KASAN: use-after-free Read in d_walk

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    d9862cfb Merge tag 'mips_5.1' of git://git.kernel.org/pub/..
git tree:       net-next
console output: https://syzkaller.appspot.com/x/log.txt?x=115d9bc7200000
kernel config:  https://syzkaller.appspot.com/x/.config?x=73d88a42238825ad
dashboard link: https://syzkaller.appspot.com/bug?extid=353f82569238b73a3cbe
compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
userspace arch: amd64

Unfortunately, I don't have any reproducer for this crash yet.

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+353f82569238b73a3cbe@xxxxxxxxxxxxxxxxxxxxxxxxx

==================================================================
BUG: KASAN: use-after-free in d_walk+0x86c/0x950 fs/dcache.c:1281
Read of size 8 at addr ffff888095e6add0 by task syz-executor.0/7708

CPU: 1 PID: 7708 Comm: syz-executor.0 Not tainted 5.0.0+ #97
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+0x172/0x1f0 lib/dump_stack.c:113
 print_address_description.cold+0x7c/0x20d mm/kasan/report.c:187
 kasan_report.cold+0x1b/0x40 mm/kasan/report.c:317
 __asan_report_load8_noabort+0x14/0x20 mm/kasan/generic_report.c:135
 d_walk+0x86c/0x950 fs/dcache.c:1281
 shrink_dcache_parent+0xa4/0x140 fs/dcache.c:1496
 d_invalidate fs/dcache.c:1590 [inline]
 d_invalidate+0xfb/0x230 fs/dcache.c:1575
 proc_flush_task_mnt fs/proc/base.c:3069 [inline]
 proc_flush_task+0x28a/0x4d0 fs/proc/base.c:3139
 release_task+0x18c/0x1620 kernel/exit.c:196
 wait_task_zombie kernel/exit.c:1164 [inline]
 wait_consider_task+0x2c95/0x3910 kernel/exit.c:1391
 do_wait_thread kernel/exit.c:1454 [inline]
 do_wait+0x439/0x9d0 kernel/exit.c:1525
 kernel_wait4+0x171/0x290 kernel/exit.c:1667
 __do_sys_wait4+0x147/0x160 kernel/exit.c:1679
 __se_sys_wait4 kernel/exit.c:1675 [inline]
 __x64_sys_wait4+0x97/0xf0 kernel/exit.c:1675
 do_syscall_64+0x103/0x610 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x4120ba
Code: 0f 83 1a 17 00 00 c3 66 0f 1f 84 00 00 00 00 00 8b 05 de 41 64 00 85  
c0 75 36 45 31 d2 48 63 d2 48 63 ff b8 3d 00 00 00 0f 05 <48> 3d 00 f0 ff  
ff 77 06 c3 0f 1f 44 00 00 48 c7 c2 d4 ff ff ff f7
RSP: 002b:00007ffe51c30ab8 EFLAGS: 00000246 ORIG_RAX: 000000000000003d
RAX: ffffffffffffffda RBX: 0000000000031389 RCX: 00000000004120ba
RDX: 0000000040000001 RSI: 00007ffe51c30af0 RDI: ffffffffffffffff
RBP: 00000000000001cb R08: 0000000000000001 R09: 0000000001aa7940
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000009
R13: 00007ffe51c30af0 R14: 000000000003106c R15: 00007ffe51c30b00

Allocated by task 11550:
 save_stack+0x45/0xd0 mm/kasan/common.c:73
 set_track mm/kasan/common.c:85 [inline]
 __kasan_kmalloc mm/kasan/common.c:495 [inline]
 __kasan_kmalloc.constprop.0+0xcf/0xe0 mm/kasan/common.c:468
 kasan_slab_alloc+0xf/0x20 mm/kasan/common.c:503
 slab_post_alloc_hook mm/slab.h:440 [inline]
 slab_alloc mm/slab.c:3388 [inline]
 kmem_cache_alloc+0x11a/0x6f0 mm/slab.c:3548
 __d_alloc+0x2e/0x8c0 fs/dcache.c:1622
 d_alloc_pseudo+0x1e/0x30 fs/dcache.c:1745
 alloc_file_pseudo+0xe2/0x280 fs/file_table.c:224
 sock_alloc_file+0x4d/0x170 net/socket.c:394
 sock_map_fd net/socket.c:417 [inline]
 __sys_socket+0x150/0x220 net/socket.c:1372
 __do_sys_socket net/socket.c:1377 [inline]
 __se_sys_socket net/socket.c:1375 [inline]
 __x64_sys_socket+0x73/0xb0 net/socket.c:1375
 do_syscall_64+0x103/0x610 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x49/0xbe

Freed by task 11539:
 save_stack+0x45/0xd0 mm/kasan/common.c:73
 set_track mm/kasan/common.c:85 [inline]
 __kasan_slab_free+0x102/0x150 mm/kasan/common.c:457
 kasan_slab_free+0xe/0x10 mm/kasan/common.c:465
 __cache_free mm/slab.c:3494 [inline]
 kmem_cache_free+0x86/0x260 mm/slab.c:3754
 __d_free fs/dcache.c:269 [inline]
 dentry_free+0xed/0x170 fs/dcache.c:348
 __dentry_kill+0x45f/0x610 fs/dcache.c:593
 dentry_kill+0xd7/0x5e0 fs/dcache.c:698
 dput+0x570/0x6a0 fs/dcache.c:859
 __fput+0x429/0x8d0 fs/file_table.c:291
 ____fput+0x16/0x20 fs/file_table.c:309
 task_work_run+0x14a/0x1c0 kernel/task_work.c:113
 tracehook_notify_resume include/linux/tracehook.h:188 [inline]
 exit_to_usermode_loop+0x273/0x2c0 arch/x86/entry/common.c:166
 prepare_exit_to_usermode arch/x86/entry/common.c:197 [inline]
 syscall_return_slowpath arch/x86/entry/common.c:268 [inline]
 do_syscall_64+0x52d/0x610 arch/x86/entry/common.c:293
 entry_SYSCALL_64_after_hwframe+0x49/0xbe

The buggy address belongs to the object at ffff888095e6ace0
 which belongs to the cache dentry(17:syz0) of size 288
The buggy address is located 240 bytes inside of
 288-byte region [ffff888095e6ace0, ffff888095e6ae00)
The buggy address belongs to the page:
page:ffffea0002579a80 count:1 mapcount:0 mapping:ffff888098f7be40 index:0x0
flags: 0x1fffc0000000200(slab)
raw: 01fffc0000000200 ffffea0002577cc8 ffffea000257df48 ffff888098f7be40
raw: 0000000000000000 ffff888095e6a080 000000010000000b ffff88806d9baec0
page dumped because: kasan: bad access detected
page->mem_cgroup:ffff88806d9baec0

Memory state around the buggy address:
 ffff888095e6ac80: 00 00 00 00 fc fc fc fc fc fc fc fc fb fb fb fb
 ffff888095e6ad00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> ffff888095e6ad80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                                                 ^
 ffff888095e6ae00: fc fc fc fc fc fc fc fc fb fb fb fb fb fb fb fb
 ffff888095e6ae80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
==================================================================


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