ext4: fix race between writepages and enabling EXT4_EXTENTS_FL

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

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index 4b986ad42b9d8db9ea875e5d94cad9d606c23724..61b37a052052b58b577ad617086dbbcdb90b48f3 100644 (file)
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -1552,7 +1552,10 @@ 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;
 #ifdef CONFIG_EXT4_DEBUG

diff --git a/fs/ext4/migrate.c b/fs/ext4/migrate.c
index 89725fa425732e4933a95b60200b33f40158537d..fb6520f37135509791c35b782ba14dfe328cfa49 100644 (file)
--- a/fs/ext4/migrate.c
+++ b/fs/ext4/migrate.c
@@ -407,6 +407,7 @@ static int free_ext_block(handle_t *handle, struct inode *inode)
 
 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;
@@ -431,6 +432,8 @@ 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
@@ -441,7 +444,7 @@ 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;
@@ -452,7 +455,7 @@ 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));
        /*
@@ -494,7 +497,7 @@ 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);
@@ -576,10 +579,11 @@ err_out:
        ext4_ext_tree_init(handle, tmp_inode);
 out_stop:
        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;
 }
 
@@ -589,7 +593,8 @@ 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;
@@ -613,9 +618,13 @@ 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);
@@ -650,5 +659,7 @@ 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;
 }