Re: KASAN: use-after-free Read in iov_iter_alignment

[Jan Kara]

On Tue 03-12-19 08:10:37, Dave Chinner wrote:
> [cc linux-ext4@xxxxxxxxxxxxxxx - this is reported from the new ext4
> dio->iomap code]
> 
> On Mon, Dec 02, 2019 at 09:15:08AM -0800, syzbot wrote:
> > Hello,
> > 
> > syzbot found the following crash on:
> > 
> > HEAD commit:    b94ae8ad Merge tag 'seccomp-v5.5-rc1' of git://git.kernel...
> > git tree:       upstream
> > console output: https://syzkaller.appspot.com/x/log.txt?x=135a8d7ae00000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=c2e464ae414aee8c
> > dashboard link: https://syzkaller.appspot.com/bug?extid=bea68382bae9490e7dd6
> > compiler:       clang version 9.0.0 (/home/glider/llvm/clang
> > 80fee25776c2fb61e74c1ecb1a523375c2500b69)
> > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1135cb36e00000
> > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=14e90abce00000
> > 
> > IMPORTANT: if you fix the bug, please add the following tag to the commit:
> > Reported-by: syzbot+bea68382bae9490e7dd6@xxxxxxxxxxxxxxxxxxxxxxxxx
> > 
> > ==================================================================
> > BUG: KASAN: use-after-free in iov_iter_alignment+0x6a1/0x7b0
> > lib/iov_iter.c:1225
> > Read of size 4 at addr ffff888098d40f54 by task loop0/8203
> > 
> > CPU: 0 PID: 8203 Comm: loop0 Not tainted 5.4.0-syzkaller #0
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
> > Google 01/01/2011
> > Call Trace:
> >  __dump_stack lib/dump_stack.c:77 [inline]
> >  dump_stack+0x1fb/0x318 lib/dump_stack.c:118
> >  print_address_description+0x75/0x5c0 mm/kasan/report.c:374
> >  __kasan_report+0x14b/0x1c0 mm/kasan/report.c:506
> >  kasan_report+0x26/0x50 mm/kasan/common.c:634
> >  __asan_report_load4_noabort+0x14/0x20 mm/kasan/generic_report.c:131
> >  iov_iter_alignment+0x6a1/0x7b0 lib/iov_iter.c:1225
> >  iomap_dio_bio_actor+0x1a7/0x11e0 fs/iomap/direct-io.c:203
> >  iomap_dio_actor+0x2b4/0x4a0 fs/iomap/direct-io.c:375
> >  iomap_apply+0x370/0x490 fs/iomap/apply.c:80
> >  iomap_dio_rw+0x8ad/0x1010 fs/iomap/direct-io.c:493
> >  ext4_dio_read_iter fs/ext4/file.c:77 [inline]
> >  ext4_file_read_iter+0x834/0xc20 fs/ext4/file.c:128
> >  lo_rw_aio+0xcbb/0xea0 include/linux/fs.h:1889
> 
> loopback -> ext4 direct IO, bad access on iov passed to iomap DIO
> code.
> 
> >  do_req_filebacked drivers/block/loop.c:616 [inline]
> >  loop_handle_cmd drivers/block/loop.c:1952 [inline]
> >  loop_queue_work+0x13ab/0x2590 drivers/block/loop.c:1966
> >  kthread_worker_fn+0x449/0x700 kernel/kthread.c:671
> >  loop_kthread_worker_fn+0x40/0x60 drivers/block/loop.c:901
> >  kthread+0x332/0x350 kernel/kthread.c:255
> >  ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:352
> > 
> > Allocated by task 4198:
> >  save_stack mm/kasan/common.c:69 [inline]
> >  set_track mm/kasan/common.c:77 [inline]
> >  __kasan_kmalloc+0x11c/0x1b0 mm/kasan/common.c:510
> >  kasan_slab_alloc+0xf/0x20 mm/kasan/common.c:518
> >  slab_post_alloc_hook mm/slab.h:584 [inline]
> >  slab_alloc mm/slab.c:3319 [inline]
