Messages in this thread | | | Date | Fri, 21 Feb 2014 22:26:31 -0800 | From | "H. Peter Anvin" <> | Subject | Re: perf_fuzzer compiled for x32 causes reboot |
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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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