On 06/02/2016 03:02 PM, Alexander Potapenko wrote: > On Wed, Jun 1, 2016 at 6:31 PM, Alexander Potapenko <gli...@google.com> wrote: >> On Wed, Jun 1, 2016 at 5:23 PM, Andrey Ryabinin <aryabi...@virtuozzo.com> >> wrote: >>> On 05/31/2016 08:49 PM, Alexander Potapenko wrote: >>>> On Tue, May 31, 2016 at 1:52 PM, Andrey Ryabinin >>>> <aryabi...@virtuozzo.com> 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.
