[PATCH v2] ext4: use fsdata to track inline data write state and fix race

[Aditya Srivastava]

From: Aditya Prakash Srivastava <aditya.ansh182@xxxxxxxxx>

Instead of checking the live inode state (ext4_has_inline_data(inode)
and ext4_test_inode_state(inode, EXT4_STATE_MAY_INLINE_DATA)) in the
write_end handlers, use the fsdata parameter of the address space
operations to explicitly pass down the state in which write_begin
prepared the write.

A concurrent thread (such as ext4_page_mkwrite()) can convert the
inline data to an extent between write_begin and write_end. If this
happens, the write_end handlers would previously miss the inline
write_end path and fall through to extent-based write_end logic.
However, since block buffers were never allocated in write_begin,
this resulted in NULL pointer dereferences or data loss because
folio_buffers(folio) was NULL.

Define EXT4_WRITE_DATA_INLINE (3) and communicate this state via
fsdata:
1) ext4_write_begin() and ext4_da_write_begin() explicitly set
   *fsdata to EXT4_WRITE_DATA_INLINE when an inline write is
   successfully prepared.
2) ext4_write_end(), ext4_journalled_write_end(), and
   ext4_da_write_end() rely solely on fsdata / write_mode to
   invoke ext4_write_inline_data_end().

Furthermore, during a buffered write, ext4_write_inline_data_end()
acquires the xattr lock after preparing the write. If a concurrent
page fault (ext4_page_mkwrite()) converts the inline data to an extent
after the write_end handlers check the state but before
ext4_write_inline_data_end() acquires the xattr write lock, the
subsequent check will trigger a kernel panic via
BUG_ON(!ext4_has_inline_data(inode)).

Replace the BUG_ON check in ext4_write_inline_data_end() with a graceful
error-handling retry path. If the inline data is cleared after locking
the xattr, we safely release all resources (releasing iloc.bh,
unlocking/putting the folio, stopping the active journal transaction
handle) and return 0 (VFS retry) to let the generic write path retry
the operation safely.

Reported-by: syzbot+0c89d865531d053abb2d@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=0c89d865531d053abb2d
Fixes: 3fdcfb668fd7 ("ext4: add journalled write support for inline data")
Suggested-by: Jan Kara <jack@xxxxxxx>
Signed-off-by: Aditya Prakash Srivastava <aditya.ansh182@xxxxxxxxx>
---
Changes in v2:
- Folded the BUG_ON fix from the second patch into the first one to
  ensure bisectability across git history, as suggested by Jan Kara.
- Removed the pointless initialization `*fsdata = NULL` on entry to
  `ext4_write_begin()`.
- Removed the redundant check `if (fsdata)` in `ext4_write_begin()`.

 fs/ext4/ext4.h   |  1 +
 fs/ext4/inline.c | 14 +++++++++++++-
 fs/ext4/inode.c  | 18 +++++++++---------
 3 files changed, 23 insertions(+), 10 deletions(-)

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index b37c136ea3ab..521bd5d6321c 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -3138,6 +3138,7 @@ int do_journal_get_write_access(handle_t *handle, struct inode *inode,
 void ext4_set_inode_mapping_order(struct inode *inode);
 #define FALL_BACK_TO_NONDELALLOC 1
 #define CONVERT_INLINE_DATA	 2
+#define EXT4_WRITE_DATA_INLINE	 3
 
 typedef enum {
 	EXT4_IGET_NORMAL =	0,
diff --git a/fs/ext4/inline.c b/fs/ext4/inline.c
index 8045e4ff270c..cfd591dc1d9c 100644
--- a/fs/ext4/inline.c
+++ b/fs/ext4/inline.c
@@ -812,7 +812,19 @@ int ext4_write_inline_data_end(struct inode *inode, loff_t pos, unsigned len,
 			goto out;
 		}
 		ext4_write_lock_xattr(inode, &no_expand);
-		BUG_ON(!ext4_has_inline_data(inode));
+		/*
+		 * We could have raced with ext4_page_mkwrite() converting
+		 * the inode and clearing the inline data flag, so we just
+		 * release resources and retry the whole write.
+		 */
+		if (unlikely(!ext4_has_inline_data(inode))) {
+			ext4_write_unlock_xattr(inode, &no_expand);
+			brelse(iloc.bh);
+			folio_unlock(folio);
+			folio_put(folio);
+			ext4_journal_stop(handle);
+			return 0;
+		}
 
 		/*
 		 * ei->i_inline_off may have changed since
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index ce99807c5f5b..d3d9e1999670 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -1316,8 +1316,10 @@ static int ext4_write_begin(const struct kiocb *iocb,
 						    foliop);
 		if (ret < 0)
 			return ret;
-		if (ret == 1)
+		if (ret == 1) {
+			*fsdata = (void *)EXT4_WRITE_DATA_INLINE;
 			return 0;
+		}
 	}
 
 	/*
@@ -1450,8 +1452,7 @@ static int ext4_write_end(const struct kiocb *iocb,
 
 	trace_ext4_write_end(inode, pos, len, copied);
 
-	if (ext4_has_inline_data(inode) &&
-	    ext4_test_inode_state(inode, EXT4_STATE_MAY_INLINE_DATA))
+	if (fsdata == (void *)EXT4_WRITE_DATA_INLINE)
 		return ext4_write_inline_data_end(inode, pos, len, copied,
 						  folio);
 
@@ -1560,8 +1561,7 @@ static int ext4_journalled_write_end(const struct kiocb *iocb,
 
 	BUG_ON(!ext4_handle_valid(handle));
 
-	if (ext4_has_inline_data(inode) &&
-	    ext4_test_inode_state(inode, EXT4_STATE_MAY_INLINE_DATA))
+	if (fsdata == (void *)EXT4_WRITE_DATA_INLINE)
 		return ext4_write_inline_data_end(inode, pos, len, copied,
 						  folio);
 
@@ -3161,8 +3161,10 @@ static int ext4_da_write_begin(const struct kiocb *iocb,
 						     foliop, fsdata, true);
 		if (ret < 0)
 			return ret;
-		if (ret == 1)
+		if (ret == 1) {
+			*fsdata = (void *)EXT4_WRITE_DATA_INLINE;
 			return 0;
+		}
 	}
 
 retry:
@@ -3299,9 +3301,7 @@ static int ext4_da_write_end(const struct kiocb *iocb,
 
 	trace_ext4_da_write_end(inode, pos, len, copied);
 
-	if (write_mode != CONVERT_INLINE_DATA &&
-	    ext4_test_inode_state(inode, EXT4_STATE_MAY_INLINE_DATA) &&
-	    ext4_has_inline_data(inode))
+	if (write_mode == EXT4_WRITE_DATA_INLINE)
 		return ext4_write_inline_data_end(inode, pos, len, copied,
 						  folio);
 
-- 
2.47.3