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Subject[PATCH 4.14 215/237] ext4: fix race between writepages and enabling EXT4_EXTENTS_FL
Date
From: Eric Biggers <ebiggers@google.com>

commit cb85f4d23f794e24127f3e562cb3b54b0803f456 upstream.

If EXT4_EXTENTS_FL is set on an inode while ext4_writepages() is running
on it, the following warning in ext4_add_complete_io() can be hit:

WARNING: CPU: 1 PID: 0 at fs/ext4/page-io.c:234 ext4_put_io_end_defer+0xf0/0x120

Here's a minimal reproducer (not 100% reliable) (root isn't required):

while true; do
sync
done &
while true; do
rm -f file
touch file
chattr -e file
echo X >> file
chattr +e file
done

The problem is that in ext4_writepages(), ext4_should_dioread_nolock()
(which only returns true on extent-based files) is checked once to set
the number of reserved journal credits, and also again later to select
the flags for ext4_map_blocks() and copy the reserved journal handle to
ext4_io_end::handle. But if EXT4_EXTENTS_FL is being concurrently set,
the first check can see dioread_nolock disabled while the later one can
see it enabled, causing the reserved handle to unexpectedly be NULL.

Since changing EXT4_EXTENTS_FL is uncommon, and there may be other races
related to doing so as well, fix this by synchronizing changing
EXT4_EXTENTS_FL with ext4_writepages() via the existing
s_writepages_rwsem (previously called s_journal_flag_rwsem).

This was originally reported by syzbot without a reproducer at
https://syzkaller.appspot.com/bug?extid=2202a584a00fffd19fbf,
but now that dioread_nolock is the default I also started seeing this
when running syzkaller locally.

Link: https://lore.kernel.org/r/20200219183047.47417-3-ebiggers@kernel.org
Reported-by: syzbot+2202a584a00fffd19fbf@syzkaller.appspotmail.com
Fixes: 6b523df4fb5a ("ext4: use transaction reservation for extent conversion in ext4_end_io")
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: stable@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
fs/ext4/ext4.h | 5 ++++-
fs/ext4/migrate.c | 27 +++++++++++++++++++--------
2 files changed, 23 insertions(+), 9 deletions(-)

--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -1532,7 +1532,10 @@ struct ext4_sb_info {
struct ratelimit_state s_warning_ratelimit_state;
struct ratelimit_state s_msg_ratelimit_state;

- /* Barrier between changing inodes' journal flags and writepages ops. */
+ /*
+ * Barrier between writepages ops and changing any inode's JOURNAL_DATA
+ * or EXTENTS flag.
+ */
struct percpu_rw_semaphore s_writepages_rwsem;
struct dax_device *s_daxdev;
};
--- a/fs/ext4/migrate.c
+++ b/fs/ext4/migrate.c
@@ -434,6 +434,7 @@ static int free_ext_block(handle_t *hand

int ext4_ext_migrate(struct inode *inode)
{
+ struct ext4_sb_info *sbi = EXT4_SB(inode->i_sb);
handle_t *handle;
int retval = 0, i;
__le32 *i_data;
@@ -458,6 +459,8 @@ int ext4_ext_migrate(struct inode *inode
*/
return retval;

+ percpu_down_write(&sbi->s_writepages_rwsem);
+
/*
* Worst case we can touch the allocation bitmaps, a bgd
* block, and a block to link in the orphan list. We do need
@@ -468,7 +471,7 @@ int ext4_ext_migrate(struct inode *inode

if (IS_ERR(handle)) {
retval = PTR_ERR(handle);
- return retval;
+ goto out_unlock;
}
goal = (((inode->i_ino - 1) / EXT4_INODES_PER_GROUP(inode->i_sb)) *
EXT4_INODES_PER_GROUP(inode->i_sb)) + 1;
@@ -479,7 +482,7 @@ int ext4_ext_migrate(struct inode *inode
if (IS_ERR(tmp_inode)) {
retval = PTR_ERR(tmp_inode);
ext4_journal_stop(handle);
- return retval;
+ goto out_unlock;
}
i_size_write(tmp_inode, i_size_read(inode));
/*
@@ -521,7 +524,7 @@ int ext4_ext_migrate(struct inode *inode
*/
ext4_orphan_del(NULL, tmp_inode);
retval = PTR_ERR(handle);
- goto out;
+ goto out_tmp_inode;
}

ei = EXT4_I(inode);
@@ -602,10 +605,11 @@ err_out:
/* Reset the extent details */
ext4_ext_tree_init(handle, tmp_inode);
ext4_journal_stop(handle);
-out:
+out_tmp_inode:
unlock_new_inode(tmp_inode);
iput(tmp_inode);
-
+out_unlock:
+ percpu_up_write(&sbi->s_writepages_rwsem);
return retval;
}

@@ -615,7 +619,8 @@ out:
int ext4_ind_migrate(struct inode *inode)
{
struct ext4_extent_header *eh;
- struct ext4_super_block *es = EXT4_SB(inode->i_sb)->s_es;
+ struct ext4_sb_info *sbi = EXT4_SB(inode->i_sb);
+ struct ext4_super_block *es = sbi->s_es;
struct ext4_inode_info *ei = EXT4_I(inode);
struct ext4_extent *ex;
unsigned int i, len;
@@ -639,9 +644,13 @@ int ext4_ind_migrate(struct inode *inode
if (test_opt(inode->i_sb, DELALLOC))
ext4_alloc_da_blocks(inode);

+ percpu_down_write(&sbi->s_writepages_rwsem);
+
handle = ext4_journal_start(inode, EXT4_HT_MIGRATE, 1);
- if (IS_ERR(handle))
- return PTR_ERR(handle);
+ if (IS_ERR(handle)) {
+ ret = PTR_ERR(handle);
+ goto out_unlock;
+ }

down_write(&EXT4_I(inode)->i_data_sem);
ret = ext4_ext_check_inode(inode);
@@ -676,5 +685,7 @@ int ext4_ind_migrate(struct inode *inode
errout:
ext4_journal_stop(handle);
up_write(&EXT4_I(inode)->i_data_sem);
+out_unlock:
+ percpu_up_write(&sbi->s_writepages_rwsem);
return ret;
}

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