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SubjectKASAN: use-after-free Read in dmabuffs_dname

syzbot found the following crash on:

HEAD commit: 63623fd4 Merge tag 'for-linus' of git://
git tree: upstream
console output:
kernel config:
dashboard link:
compiler: gcc (GCC) 9.0.0 20181231 (experimental)
userspace arch: i386

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:

BUG: KASAN: use-after-free in dmabuffs_dname+0x4f4/0x560 drivers/dma-buf/dma-buf.c:48
Read of size 8 at addr ffff8880a6b390e8 by task syz-executor.1/2394

CPU: 1 PID: 2394 Comm: syz-executor.1 Not tainted 5.6.0-rc3-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+0x197/0x210 lib/dump_stack.c:118
print_address_description.constprop.0.cold+0xd4/0x30b mm/kasan/report.c:374
__kasan_report.cold+0x1b/0x32 mm/kasan/report.c:506
kasan_report+0x12/0x20 mm/kasan/common.c:641
__asan_report_load8_noabort+0x14/0x20 mm/kasan/generic_report.c:135
dmabuffs_dname+0x4f4/0x560 drivers/dma-buf/dma-buf.c:48
tomoyo_realpath_from_path+0x165/0x660 security/tomoyo/realpath.c:259
tomoyo_get_realpath security/tomoyo/file.c:151 [inline]
tomoyo_check_open_permission+0x2a3/0x3e0 security/tomoyo/file.c:771
tomoyo_file_open security/tomoyo/tomoyo.c:319 [inline]
tomoyo_file_open+0xa9/0xd0 security/tomoyo/tomoyo.c:314
security_file_open+0x71/0x300 security/security.c:1529
do_dentry_open+0x37a/0x1380 fs/open.c:784
vfs_open+0xa0/0xd0 fs/open.c:914
do_last fs/namei.c:3490 [inline]
path_openat+0x12ee/0x3490 fs/namei.c:3607
do_filp_open+0x192/0x260 fs/namei.c:3637
do_sys_openat2+0x5eb/0x7e0 fs/open.c:1149
do_sys_open+0xf2/0x180 fs/open.c:1165
__do_compat_sys_open fs/open.c:1212 [inline]
__se_compat_sys_open fs/open.c:1210 [inline]
__ia32_compat_sys_open+0x79/0xb0 fs/open.c:1210
do_syscall_32_irqs_on arch/x86/entry/common.c:337 [inline]
do_fast_syscall_32+0x27b/0xe16 arch/x86/entry/common.c:408
entry_SYSENTER_compat+0x70/0x7f arch/x86/entry/entry_64_compat.S:139
RIP: 0023:0xf7fd8e39
Code: 1d 00 00 00 89 d3 5b 5e 5d c3 8b 04 24 c3 8b 1c 24 c3 8b 3c 24 c3 90 90 90 90 90 90 90 90 90 90 90 51 52 55 89 e5 0f 34 cd 80 <5d> 5a 59 c3 90 90 90 90 eb 0d 90 90 90 90 90 90 90 90 90 90 90 90
RSP: 002b:00000000f5db2014 EFLAGS: 00000296 ORIG_RAX: 0000000000000005
RAX: ffffffffffffffda RBX: 00000000f5db204c RCX: 0000000000000000
RDX: 0000000000000000 RSI: 0000000000000c09 RDI: 00000000f5db204c
RBP: 00000000f5db2168 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000

Allocated by task 2388:
save_stack+0x23/0x90 mm/kasan/common.c:72
set_track mm/kasan/common.c:80 [inline]
__kasan_kmalloc mm/kasan/common.c:515 [inline]
__kasan_kmalloc.constprop.0+0xcf/0xe0 mm/kasan/common.c:488
kasan_kmalloc+0x9/0x10 mm/kasan/common.c:529
__do_kmalloc mm/slab.c:3656 [inline]
__kmalloc+0x163/0x770 mm/slab.c:3665
kmalloc include/linux/slab.h:560 [inline]
kzalloc include/linux/slab.h:669 [inline]
dma_buf_export+0x24d/0xa80 drivers/dma-buf/dma-buf.c:533
ion_alloc drivers/staging/android/ion/ion.c:386 [inline]
ion_ioctl+0x5a9/0xd20 drivers/staging/android/ion/ion.c:495
compat_ptr_ioctl+0x6e/0xa0 fs/ioctl.c:804
__do_compat_sys_ioctl fs/ioctl.c:857 [inline]
__se_compat_sys_ioctl fs/ioctl.c:808 [inline]
__ia32_compat_sys_ioctl+0x245/0x2c0 fs/ioctl.c:808
do_syscall_32_irqs_on arch/x86/entry/common.c:337 [inline]
do_fast_syscall_32+0x27b/0xe16 arch/x86/entry/common.c:408
entry_SYSENTER_compat+0x70/0x7f arch/x86/entry/entry_64_compat.S:139

Freed by task 2380:
save_stack+0x23/0x90 mm/kasan/common.c:72
set_track mm/kasan/common.c:80 [inline]
kasan_set_free_info mm/kasan/common.c:337 [inline]
__kasan_slab_free+0x102/0x150 mm/kasan/common.c:476
kasan_slab_free+0xe/0x10 mm/kasan/common.c:485
__cache_free mm/slab.c:3426 [inline]
kfree+0x10a/0x2c0 mm/slab.c:3757
dma_buf_release+0x343/0x420 drivers/dma-buf/dma-buf.c:111
__fput+0x2ff/0x890 fs/file_table.c:280
____fput+0x16/0x20 fs/file_table.c:313
task_work_run+0x145/0x1c0 kernel/task_work.c:113
tracehook_notify_resume include/linux/tracehook.h:188 [inline]
exit_to_usermode_loop+0x316/0x380 arch/x86/entry/common.c:164
prepare_exit_to_usermode arch/x86/entry/common.c:195 [inline]
syscall_return_slowpath arch/x86/entry/common.c:278 [inline]
do_syscall_32_irqs_on arch/x86/entry/common.c:352 [inline]
do_fast_syscall_32+0xbbd/0xe16 arch/x86/entry/common.c:408
entry_SYSENTER_compat+0x70/0x7f arch/x86/entry/entry_64_compat.S:139

The buggy address belongs to the object at ffff8880a6b39000
which belongs to the cache kmalloc-1k of size 1024
The buggy address is located 232 bytes inside of
1024-byte region [ffff8880a6b39000, ffff8880a6b39400)
The buggy address belongs to the page:
page:ffffea00029ace40 refcount:1 mapcount:0 mapping:ffff8880aa400c40 index:0x0
flags: 0xfffe0000000200(slab)
raw: 00fffe0000000200 ffffea0002346b48 ffffea00022f49c8 ffff8880aa400c40
raw: 0000000000000000 ffff8880a6b39000 0000000100000002 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
ffff8880a6b38f80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
ffff8880a6b39000: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>ffff8880a6b39080: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff8880a6b39100: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff8880a6b39180: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb

This bug is generated by a bot. It may contain errors.
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