On Thu, Jun 2, 2016 at 2:17 PM, Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx> wrote: > > > On 06/02/2016 03:02 PM, Alexander Potapenko wrote: >> On Wed, Jun 1, 2016 at 6:31 PM, Alexander Potapenko <glider@xxxxxxxxxx> wrote: >>> On Wed, Jun 1, 2016 at 5:23 PM, Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx> wrote: >>>> On 05/31/2016 08:49 PM, Alexander Potapenko wrote: >>>>> On Tue, May 31, 2016 at 1:52 PM, Andrey Ryabinin >>>>> <aryabinin@xxxxxxxxxxxxx> wrote: >>>>>> >>>>>> >>>>>> On 05/31/2016 01:44 PM, Alexander Potapenko wrote: >>>>>>> Add a special shadow value to distinguish accesses to KASAN-specific >>>>>>> allocator metadata. >>>>>>> >>>>>>> Unlike AddressSanitizer in the userspace, KASAN lets the kernel proceed >>>>>>> after a memory error. However a write to the kmalloc metadata may cause >>>>>>> memory corruptions that will make the tool itself unreliable and induce >>>>>>> crashes later on. Warning about such corruptions will ease the >>>>>>> debugging. >>>>>> >>>>>> It will not. Whether out-of-bounds hits metadata or not is absolutely irrelevant >>>>>> to the bug itself. This information doesn't help to understand, analyze or fix the bug. >>>>>> >>>>> Here's the example that made me think the opposite. >>>>> >>>>> I've been reworking KASAN hooks for mempool and added a test that did >>>>> a write-after-free to an object allocated from a mempool. >>>>> This resulted in flaky kernel crashes somewhere in quarantine >>>>> shrinking after several attempts to `insmod test_kasan.ko`. >>>>> Because there already were numerous KASAN errors in the test, it >>>>> wasn't evident that the crashes were related to the new test, so I >>>>> thought the problem was in the buggy quarantine implementation. >>>>> However the problem was indeed in the new test, which corrupted the >>>>> quarantine pointer in the object and caused a crash while traversing >>>>> the quarantine list. >>>>> >>>>> My previous experience with userspace ASan shows that crashes in the >>>>> tool code itself puzzle the developers. >>>>> As a result, the users think that the tool is broken and don't believe >>>>> its reports. >>>>> >>>>> I first thought about hardening the quarantine list by checksumming >>>>> the pointers and validating them on each traversal. >>>>> This prevents the crashes, but doesn't give the users any idea about >>>>> what went wrong. >>>>> On the other hand, reporting the pointer corruption right when it happens does. >>>>> Distinguishing between a regular UAF and a quarantine corruption >>>>> (which is what the patch in question is about) helps to prioritize the >>>>> KASAN reports and give the developers better understanding of the >>>>> consequences. >>>>> >>>> >>>> After the first report we have memory in a corrupted state, so we are done here. >>> This is theoretically true, that's why we crash after the first report >>> in the userspace ASan. >>> But since the kernel proceeds after the first KASAN report, it's >>> possible that we see several different reports, and they are sometimes >>> worth looking at. >>> >>>> Anything that happens after the first report can't be trusted since it can be an after-effect, >>>> just like in your case. Such crashes are not worthy to look at. >>>> Out-of-bounds that doesn't hit metadata as any other memory corruption also can lead to after-effects crashes, >>>> thus distinguishing such bugs doesn't make a lot of sense. >>> Unlike the crashes in the kernel itself, crashes with KASAN functions >>> in the stack trace may make the developer think the tool is broken. >>>> >>>> test_kasan module is just a quick hack, made only to make sure that KASAN works. >>>> It does some crappy thing, and may lead to crash as well. So I would recommend an immediate >>>> reboot even after single attempt to load it. >>> Agreed. However a plain write into the first byte of the freed object >>> will cause similar problems. >> >> On a second thought, we could do without the additional shadow byte >> value, by just comparing the address to the metadata offset. >> > > We could. But still, there is no point in doing anything like that. Ok, got it. -- Alexander Potapenko Software Engineer Google Germany GmbH Erika-Mann-Straße, 33 80636 München Geschäftsführer: Matthew Scott Sucherman, Paul Terence Manicle Registergericht und -nummer: Hamburg, HRB 86891 Sitz der Gesellschaft: Hamburg -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href