From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Subject: Re: [GIT PULL] Block pull request for- 4.11-rc1 To: Linus Torvalds , Bart Van Assche References: <1D08B61A9CF0974AA09887BE32D889DA0A74F3@ULS-OP-MBXIP03.sdcorp.global.sandisk.com> <633d226d-cec3-a01f-a069-ffff307e9715@kernel.dk> <20170220073539.GA17687@lst.de> <1D08B61A9CF0974AA09887BE32D889DA0A8CBD@ULS-OP-MBXIP03.sdcorp.global.sandisk.com> <1487957968.2575.6.camel@sandisk.com> Cc: "hch@lst.de" , "linux-kernel@vger.kernel.org" , "linux-block@vger.kernel.org" , "snitzer@redhat.com" From: Jens Axboe Message-ID: <0af4ed69-a9d0-44be-701c-ced76b84bfef@kernel.dk> Date: Fri, 24 Feb 2017 13:00:39 -0700 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 List-ID: On 02/24/2017 12:43 PM, Linus Torvalds wrote: > On Fri, Feb 24, 2017 at 9:39 AM, Bart Van Assche > wrote: >> >> So the crash is caused by an attempt to dereference address 0x6b6b6b6b6b6b6b6b >> at offset 0x270. I think this means the crash is caused by a use-after-free. > > Yeah, that's POISON_FREE, and that might explain why you see crashes > that others don't - you obviously have SLAB poisoning enabled. Jens > may not have. > > %rdi is "struct mapped_device *md", which came from dm_softirq_done() doing > > struct dm_rq_target_io *tio = tio_from_request(rq); > struct request *clone = tio->clone; > int rw; > > if (!clone) { > rq_end_stats(tio->md, rq); > rw = rq_data_dir(rq); > if (!rq->q->mq_ops) > blk_end_request_all(rq, tio->error); > else > blk_mq_end_request(rq, tio->error); > rq_completed(tio->md, rw, false); > return; > } > > so it's the 'tio' pointer that has been free'd. But it's worth noting > that we did apparently successfully dereference "tio" earlier in that > dm_softirq_done() *without* getting the poison value, so what I think > might be going on is that the 'tio' thing gets free'd when the code > does the blk_end_request_all()/blk_mq_end_request() call. > > Which makes sense - that ends the lifetime of the request, which in > turn also ends the lifetime of the "tio_from_request()", no? > > So the fix may be as simple as just doing > > if (!clone) { > struct mapped_device *md = tio->md; > > rq_end_stats(md, rq); > ... > rq_completed(md, rw, false); > return; > } > > because the 'mapped_device' pointer hopefully is still valid, it's > just 'tio' that has been freed. > > Jens? Bart? Christoph? Somebody who knows this code should > double-check my thinking above. I don't actually know the tio > lifetimes, I'm just going by looking at how earlier accesses seemed to > be fine (eg that "tio->clone" got us NULL, not a POISON_FREE pointer, > for example). I think that is spot on. With the request changes for CDBs, for non blk-mq, we know also carry the payload after the request. But since blk-mq never frees the request, the above use-after-free with poison will only happen for !mq. Caching 'md' and avoiding a dereference of 'tio' after calling blk_end_request_all() will likely fix it. Bart, can you test that? -- Jens Axboe