Messages in this thread | | | Date | Mon, 28 Sep 2015 11:37:41 +0200 | From | Borislav Petkov <> | Subject | Re: [PATCH] arch/x86: fix out-of-bounds in get_wchan() |
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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.
ECO tip #101: Trim your mails when you reply.
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