Re: [PATCH v2] debugfs: Fix lockdown check for mmap_prepare

[Disha Goel]

On 15/06/26 4:17 pm, Chun-Yi Lee wrote:
> From: Chun-Yi Lee <jlee@xxxxxxxx>
>
> 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`.
>
> Since the conversion to `mmap_prepare` across the kernel is not yet
> complete, update the lockdown check to look for both `mmap` and
> `mmap_prepare` to ensure comprehensive coverage.
>
> Fixes: 651fdda8406d ("relay: update relay to use mmap_prepare")
> Signed-off-by: Chun-Yi Lee <jlee@xxxxxxxx>
> Cc: David Howells <dhowells@xxxxxxxxxx>
> Cc: Lorenzo Stoakes <ljs@xxxxxxxxxx>
> Cc: Andy Shevchenko <andy.shevchenko@xxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Rafael J. Wysocki <rafael@xxxxxxxxxx>
> Cc: Matthew Garrett <mjg59@xxxxxxxxxxxxx>
> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> Cc: Danilo Krummrich <dakr@xxxxxxxxxx>
> Cc: driver-core@xxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: stable@xxxxxxxxxxxxxxx
> ---

Hi,

I tested this patch on ppc64le with lockdown enabled. It correctly fixes 
the security issue where debugfs files using mmap_prepare were not being 
restricted.

Test: blktrace/001 from blktests (uses relayfs via debugfs)
- Before patch: blktrace bypassed lockdown and accessed debugfs
- After patch: blktrace properly blocked from accessing debugfs

Environment:
Kernel: 7.1.0-rc7
Lockdown: integrity mode

Feel free to add:
Tested-by: Disha Goel <disgoel@xxxxxxxxxxxxx>

> v2:
> - Add explicit From tag to match Signed-off-by.
> - Fix Lorenzo's email address.
> - Add Cc stable for backporting.
> - Check both mmap and mmap_prepare as suggested by Lorenzo.
>
>   fs/debugfs/file.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/fs/debugfs/file.c b/fs/debugfs/file.c
> index edd6aafbfbaa..08de6652a4f3 100644
> --- a/fs/debugfs/file.c
> +++ b/fs/debugfs/file.c
> @@ -273,7 +273,8 @@ static int debugfs_locked_down(struct inode *inode,
>   	    (!real_fops ||
>   	     (!real_fops->unlocked_ioctl &&
>   	      !real_fops->compat_ioctl &&
> -	      !real_fops->mmap)))
> +	      !real_fops->mmap &&
> +	      !real_fops->mmap_prepare)))
>   		return 0;
>   
>   	if (security_locked_down(LOCKDOWN_DEBUGFS))

--
Regards,
Disha