Re: [REGRESSION] ext4: oops in ext4_finish_bio() after enabling encryption on a mounted fs

[Alberto Garcia <berto@igalia.com>]

On Wed, Sep 23, 2026 at 06:01:19PM +0000, Eric Biggers wrote:
> The proposed patch looks good

As I said, I think we need an additional check to detect if encryption
is set on a mounted filesystem that has blocksize > pagesize.

Such a filesystem cannot be mounted (ext4_check_large_folio refuses),
but if you do it after the fs is mounted you can crash the kernel
using the same method.

# mkfs.ext4 -b 16384 /dev/vdb 
# mount /dev/vdb /mnt/
# fscrypt setup /mnt
# tune2fs -O encrypt /dev/vdb 
# mkdir /mnt/foo
# fscrypt encrypt /mnt/foo/

The other patch alone does not prevent this.

With this additional change, FS_IOC_SET_ENCRYPTION_POLICY returns
-EOPNOTSUPP, so tune2fs succeeds, but 'fscrypt encrypt' fails:

   [ERROR] fscrypt encrypt: encryption not enabled on filesystem /mnt (/dev/vdb).

--- a/fs/ext4/crypto.c
+++ b/fs/ext4/crypto.c
@@ -156,6 +156,9 @@ static int ext4_set_context(struct inode *inode, const void *ctx, size_t len,
 	if (ext4_test_inode_flag(inode, EXT4_INODE_DAX))
 		return -EOPNOTSUPP;
 
+	if (inode->i_sb->s_blocksize > PAGE_SIZE)
+		return -EOPNOTSUPP;
+
 	res = ext4_convert_inline_data(inode);
 	if (res)
 		return res;

That prevents the crash, but the filesystem cannot be remounted:

   kernel: EXT4-fs (vdb): bs(16384) > ps(4096) unsupported for encrypt

Would this be enough or is there anything else to take into account?

Regards,

Berto