ext4_split_extent_at() retries after ext4_ext_insert_extent() fails by refinding the original extent and restoring its length. That recovery is only safe for transient resource failures such as -ENOSPC, -EDQUOT, and -ENOMEM. When ext4_ext_insert_extent() fails because the extent tree is already corrupted, ext4_find_extent() can return a leaf path without p_ext. ext4_split_extent_at() then dereferences path[depth].p_ext while trying to fix up the original extent length, causing a NULL pointer dereference while handling a pre-existing filesystem corruption. Do not enter the recovery path for corruption errors, and validate p_ext after refinding the extent before touching it. This keeps the recovery path limited to cases it can actually repair and turns the syzbot-triggered crash into a proper corruption report. Fixes: 716b9c23b862 ("ext4: refactor split and convert extents") Reported-by: syzbot+1ffa5d865557e51cb604@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=1ffa5d865557e51cb604 Reviewed-by: Jan Kara Reviewed-by: Zhang Yi Signed-off-by: hongao --- v2: - move the p_ext validation before ext4_ext_get_access() - add Reviewed-by tags from Jan Kara and Zhang Yi fs/ext4/extents.c | 16 +++++++++++++--- 1 file changed, 13 insertions(+), 3 deletions(-) diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c index ae3804f36535..10af8dee0f1a 100644 --- a/fs/ext4/extents.c +++ b/fs/ext4/extents.c @@ -3239,6 +3239,9 @@ static struct ext4_ext_path *ext4_split_extent_at(handle_t *handle, insert_err = PTR_ERR(path); err = 0; + if (insert_err != -ENOSPC && insert_err != -EDQUOT && + insert_err != -ENOMEM) + goto out_path; /* * Get a new path to try to zeroout or fix the extent length. @@ -3255,13 +3258,20 @@ static struct ext4_ext_path *ext4_split_extent_at(handle_t *handle, goto out_path; } + depth = ext_depth(inode); + ex = path[depth].p_ext; + if (!ex) { + EXT4_ERROR_INODE(inode, + "bad extent address lblock: %lu, depth: %d pblock %lld", + (unsigned long)ee_block, depth, path[depth].p_block); + err = -EFSCORRUPTED; + goto out; + } + err = ext4_ext_get_access(handle, inode, path + depth); if (err) goto out; - depth = ext_depth(inode); - ex = path[depth].p_ext; - fix_extent_len: ex->ee_len = orig_ex.ee_len; err = ext4_ext_dirty(handle, inode, path + path->p_depth); -- 2.51.0