> >  kmem_cache_alloc+0x1f5/0x2e0 mm/slab.c:3483
> >  mempool_alloc_slab+0x4d/0x70 mm/mempool.c:513
> >  mempool_alloc+0x104/0x5e0 mm/mempool.c:393
> >  bio_alloc_bioset+0x1b0/0x5f0 block/bio.c:477
> >  bio_alloc include/linux/bio.h:400 [inline]
> >  mpage_alloc fs/mpage.c:79 [inline]
> >  do_mpage_readpage+0x1685/0x1d10 fs/mpage.c:306
> >  mpage_readpages+0x2a9/0x440 fs/mpage.c:404
> >  blkdev_readpages+0x2c/0x40 fs/block_dev.c:620
> >  read_pages+0xad/0x4d0 mm/readahead.c:126
> >  __do_page_cache_readahead+0x480/0x530 mm/readahead.c:212
> >  force_page_cache_readahead mm/readahead.c:243 [inline]
> >  page_cache_sync_readahead+0x329/0x3b0 mm/readahead.c:522
> >  generic_file_buffered_read+0x41d/0x2570 mm/filemap.c:2051
> >  generic_file_read_iter+0xa9/0x450 mm/filemap.c:2324
> >  blkdev_read_iter+0x12e/0x140 fs/block_dev.c:2039
> >  call_read_iter include/linux/fs.h:1889 [inline]
> >  new_sync_read fs/read_write.c:414 [inline]
> >  __vfs_read+0x59e/0x730 fs/read_write.c:427
> >  vfs_read+0x1dd/0x420 fs/read_write.c:461
> >  ksys_read+0x117/0x220 fs/read_write.c:587
> >  __do_sys_read fs/read_write.c:597 [inline]
> >  __se_sys_read fs/read_write.c:595 [inline]
> >  __x64_sys_read+0x7b/0x90 fs/read_write.c:595
> >  do_syscall_64+0xf7/0x1c0 arch/x86/entry/common.c:294
> >  entry_SYSCALL_64_after_hwframe+0x49/0xbe
> > 
> > Freed by task 4205:
> >  save_stack mm/kasan/common.c:69 [inline]
> >  set_track mm/kasan/common.c:77 [inline]
> >  kasan_set_free_info mm/kasan/common.c:332 [inline]
> >  __kasan_slab_free+0x12a/0x1e0 mm/kasan/common.c:471
> >  kasan_slab_free+0xe/0x10 mm/kasan/common.c:480
> >  __cache_free mm/slab.c:3425 [inline]
> >  kmem_cache_free+0x81/0xf0 mm/slab.c:3693
> >  mempool_free_slab+0x1d/0x30 mm/mempool.c:520
> >  mempool_free+0xd5/0x350 mm/mempool.c:502
> >  bio_put+0x38b/0x460 block/bio.c:255
> >  mpage_end_io+0x2f5/0x330 fs/mpage.c:58
> >  bio_endio+0x4ff/0x570 block/bio.c:1818
> >  req_bio_endio block/blk-core.c:245 [inline]
> >  blk_update_request+0x438/0x10d0 block/blk-core.c:1464
> >  scsi_end_request+0x8c/0xa20 drivers/scsi/scsi_lib.c:579
> >  scsi_io_completion+0x17c/0x1b80 drivers/scsi/scsi_lib.c:963
> >  scsi_finish_command+0x3b3/0x560 drivers/scsi/scsi.c:228
> >  scsi_softirq_done+0x289/0x310 drivers/scsi/scsi_lib.c:1477
> >  blk_done_softirq+0x312/0x370 block/blk-softirq.c:37
> >  __do_softirq+0x333/0x7c4 arch/x86/include/asm/paravirt.h:762
> 
> Looks like buffered read IO on a loopback device on an ext4 image
> file, and something is being tripped over in the new ext4 direct IO
> path.  Might be an iomap issue, might be an ext4 issue, but it looks
> like the buffered read bio completion is running while the iov is
> still being submitted...

Looking a bit more into this, I'm pretty sure this is caused by commit
8cefc107ca54c "pipe: Use head and tail pointers for the ring, not cursor
and length". The pipe dereference it has added to iov_iter_alignment() is
just bogus for all iter types except for pipes. I'll send a fix.

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR