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ext4: fix bh->b_state corruption

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ext4 can update bh->b_state non-atomically in _ext4_get_block() and
ext4_da_get_block_prep(). Usually this is fine since bh is just a
temporary storage for mapping information on stack but in some cases it
can be fully living bh attached to a page. In such case non-atomic
update of bh->b_state can race with an atomic update which then gets
lost. Usually when we are mapping bh and thus updating bh->b_state
non-atomically, nobody else touches the bh and so things work out fine
but there is one case to especially worry about: ext4_finish_bio() uses
BH_Uptodate_Lock on the first bh in the page to synchronize handling of
PageWriteback state. So when blocksize < pagesize, we can be atomically
modifying bh->b_state of a buffer that actually isn't under IO and thus
can race e.g. with delalloc trying to map that buffer. The result is
that we can mistakenly set / clear BH_Uptodate_Lock bit resulting in the
corruption of PageWriteback state or missed unlock of BH_Uptodate_Lock.

Fix the problem by always updating bh->b_state bits atomically.

CC: stable@vger.kernel.org
Reported-by: Nikolay Borisov <kernel@kyup.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
This commit is contained in:
Jan Kara 2016-02-19 00:18:25 -05:00 committed by Theodore Ts'o
parent c906f38e88
commit ed8ad83808
1 changed files with 30 additions and 2 deletions
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32
fs/ext4/inode.c
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@ -686,6 +686,34 @@ out_sem:
return retval;
}
/*
* Update EXT4_MAP_FLAGS in bh->b_state. For buffer heads attached to pages
* we have to be careful as someone else may be manipulating b_state as well.
*/
static void ext4_update_bh_state(struct buffer_head *bh, unsigned long flags)
{
unsigned long old_state;
unsigned long new_state;
flags &= EXT4_MAP_FLAGS;
/* Dummy buffer_head? Set non-atomically. */
if (!bh->b_page) {
bh->b_state = (bh->b_state & ~EXT4_MAP_FLAGS) | flags;
return;
}
/*
* Someone else may be modifying b_state. Be careful! This is ugly but
* once we get rid of using bh as a container for mapping information
* to pass to / from get_block functions, this can go away.
*/
do {
old_state = READ_ONCE(bh->b_state);
new_state = (old_state & ~EXT4_MAP_FLAGS) | flags;
} while (unlikely(
cmpxchg(&bh->b_state, old_state, new_state) != old_state));
}
/* Maximum number of blocks we map for direct IO at once. */
#define DIO_MAX_BLOCKS 4096
@ -722,7 +750,7 @@ static int _ext4_get_block(struct inode *inode, sector_t iblock,
ext4_io_end_t *io_end = ext4_inode_aio(inode);
map_bh(bh, inode->i_sb, map.m_pblk);
bh->b_state = (bh->b_state & ~EXT4_MAP_FLAGS) | map.m_flags;
ext4_update_bh_state(bh, map.m_flags);
if (io_end && io_end->flag & EXT4_IO_END_UNWRITTEN)
set_buffer_defer_completion(bh);
bh->b_size = inode->i_sb->s_blocksize * map.m_len;
@ -1685,7 +1713,7 @@ int ext4_da_get_block_prep(struct inode *inode, sector_t iblock,
return ret;
map_bh(bh, inode->i_sb, map.m_pblk);
bh->b_state = (bh->b_state & ~EXT4_MAP_FLAGS) | map.m_flags;
ext4_update_bh_state(bh, map.m_flags);
if (buffer_unwritten(bh)) {
/* A delayed write to unwritten bh should be marked