[PATCH AUTOSEL 5.4 077/134] ext4: fix a bug in ext4_wait_for_tail_page_commit

Sasha Levin
2019-12-11T16:02:18Z

From: yangerkun <yangerkun@huawei.com>

[ Upstream commit 565333a1554d704789e74205989305c811fd9c7a ]

No need to wait for any commit once the page is fully truncated.
Besides, it may confuse e.g. concurrent ext4_writepage() with the page
still be dirty (will be cleared by truncate_pagecache() in
ext4_setattr()) but buffers has been freed; and then trigger a bug
show as below:

[ 26.057508] ------------[ cut here ]------------
[ 26.058531] kernel BUG at fs/ext4/inode.c:2134!
...
[ 26.088130] Call trace:
[ 26.088695] ext4_writepage+0x914/0xb28
[ 26.089541] writeout.isra.4+0x1b4/0x2b8
[ 26.090409] move_to_new_page+0x3b0/0x568
[ 26.091338] __unmap_and_move+0x648/0x988
[ 26.092241] unmap_and_move+0x48c/0xbb8
[ 26.093096] migrate_pages+0x220/0xb28
[ 26.093945] kernel_mbind+0x828/0xa18
[ 26.094791] __arm64_sys_mbind+0xc8/0x138
[ 26.095716] el0_svc_common+0x190/0x490
[ 26.096571] el0_svc_handler+0x60/0xd0
[ 26.097423] el0_svc+0x8/0xc

Run the procedure (generate by syzkaller) parallel with ext3.

void main()
{
int fd, fd1, ret;
void *addr;
size_t length = 4096;
int flags;
off_t offset = 0;
char *str = "12345";

fd = open("a", O_RDWR | O_CREAT);
assert(fd >= 0);

/* Truncate to 4k */
ret = ftruncate(fd, length);
assert(ret == 0);

/* Journal data mode */
flags = 0xc00f;
ret = ioctl(fd, _IOW('f', 2, long), &flags);
assert(ret == 0);

/* Truncate to 0 */
fd1 = open("a", O_TRUNC | O_NOATIME);
assert(fd1 >= 0);

addr = mmap(NULL, length, PROT_WRITE | PROT_READ,
MAP_SHARED, fd, offset);
assert(addr != (void *)-1);

memcpy(addr, str, 5);
mbind(addr, length, 0, 0, 0, MPOL_MF_MOVE);
}

And the bug will be triggered once we seen the below order.

reproduce1 reproduce2

... | ...
truncate to 4k |
change to journal data mode |
| memcpy(set page dirty)
truncate to 0: |
ext4_setattr: |
... |
ext4_wait_for_tail_page_commit |
| mbind(trigger bug)
truncate_pagecache(clean dirty)| ...
... |

mbind will call ext4_writepage() since the page still be dirty, and then
report the bug since the buffers has been free. Fix it by return
directly once offset equals to 0 which means the page has been fully
truncated.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: yangerkun <yangerkun@huawei.com>
Link: https://lore.kernel.org/r/20190919063508.1045-1-yangerkun@huawei.com
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
fs/ext4/inode.c | 12 ++++++++----
1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 02313f10e8897..8b7c704b0de5e 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -5462,11 +5462,15 @@ static void ext4_wait_for_tail_page_commit(struct inode *inode)

offset = inode->i_size & (PAGE_SIZE - 1);
/*
- * All buffers in the last page remain valid? Then there's nothing to
- * do. We do the check mainly to optimize the common PAGE_SIZE ==
- * blocksize case
+ * If the page is fully truncated, we don't need to wait for any commit
+ * (and we even should not as __ext4_journalled_invalidatepage() may
+ * strip all buffers from the page but keep the page dirty which can then
+ * confuse e.g. concurrent ext4_writepage() seeing dirty page without
+ * buffers). Also we don't need to wait for any commit if all buffers in
+ * the page remain valid. This is most beneficial for the common case of
+ * blocksize == PAGESIZE.
*/
- if (offset > PAGE_SIZE - i_blocksize(inode))
+ if (!offset || offset > (PAGE_SIZE - i_blocksize(inode)))
return;
while (1) {
page = find_lock_page(inode->i_mapping,
--
2.20.1