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

[Dmitry Vyukov]

On Thu, Dec 28, 2017 at 1:02 PM, syzbot
<syzbot+3657e9e2c5ad99152f32@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
> Hello,
>
> syzkaller hit the following crash on
> 464e1d5f23cca236b930ef068c328a64cab78fb1
> git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/master
> compiler: gcc (GCC) 7.1.1 20170620
> .config is attached
> Raw console output is attached.
> Unfortunately, I don't have any reproducer for this bug yet.
>
>
> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> Reported-by: syzbot+3657e9e2c5ad99152f32@xxxxxxxxxxxxxxxxxxxxxxxxx
> It will help syzbot understand when the bug is fixed. See footer for
> details.
> If you forward the report, please keep this part and the footer.
>
> device gre0 entered promiscuous mode
> kasan: CONFIG_KASAN_INLINE enabled
> ==================================================================
> BUG: KASAN: use-after-free in __fcheck_files include/linux/fdtable.h:85
> [inline]
> BUG: KASAN: use-after-free in fcheck_files include/linux/fdtable.h:95
> [inline]
> BUG: KASAN: use-after-free in __fget+0x547/0x570 fs/file.c:690
> Read of size 4 at addr ffff8801d4302f00 by task syz-executor4/22370
>
> CPU: 0 PID: 22370 Comm: syz-executor4 Not tainted 4.15.0-rc5+ #147
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
> Google 01/01/2011
> Call Trace:
>  __dump_stack lib/dump_stack.c:17 [inline]
>  dump_stack+0x194/0x257 lib/dump_stack.c:53
>  print_address_description+0x73/0x250 mm/kasan/report.c:252
>  kasan_report_error mm/kasan/report.c:351 [inline]
>  kasan_report+0x25b/0x340 mm/kasan/report.c:409
>  __asan_report_load4_noabort+0x14/0x20 mm/kasan/report.c:429
>  __fcheck_files include/linux/fdtable.h:85 [inline]
>  fcheck_files include/linux/fdtable.h:95 [inline]
>  __fget+0x547/0x570 fs/file.c:690
>  __fget_light+0x2eb/0x380 fs/file.c:745
>  __fdget+0x18/0x20 fs/file.c:753
>  fdget include/linux/file.h:57 [inline]
>  sockfd_lookup_light+0x21/0x150 net/socket.c:495
>  SYSC_sendto+0x144/0x5c0 net/socket.c:1709
>  SyS_sendto+0x40/0x50 net/socket.c:1695
>  do_syscall_32_irqs_on arch/x86/entry/common.c:327 [inline]
>  do_fast_syscall_32+0x3ee/0xf9d arch/x86/entry/common.c:389
>  entry_SYSENTER_compat+0x54/0x63 arch/x86/entry/entry_64_compat.S:125
> RIP: 0023:0xf7f53c79
> RSP: 002b:00000000f772e08c EFLAGS: 00000296 ORIG_RAX: 0000000000000171
> RAX: ffffffffffffffda RBX: 0000000000000013 RCX: 0000000020ab7000
> RDX: 0000000000000001 RSI: 0000000000000000 RDI: 0000000020ab8000
> RBP: 000000000000001c R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
> R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
>
> Allocated by task 22368:
>  save_stack+0x43/0xd0 mm/kasan/kasan.c:447
>  set_track mm/kasan/kasan.c:459 [inline]
>  kasan_kmalloc+0xad/0xe0 mm/kasan/kasan.c:551
>  __do_kmalloc mm/slab.c:3708 [inline]
>  __kmalloc+0x162/0x760 mm/slab.c:3717
>  kmalloc include/linux/slab.h:504 [inline]
>  kzalloc include/linux/slab.h:688 [inline]
>  ext4_htree_store_dirent+0x8b/0x580 fs/ext4/dir.c:451
>  htree_dirblock_to_tree+0x4e8/0xa00 fs/ext4/namei.c:1019
>  ext4_htree_fill_tree+0x2bb/0xcb0 fs/ext4/namei.c:1096
>  ext4_dx_readdir fs/ext4/dir.c:575 [inline]
>  ext4_readdir+0x1d03/0x3050 fs/ext4/dir.c:122
>  iterate_dir+0x1ca/0x530 fs/readdir.c:51
>  SYSC_getdents fs/readdir.c:231 [inline]
>  SyS_getdents+0x225/0x450 fs/readdir.c:212
>  entry_SYSCALL_64_fastpath+0x1f/0x96
>
> Freed by task 22368:
>  save_stack+0x43/0xd0 mm/kasan/kasan.c:447
>  set_track mm/kasan/kasan.c:459 [inline]
>  kasan_slab_free+0x71/0xc0 mm/kasan/kasan.c:524
>  __cache_free mm/slab.c:3488 [inline]
>  kfree+0xd6/0x260 mm/slab.c:3803
>  free_rb_tree_fname+0x85/0xe0 fs/ext4/dir.c:403
>  ext4_htree_free_dir_info fs/ext4/dir.c:425 [inline]
>  ext4_release_dir+0x44/0x60 fs/ext4/dir.c:623
>  __fput+0x327/0x7e0 fs/file_table.c:210
>  ____fput+0x15/0x20 fs/file_table.c:244
>  task_work_run+0x199/0x270 kernel/task_work.c:113
>  tracehook_notify_resume include/linux/tracehook.h:191 [inline]
>  exit_to_usermode_loop+0x296/0x310 arch/x86/entry/common.c:162
>  prepare_exit_to_usermode arch/x86/entry/common.c:195 [inline]
>  syscall_return_slowpath+0x490/0x550 arch/x86/entry/common.c:264
>  entry_SYSCALL_64_fastpath+0x94/0x96
>
> The buggy address belongs to the object at ffff8801d4302f00
>  which belongs to the cache kmalloc-64 of size 64
> The buggy address is located 0 bytes inside of
>  64-byte region [ffff8801d4302f00, ffff8801d4302f40)
> The buggy address belongs to the page:
> page:0000000089700587 count:1 mapcount:0 mapping:00000000159189cf
> index:0xffff8801d4302400
> flags: 0x2fffc0000000100(slab)
> raw: 02fffc0000000100 ffff8801d4302000 ffff8801d4302400 000000010000001b
> raw: ffffea00070cfda0 ffffea0006cb16e0 ffff8801db000340 0000000000000000
> page dumped because: kasan: bad access detected
>
> Memory state around the buggy address:
>  ffff8801d4302e00: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
>  ffff8801d4302e80: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
>>
>> ffff8801d4302f00: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
>
>                    ^
>  ffff8801d4302f80: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
>  ffff8801d4303000: 00 00 00 00 fc fc fc fc 00 00 00 fc fc fc fc fc
> ==================================================================


This happened only once so far. Can be a subtle race, can be a
previous silent memory corruption. So just in case you see something
obvious in the code.

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