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Subject[PATCH] ext4: fix a data race in EXT4_I(inode)->i_disksize
Date
EXT4_I(inode)->i_disksize could be accessed concurrently as noticed by
KCSAN,

BUG: KCSAN: data-race in ext4_write_end [ext4] / ext4_writepages [ext4]

write to 0xffff91c6713b00f8 of 8 bytes by task 49268 on cpu 127:
ext4_write_end+0x4e3/0x750 [ext4]
ext4_update_i_disksize at fs/ext4/ext4.h:3032
(inlined by) ext4_update_inode_size at fs/ext4/ext4.h:3046
(inlined by) ext4_write_end at fs/ext4/inode.c:1287
generic_perform_write+0x208/0x2a0
ext4_buffered_write_iter+0x11f/0x210 [ext4]
ext4_file_write_iter+0xce/0x9e0 [ext4]
new_sync_write+0x29c/0x3b0
__vfs_write+0x92/0xa0
vfs_write+0x103/0x260
ksys_write+0x9d/0x130
__x64_sys_write+0x4c/0x60
do_syscall_64+0x91/0xb47
entry_SYSCALL_64_after_hwframe+0x49/0xbe

read to 0xffff91c6713b00f8 of 8 bytes by task 24872 on cpu 37:
ext4_writepages+0x10ac/0x1d00 [ext4]
mpage_map_and_submit_extent at fs/ext4/inode.c:2468
(inlined by) ext4_writepages at fs/ext4/inode.c:2772
do_writepages+0x5e/0x130
__writeback_single_inode+0xeb/0xb20
writeback_sb_inodes+0x429/0x900
__writeback_inodes_wb+0xc4/0x150
wb_writeback+0x4bd/0x870
wb_workfn+0x6b4/0x960
process_one_work+0x54c/0xbe0
worker_thread+0x80/0x650
kthread+0x1e0/0x200
ret_from_fork+0x27/0x50

Reported by Kernel Concurrency Sanitizer on:
CPU: 37 PID: 24872 Comm: kworker/u261:2 Tainted: G W O L 5.5.0-next-20200204+ #5
Hardware name: HPE ProLiant DL385 Gen10/ProLiant DL385 Gen10, BIOS A40 07/10/2019
Workqueue: writeback wb_workfn (flush-7:0)

Since only the read is operating as lockless (outside of the
"i_data_sem"), a load tearing could introduce a logic bug. Fix it by
adding READ_ONCE() for the read.

Signed-off-by: Qian Cai <cai@lca.pw>
---
fs/ext4/inode.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 3313168b680f..6f9862bf63f1 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -2465,7 +2465,7 @@ static int mpage_map_and_submit_extent(handle_t *handle,
* truncate are avoided by checking i_size under i_data_sem.
*/
disksize = ((loff_t)mpd->first_page) << PAGE_SHIFT;
- if (disksize > EXT4_I(inode)->i_disksize) {
+ if (disksize > READ_ONCE(EXT4_I(inode)->i_disksize)) {
int err2;
loff_t i_size;

--
1.8.3.1
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