Re: [PATCH] debugfs: Fix lockdown check for mmap_prepare

[Lorenzo Stoakes]

-cc wrong email

(Please make sure to use my correct email address - ljs@xxxxxxxxxx -
 I usually do not see emails sent to the incorrect one)

+From: Chun-Yi Lee <jlee@xxxxxxxx>

(You're sending from a different email address so you need to explicitly
add a From: xxx tag to match the S-o-b at the top of the message)


On Fri, Jun 12, 2026 at 01:54:55PM +0800, Chun-Yi Lee wrote:
> Commit 651fdda8406d ("relay: update relay to use mmap_prepare")
> changed the `mmap` file operation to `mmap_prepare` for relayfs, but
> the lockdown check in debugfs was not updated accordingly.
>
> This prevents debugfs from being locked down when the kernel is in
> integrity mode if a file uses `mmap_prepare` but not `mmap`.
>
> Update the lockdown check to look for `mmap_prepare` instead of `mmap`
> to fix this.
>
> Fixes: 651fdda8406d ("relay: update relay to use mmap_prepare")

You need to explicitly add:

Cc: stable@xxxxxxxxxxxxxxx

As we will want this backported.

> Signed-off-by: Chun-Yi Lee <jlee@xxxxxxxx>

I think the idea is correct, but needs tweaking to cover _both_ mmap and
mmap_prepare, until the mmap_prepare conversion is complete, see below.

> Cc: David Howells <dhowells@xxxxxxxxxx>
> Cc: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
> cc: Andy Shevchenko <andy.shevchenko@xxxxxxxxx>
> cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Rafael J. Wysocki <rafael@xxxxxxxxxx>
> Cc: Matthew Garrett <matthewgarrett@xxxxxxxxxx>
> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> Cc: Danilo Krummrich <dakr@xxxxxxxxxx>
> Cc: driver-core@xxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> ---
>  fs/debugfs/file.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fs/debugfs/file.c b/fs/debugfs/file.c
> index edd6aafbfbaa..d5e187895dd8 100644
> --- a/fs/debugfs/file.c
> +++ b/fs/debugfs/file.c
> @@ -273,7 +273,7 @@ static int debugfs_locked_down(struct inode *inode,
>  	    (!real_fops ||
>  	     (!real_fops->unlocked_ioctl &&
>  	      !real_fops->compat_ioctl &&
> -	      !real_fops->mmap)))
> +	      !real_fops->mmap_prepare)))

This is not only applicable to relayfs.

So while we both have mmap and mmap_prepare, this should be:

-	      !real_fops->mmap)))
+	      !real_fops->mmap &&
+	      !real_fops->mmap_prepare)))


>  		return 0;
>
>  	if (security_locked_down(LOCKDOWN_DEBUGFS))
> --
> 2.43.0
>

Thanks, Lorenzo