Re: [PATCH] arch/x86: fix out-of-bounds in get_wchan()

[Borislav Petkov <bp@alien8.de>]

On Mon, Sep 28, 2015 at 11:00:39AM +0200, Dmitry Vyukov wrote:
> get_wchan() checks that fp is within stack bounds,
> but then dereferences fp+8. This can crash kernel
> or leak sensitive information. Also the function
> operates on a potentially running stack, but does
> not use READ_ONCE. As the result it can check that
> one value is within stack bounds, but then deref
> another value.
> 
> Fix the bounds check and use READ_ONCE for all
> volatile data.
> 
> The bug was discovered with KASAN.
> 
> Signed-off-by: Dmitry Vyukov <dvyukov@google.com>
> ---
> FTR, here is the KASAN report:
> 
> [  124.575597] ERROR: AddressSanitizer: heap-buffer-overflow on address ffff88002e280000
> [  124.578633] Accessed by thread T10915:
> [  124.581050]   #2 ffffffff810dd423 in __tsan_read8 ??:0
> [  124.581893]   #3 ffffffff8107c093 in get_wchan ./arch/x86/kernel/process_64.c:444
> [  124.582763]   #4 ffffffff81342108 in do_task_stat array.c:0
> [  124.583634]   #5 ffffffff81342dcc in proc_tgid_stat ??:0
> [  124.584548]   #6 ffffffff8133c984 in proc_single_show base.c:0
> [  124.585461]   #7 ffffffff812d18cc in seq_read ./fs/seq_file.c:222
> [  124.586313]   #8 ffffffff8129e503 in vfs_read ??:0
> [  124.587137]   #9 ffffffff8129f800 in SyS_read ??:0
> [  124.587827]   #10 ffffffff81929bf5 in sysenter_dispatch ./arch/x86/ia32/ia32entry.S:164
> [  124.588738]
> [  124.593434] Shadow bytes around the buggy address:
> [  124.594270]   ffff88002e27fd80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> [  124.595339]   ffff88002e27fe00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> [  124.596453]   ffff88002e27fe80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> [  124.597466]   ffff88002e27ff00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> [  124.598501]   ffff88002e27ff80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> [  124.599629] =>ffff88002e280000:[fa]fa fa fa fa fa fa fa fa fa 00 00 00 00 00 00
> [  124.600873]   ffff88002e280080: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> [  124.601892]   ffff88002e280100: 00 fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
> [  124.603037]   ffff88002e280180: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
> [  124.604047]   ffff88002e280200: fa fa fa fa fa fa fa fd fd fd fd fd fd fd fd fd
> [  124.605054]   ffff88002e280280: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fa fa
> [  124.605993] Shadow byte legend (one shadow byte represents 8 application bytes):
> [  124.606958]   Addressable:   00
> [  124.607483]   Partially addressable: 01 02 03 04 05 06 07
> [  124.608219]   Heap redzone:  fa
> [  124.608724]   Heap kmalloc redzone:  fb
> [  124.609249]   Freed heap region: fd
> [  124.609753]   Shadow gap:fe
> [  124.610292] =========================================================================
> ---
>  arch/x86/kernel/process_64.c | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
> index 71d7849..a1fce34 100644
> --- a/arch/x86/kernel/process_64.c
> +++ b/arch/x86/kernel/process_64.c
> @@ -506,17 +506,19 @@ unsigned long get_wchan(struct task_struct *p)
>  	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)
> +	/* The task can be already running at this point, so tread carefully. */
> +	fp = READ_ONCE(p->thread.sp);
> +	if (fp < stack || fp >= stack+THREAD_SIZE)
>  		return 0;
> -	fp = *(u64 *)(p->thread.sp);
> +	fp = READ_ONCE(*(u64 *)fp);

Why isn't this:

	fp = READ_ONCE(*(u64 *)p->thread.sp);

like the original code did?

Actually, the original code looks fishy to me too - it did access live
stack three times. And shouldn't we be accessing it only once?

I.e.,

	fp_st = READ_ONCE(p->thread.sp);
	if (fp_st < stack || fp_st >= stack + THREAD_SIZE)
		return 0;
	fp = *(u64 *)fp_st;

Hmm?

Maybe I'm not completely clear on how the whole locking happens here
because we do

        if (!p || p == current || p->state == TASK_RUNNING)
                return 0;

earlier but apparently we can become TASK_RUNNING after the check...

Also, shouldn't this one have a CVE number assigned or so due to the
leakage potential?

Thanks.

-- 
Regards/Gruss,
    Boris.

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