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ext4: fix a bug in ext4_wait_for_tail_page_commit

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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>
This commit is contained in:
yangerkun 2019-09-19 14:35:08 +08:00 committed by Theodore Ts'o
parent 5500221ea1
commit 565333a155
1 changed files with 8 additions and 4 deletions
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12
fs/ext4/inode.c
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@ -5114,11 +5114,15 @@ 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,