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

[Eric Biggers <ebiggers@kernel.org>]

On Thu, Sep 24, 2026 at 05:15:30PM +0200, Alberto Garcia wrote:
> 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?

Right, encryption should work with large folios now, but not with
fs_block_size > PAGE_SIZE.  For the latter, for now I think we need a
few different things to prevent it:

* (Already present) A mount-time check, to prevent mounting when the
  encrypt feature flag is set and fs_block_size > PAGE_SIZE;

* A check in ext4_set_context(), as you suggested, to prevent new
  encrypted files from being created when fs_block_size > PAGE_SIZE
  after the encrypt feature flag was set at runtime;

* tune2fs should disallow setting the encrypt feature flag in the first
  place when fs_block_size > PAGE_SIZE;

* And maybe a check in __ext4_iget() to prevent existing encrypted
  inodes from being loaded when fs_block_size > PAGE_SIZE.  This would
  be needed for fuzzing robustness, where a fuzzer generates a
  filesystem that has encrypted files without the encrypt feature flag.
  (I don't remember whether ext4 claims to support this level of fuzzing
  robustness, though.  __ext4_iget() has a few similar checks for other
  filesystem features, but it's very incomplete, so I'm not sure.)

- Eric