[PATCH] ext4: fix race between inline dir conversion and folio read

[Sandeep Dhavale <dhavale@google.com>]

Commit 90f097b1403f ("ext4: refactor the inline directory conversion and
new directory codepaths") moved directory block initialization in
ext4_convert_inline_data_nolock() into ext4_init_dirblock(), unlocking
data_bh before setting BH_Uptodate.

When block size is smaller than PAGE_SIZE, multiple buffer heads share a
single block device folio. If a concurrent read on that folio runs via
block_read_full_folio() while data_bh is unlocked and not uptodate, it
locks data_bh and submits a disk read for the uninitialized block.

Because ext4_init_dirblock() initializes bh->b_data without holding the
buffer lock, the disk read races with and overwrites the initialized
directory entries and checksum tail, corrupting the directory block:

  EXT4-fs warning: ext4_dirblock_csum_verify:375: inode #...:
    No space for directory leaf checksum. Please run e2fsck -D.
  EXT4-fs error: __ext4_find_entry:1626: inode #...:
    checksumming directory block 0

To fix this, mark data_bh uptodate before unlocking it in
ext4_convert_inline_data_nolock(), just like the regular file path
does. Additionally, hold lock_buffer(bh) in ext4_init_dirblock() while
populating the directory entries and checksum tail so any buffer
modifications are properly serialized.

Fixes: 90f097b1403f ("ext4: refactor the inline directory conversion and new directory codepaths")
Cc: stable@vger.kernel.org
Assisted-by: Antigravity:gemini-3.8-flash
Signed-off-by: Sandeep Dhavale <dhavale@google.com>
---
Note:
This issue was reported by an Android partner encountering kernel panics
in ext4_dirblock_csum_verify() ("No space for directory leaf checksum")
on 16KB page size kernels mounting a 4KB block size ext4 filesystem.

We verified with a standalone reproducer that the race reproduces
deterministically on Iteration 0 (< 1s) on both ARM64 (16KB page size,
4KB ext4 blocks) and upstream ext4-tree/dev on x86_64 (4KB page size,
1KB ext4 blocks), and confirmed that this patch resolves the issue.

 fs/ext4/inline.c | 1 +
 fs/ext4/namei.c  | 2 ++
 2 files changed, 3 insertions(+)

diff --git a/fs/ext4/inline.c b/fs/ext4/inline.c
index ceee69a66482..0aa14cee3310 100644
--- a/fs/ext4/inline.c
+++ b/fs/ext4/inline.c
@@ -1167,6 +1167,7 @@ static int ext4_convert_inline_data_nolock(handle_t *handle,
 		error = ext4_handle_dirty_metadata(handle,
 						   inode, data_bh);
 	} else {
+		set_buffer_uptodate(data_bh);
 		unlock_buffer(data_bh);
 		inode->i_size = inode->i_sb->s_blocksize;
 		i_size_write(inode, inode->i_sb->s_blocksize);
diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
index 3b9740c1c16d..6550102fb56b 100644
--- a/fs/ext4/namei.c
+++ b/fs/ext4/namei.c
@@ -2933,6 +2933,7 @@ int ext4_init_dirblock(handle_t *handle, struct inode *inode,
 	if (ext4_has_feature_metadata_csum(inode->i_sb))
 		csum_size = sizeof(struct ext4_dir_entry_tail);
 
+	lock_buffer(bh);
 	de->inode = cpu_to_le32(inode->i_ino);
 	de->name_len = 1;
 	de->rec_len = ext4_rec_len_to_disk(ext4_dir_rec_len(de->name_len, NULL),
@@ -2965,6 +2966,7 @@ int ext4_init_dirblock(handle_t *handle, struct inode *inode,
 	BUFFER_TRACE(dir_block, "call ext4_handle_dirty_metadata");
 	set_buffer_uptodate(bh);
 	set_buffer_verified(bh);
+	unlock_buffer(bh);
 	return ext4_handle_dirty_dirblock(handle, inode, bh);
 }
 
-- 
2.55.0.1082.g2b9226bbc0-goog