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SubjectRe: KASAN: use-after-free Read in generic_perform_write
From
syzbot has found a reproducer for the following crash on:

HEAD commit: d1e0b8e0cb7a Add linux-next specific files for 20180725
git tree: linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=10eff978400000
kernel config: https://syzkaller.appspot.com/x/.config?x=eef3552c897e4d33
dashboard link: https://syzkaller.appspot.com/bug?extid=b173e77096a8ba815511
compiler: gcc (GCC) 8.0.1 20180413 (experimental)
syzkaller repro:https://syzkaller.appspot.com/x/repro.syz?x=171d9578400000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=132d14b4400000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+b173e77096a8ba815511@syzkaller.appspotmail.com

RDX: 0000000000000030 RSI: 0000000020011fd2 RDI: 0000000000000004
RBP: 00000000006dbc40 R08: 00000000006dbc40 R09: 0000000000000000
R10: 00007fc1cd74ccf0 R11: 0000000000000246 R12: 00000000006dbc4c
R13: 00007fffabd5ea5f R14: 00007fc1cd74d9c0 R15: 00000000006dbd4c
==================================================================
BUG: KASAN: use-after-free in memcpy include/linux/string.h:345 [inline]
BUG: KASAN: use-after-free in iov_iter_copy_from_user_atomic+0xb8d/0xfa0
lib/iov_iter.c:916
Read of size 21 at addr ffff8801ad780d60 by task kworker/0:1/13

CPU: 0 PID: 13 Comm: kworker/0:1 Not tainted 4.18.0-rc6-next-20180725+ #18
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
Workqueue: events p9_write_work
Call Trace:
__dump_stack lib/dump_stack.c:77 [inline]
dump_stack+0x1c9/0x2b4 lib/dump_stack.c:113
print_address_description+0x6c/0x20b mm/kasan/report.c:256
kasan_report_error mm/kasan/report.c:354 [inline]
kasan_report.cold.7+0x242/0x30d mm/kasan/report.c:412
check_memory_region_inline mm/kasan/kasan.c:260 [inline]
check_memory_region+0x13e/0x1b0 mm/kasan/kasan.c:267
memcpy+0x23/0x50 mm/kasan/kasan.c:302
memcpy include/linux/string.h:345 [inline]
iov_iter_copy_from_user_atomic+0xb8d/0xfa0 lib/iov_iter.c:916
generic_perform_write+0x469/0x6c0 mm/filemap.c:3147
__generic_file_write_iter+0x26e/0x630 mm/filemap.c:3264
ext4_file_write_iter+0x390/0x1450 fs/ext4/file.c:266
call_write_iter include/linux/fs.h:1807 [inline]
new_sync_write fs/read_write.c:474 [inline]
__vfs_write+0x6af/0x9d0 fs/read_write.c:487
vfs_write+0x1fc/0x560 fs/read_write.c:549
kernel_write+0xab/0x120 fs/read_write.c:526
p9_fd_write net/9p/trans_fd.c:432 [inline]
p9_write_work+0x6f1/0xd50 net/9p/trans_fd.c:481
process_one_work+0xc73/0x1ba0 kernel/workqueue.c:2153
worker_thread+0x189/0x13c0 kernel/workqueue.c:2296
kthread+0x345/0x410 kernel/kthread.c:246
ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:415

Allocated by task 4438:
save_stack+0x43/0xd0 mm/kasan/kasan.c:448
set_track mm/kasan/kasan.c:460 [inline]
kasan_kmalloc+0xc4/0xe0 mm/kasan/kasan.c:553
__do_kmalloc mm/slab.c:3718 [inline]
__kmalloc+0x14e/0x760 mm/slab.c:3727
kmalloc include/linux/slab.h:518 [inline]
p9_fcall_alloc+0x1e/0x90 net/9p/client.c:237
p9_tag_alloc net/9p/client.c:266 [inline]
p9_client_prepare_req.part.8+0x107/0xa00 net/9p/client.c:647
p9_client_prepare_req net/9p/client.c:682 [inline]
p9_client_rpc+0x247/0x1420 net/9p/client.c:682
p9_client_version net/9p/client.c:897 [inline]
p9_client_create+0xd76/0x1631 net/9p/client.c:981
v9fs_session_init+0x21a/0x1a80 fs/9p/v9fs.c:400
v9fs_mount+0x7c/0x900 fs/9p/vfs_super.c:135
legacy_get_tree+0x131/0x460 fs/fs_context.c:674
vfs_get_tree+0x1cb/0x5c0 fs/super.c:1762
do_new_mount fs/namespace.c:2629 [inline]
do_mount+0x6f2/0x1e20 fs/namespace.c:2953
ksys_mount+0x12d/0x140 fs/namespace.c:3169
__do_sys_mount fs/namespace.c:3183 [inline]
__se_sys_mount fs/namespace.c:3180 [inline]
__x64_sys_mount+0xbe/0x150 fs/namespace.c:3180
do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
entry_SYSCALL_64_after_hwframe+0x49/0xbe

Freed by task 4438:
save_stack+0x43/0xd0 mm/kasan/kasan.c:448
set_track mm/kasan/kasan.c:460 [inline]
__kasan_slab_free+0x11a/0x170 mm/kasan/kasan.c:521
kasan_slab_free+0xe/0x10 mm/kasan/kasan.c:528
__cache_free mm/slab.c:3498 [inline]
kfree+0xd9/0x260 mm/slab.c:3813
p9_free_req+0xb5/0x120 net/9p/client.c:338
p9_client_rpc+0xb20/0x1420 net/9p/client.c:746
p9_client_version net/9p/client.c:897 [inline]
p9_client_create+0xd76/0x1631 net/9p/client.c:981
v9fs_session_init+0x21a/0x1a80 fs/9p/v9fs.c:400
v9fs_mount+0x7c/0x900 fs/9p/vfs_super.c:135
legacy_get_tree+0x131/0x460 fs/fs_context.c:674
vfs_get_tree+0x1cb/0x5c0 fs/super.c:1762
do_new_mount fs/namespace.c:2629 [inline]
do_mount+0x6f2/0x1e20 fs/namespace.c:2953
ksys_mount+0x12d/0x140 fs/namespace.c:3169
__do_sys_mount fs/namespace.c:3183 [inline]
__se_sys_mount fs/namespace.c:3180 [inline]
__x64_sys_mount+0xbe/0x150 fs/namespace.c:3180
do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
entry_SYSCALL_64_after_hwframe+0x49/0xbe

The buggy address belongs to the object at ffff8801ad780d40
which belongs to the cache kmalloc-16384 of size 16384
The buggy address is located 32 bytes inside of
16384-byte region [ffff8801ad780d40, ffff8801ad784d40)
The buggy address belongs to the page:
page:ffffea0006b5e000 count:1 mapcount:0 mapping:ffff8801dac02200 index:0x0
compound_mapcount: 0
flags: 0x2fffc0000008100(slab|head)
raw: 02fffc0000008100 ffffea00072e3008 ffffea0006c75408 ffff8801dac02200
raw: 0000000000000000 ffff8801ad780d40 0000000100000001 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
ffff8801ad780c00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
ffff8801ad780c80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> ffff8801ad780d00: fc fc fc fc fc fc fc fc fb fb fb fb fb fb fb fb
^
ffff8801ad780d80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff8801ad780e00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
==================================================================

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