When a write fault occurs on a memory-mapped ext4 file, ext4_page_mkwrite() is called to prepare the folio for writing. However, if the folio could not be read successfully due to filesystem corruption or I/O errors, it will not be marked uptodate. Attempting to write to a non-uptodate folio is problematic because: 1. We don't have valid data from the backing store to preserve 2. A subsequent writeback could write uninitialized data to disk 3. It triggers a warning in __folio_mark_dirty(): WARN_ON_ONCE(warn && !folio_test_uptodate(folio)) This issue can be reproduced by: 1. Creating a corrupted ext4 filesystem with invalid extent entries 2. Memory-mapping a file on that filesystem 3. Attempting to write to the mapped region The sequence of events is: - User accesses mmap region -> page fault - ext4_filemap_fault() -> ext4_map_blocks() detects corruption - Returns error, folio allocated but NOT marked uptodate - User writes to same region -> ext4_page_mkwrite() called - Without check: folio marked dirty -> WARNING - With check: return VM_FAULT_SIGBUS immediately Fix this by checking folio_test_uptodate() early in ext4_page_mkwrite(), before any code paths (delalloc, journal data, or normal). This ensures all paths are protected. If the folio is not uptodate, unlock it and return VM_FAULT_SIGBUS to signal the error to userspace. Reported-by: syzbot+b0a0670332b6b3230a0a@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=b0a0670332b6b3230a0a Signed-off-by: Deepanshu Kartikey --- fs/ext4/inode.c | 8 ++++++++ 1 file changed, 8 insertions(+) diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c index e99306a8f47c..18a029362c1f 100644 --- a/fs/ext4/inode.c +++ b/fs/ext4/inode.c @@ -6688,6 +6688,14 @@ vm_fault_t ext4_page_mkwrite(struct vm_fault *vmf) if (err) goto out_ret; + folio_lock(folio); + if (!folio_test_uptodate(folio)) { + folio_unlock(folio); + ret = VM_FAULT_SIGBUS; + goto out; + } + folio_unlock(folio); + /* * On data journalling we skip straight to the transaction handle: * there's no delalloc; page truncated will be checked later; the -- 2.43.0