffff88048a733380: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> 
> 
> It looks like that pte pointer is invalid in do_fault_around().
> This could happen if fault_around_bytes is set to 0.
> fault_around_pages() and fault_around_mask() calls rounddown_pow_of_to(fault_around_bytes)
> The result of rounddown_pow_of_to is undefined if parameter == 0
> (in my environment it returns 0x8000000000000000).

Ouch. Good catch!

Although, I'm not convinced that it caused the issue. Sasha, did you touch the
debugfs handle?

> One way to fix this would be to return 0 from fault_around_pages() if fault_around_bytes == 0,
> however this would add extra code on fault path.
> 
> So let's just forbid to set fault_around_bytes to zero.
> Fault around is not used if fault_around_pages() <= 1, so if anyone doesn't want to use
> it, fault_around_bytes could be set to any value in range [1, 2*PAGE_SIZE - 1]
> instead of 0.

>From user point of view, 0 is perfectly fine. What about untested patch
below?

Other option: get rid of debugfs interface, so fault_around_pages() and
fault_around_mask() will always be known compile time.

There's other problem with the debugfs handle: we don't have serialization
between fault_around_bytes_set() and do_fault_around(). It can end up
badly if fault_around_bytes will be changed under do_fault_around()...

I don't think it worth adding the serialization to hot path to protect
against debug interface.
Any thoughts?

>From 2932fbcefe4ec21c046348e21981149ecce5d161 Mon Sep 17 00:00:00 2001
From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Date: Mon, 28 Jul 2014 12:16:49 +0300
Subject: [PATCH] mm, debugfs: workaround undefined behaviour of
 rounddown_pow_of_two(0)

Result of rounddown_pow_of_two(0) is not defined. It can cause a bug if
user will set fault_around_bytes to 0 via debugfs interface.

Let's set fault_around_bytes to PAGE_SIZE if user tries to set it to
something below PAGE_SIZE.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
 mm/memory.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/memory.c b/mm/memory.c
index 7e8d8205b610..2d8fa7a7b0ee 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2786,7 +2786,8 @@ static int fault_around_bytes_set(void *data, u64 val)
 {
 	if (val / PAGE_SIZE > PTRS_PER_PTE)
 		return -EINVAL;
-	fault_around_bytes = val;
+	/* rounddown_pow_of_two(0) is not defined */
+	fault_around_bytes = max(val, PAGE_SIZE);
 	return 0;
 }
 DEFINE_SIMPLE_ATTRIBUTE(fault_around_bytes_fops,
-- 
 Kirill A. Shutemov

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