Message-ID: <53084317.4090304@zytor.com>
Date: Fri, 21 Feb 2014 22:26:31 -0800
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Vince Weaver <vincent.weaver@maine.edu>
CC: Linux Kernel <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@redhat.com>,
        "H.J. Lu" <hjl.tools@gmail.com>
Subject: Re: perf_fuzzer compiled for x32 causes reboot
References: <alpine.DEB.2.10.1402211521040.6395@vincent-weaver-1.um.maine.edu> <alpine.DEB.2.10.1402211701380.6395@vincent-weaver-1.um.maine.edu> <alpine.DEB.2.10.1402211732290.11615@vincent-weaver-1.um.maine.edu> <alpine.DEB.2.10.1402212342090.17416@vincent-weaver-1.um.maine.edu>
In-Reply-To: <alpine.DEB.2.10.1402212342090.17416@vincent-weaver-1.um.maine.edu>
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On 02/21/2014 08:50 PM, Vince Weaver wrote:
>
> So I changed the perf_fuzzer so when it randomly stomps all over the
> perf_event_mmap_page, it uses a constant value of 0xdeadbeef rather
> than a random value.
>
> The result is below.  The segfaults make a bit more sense now, it
> almost looks like what is happening is we are corrupting an address
> value somehow (head? tail?) and the kernel then uses the corrupt address
> and writes to memory outside of the mmap ring buffer.
>

That seems unlikely:

	handle->page = (offset >> page_shift) & (rb->nr_pages - 1);
	offset &= (1UL << page_shift) - 1;

The masking to the number of pages should make that not possible, even 
if a completely bogus value is written.

> I still haven't figured out how to trigger this exactly, but you can
> see when over-written with 0xdeadbeef the memory address written to is
> consistently some small multiple of 0x120.
>
> I imagine it would be a bad thing if it turned out to be possible to
> select what memory address got written to.  Although since I've
> only reproduced this on x32 maybe it won't be possible to over-write
> the kernel; but I have seen this bug cause a reboot when the
> wrong thing got over-written.
>
> [28002.850192] perf_fuzzer[7083]: segfault at 2be0 ip 000000000041efab sp 00000000ff826748 error 6 in perf_fuzzer[400000+d1000]
> [28639.769869] perf_fuzzer[7100]: segfault at 1320 ip 000000000041efab sp 00000000ffa65038 error 6 in perf_fuzzer[400000+d1000]
> [29396.986242] perf_fuzzer[7120]: segfault at 10e0 ip 000000000041efab sp 00000000ffd48e68 error 6 in perf_fuzzer[400000+d1000]
> [29738.892931] perf_fuzzer[7128]: segfault at 18c0 ip 000000000041efab sp 00000000ffcdcd88 error 6 in perf_fuzzer[400000+d1000]
> [29815.550210] perf_fuzzer[7132]: segfault at 120 ip 000000000041efab sp 00000000ffe673b8 error 6 in perf_fuzzer[400000+d1000]
> [30173.455348] perf_fuzzer[7141]: segfault at 120 ip 000000000041efab sp 00000000ffda1948 error 6 in perf_fuzzer[400000+d1000]
> [30570.625642] perf_fuzzer[7156]: segfault at 1680 ip 000000000041efab sp 00000000ffaad028 error 6 in perf_fuzzer[400000+d1000]
> [31047.887784] perf_fuzzer[7169]: segfault at 60c0 ip 000000000041efab sp 00000000ffaa86e8 error 6 in perf_fuzzer[400000+d1000]
> [31300.168714] perf_fuzzer[7175]: segfault at 3a80 ip 000000000041efab sp 00000000ffd83228 error 6 in perf_fuzzer[400000+d1000]
> [31984.727278] perf_fuzzer[7193]: segfault at 7e0 ip 000000000041efab sp 00000000ff9db1f8 error 6 in perf_fuzzer[400000+d1000]

Error 6 reflects a write in userspace to a not-present page.

Since your previous trace indicates that the value of the register in 
question is a different one, I'm guessing that what we have here is PEBS 
getting activated.  0x120 is 2*0x90, and 0x90 is the size of a 64-bit 
PEBS record.

	-hpa
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