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Message-ID: <20150928093741.GA3556@pd.tnic>
Date: Mon, 28 Sep 2015 11:37:41 +0200
From: Borislav Petkov <bp@...en8.de>
To: Dmitry Vyukov <dvyukov@...gle.com>
Cc: tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
luto@...nel.org, dvlasenk@...hat.com, x86@...nel.org,
linux-kernel@...r.kernel.org, kcc@...gle.com, glider@...gle.com,
andreyknvl@...gle.com, ryabinin.a.a@...il.com,
sasha.levin@...cle.com, ak@...ux.intel.com,
kasan-dev@...glegroups.com
Subject: Re: [PATCH] arch/x86: fix out-of-bounds in get_wchan()
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@...gle.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.
ECO tip #101: Trim your mails when you reply.
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