On Thu 07-01-16 17:55:21, Jan Kara wrote:
> From: Jan Kara <jack@xxxxxxxx>
>
> 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@xxxxxxxxxxxxxxx
> Reported-by: Nikolay Borisov <kernel@xxxxxxxx>
> Signed-off-by: Jan Kara <jack@xxxxxxx>
Ping Ted? This seems to have fallen through the cracks?
Honza
> ---
> fs/ext4/inode.c | 32 ++++++++++++++++++++++++++++++--
> 1 file changed, 30 insertions(+), 2 deletions(-)
>
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index ea433a7f4bca..06bda0361e7c 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -657,6 +657,34 @@ has_zeroout:
> 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
>
> @@ -693,7 +721,7 @@ 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 (IS_DAX(inode) && buffer_unwritten(bh)) {
> /*
> * dgc: I suspect unwritten conversion on ext4+DAX is
> @@ -1669,7 +1697,7 @@ 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
> --
> 2.6.2
>
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
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