[PATCH v2] ext4: check folio uptodate state in ext4_page_mkwrite()

[Deepanshu Kartikey <kartikey406@gmail.com>]

When delayed block allocation fails due to filesystem corruption,
ext4's writeback error handling invalidates affected folios by calling
mpage_release_unused_pages() with invalidate=true, which explicitly
clears the uptodate flag:

    static void mpage_release_unused_pages(..., bool invalidate)
    {
        ...
        if (invalidate) {
            block_invalidate_folio(folio, 0, folio_size(folio));
            folio_clear_uptodate(folio);
        }
    }

If ext4_page_mkwrite() is subsequently called on such a non-uptodate
folio, it can proceed to mark the folio dirty without checking its
state. This triggers a warning in __folio_mark_dirty():

    WARNING: CPU: 0 PID: 5 at mm/page-writeback.c:2960
    __folio_mark_dirty+0x578/0x880

    Call Trace:
     fault_dirty_shared_page+0x16e/0x2d0
     do_wp_page+0x38b/0xd20
     handle_pte_fault+0x1da/0x450
     __handle_mm_fault+0x652/0x13b0
     handle_mm_fault+0x22a/0x6f0
     do_user_addr_fault+0x200/0x8a0
     exc_page_fault+0x81/0x1b0

This scenario occurs when:
1. A write with delayed allocation marks a folio dirty (uptodate=1)
2. Writeback attempts block allocation but detects filesystem corruption
3. Error handling calls mpage_release_unused_pages(invalidate=true),
   which clears the uptodate flag via folio_clear_uptodate()
4. A subsequent ftruncate() triggers ext4_truncate()
5. ext4_block_truncate_page() attempts to zero the page tail
6. This triggers a write fault on the mmap'd page
7. ext4_page_mkwrite() is called with the non-uptodate folio
8. Without checking uptodate, it proceeds to mark the folio dirty
9. __folio_mark_dirty() triggers: WARN_ON_ONCE(!folio_test_uptodate())

Fix this by checking folio_test_uptodate() early in ext4_page_mkwrite()
and returning VM_FAULT_SIGBUS if the folio is not uptodate. This prevents
attempting to write to invalidated folios and properly signals the error
to userspace.

The check is placed early, before the delalloc/journal/normal code paths,
as none of these paths should proceed with a non-uptodate folio.

Reported-by: syzbot+b0a0670332b6b3230a0a@syzkaller.appspotmail.com
Tested-by: syzbot+b0a0670332b6b3230a0a@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=b0a0670332b6b3230a0a
Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
---
 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