[PATCH 1/2] apparmor: shift ouid when mediating hard links in userns

[John Johansen]

On 4/16/25 15:42, Gabriel Totev wrote:
> When using AppArmor profiles inside an unprivileged container,
> the link operation observes an unshifted ouid.
> (tested with LXD and Incus)
> 
> For example, root inside container and uid 1000000 outside, with
> `owner /root/link l,` profile entry for ln:
> 
> /root$ touch chain && ln chain link
> ==> dmesg
> apparmor="DENIED" operation="link" class="file"
> namespace="root//lxd-feet_<var-snap-lxd-common-lxd>" profile="linkit"
> name="/root/link" pid=1655 comm="ln" requested_mask="l" denied_mask="l"
> fsuid=1000000 ouid=0 [<== should be 1000000] target="/root/chain"
> 
> Fix by mapping inode uid of old_dentry in aa_path_link() rather than
> using it directly, similarly to how it's mapped in __file_path_perm()
> later in the file.

so unfortunately this isn't correct. Yes some mapping needs to be
done but it needs to be relative to different policy namespaces. I
need to spend some time on this


> 
> Signed-off-by: Gabriel Totev <gabriel.totev at zetier.com>
> ---
>   security/apparmor/file.c | 6 ++++--
>   1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/security/apparmor/file.c b/security/apparmor/file.c
> index 5c984792cbf0..ecd36199337c 100644
> --- a/security/apparmor/file.c
> +++ b/security/apparmor/file.c
> @@ -430,9 +430,11 @@ int aa_path_link(const struct cred *subj_cred,
>   {
>   	struct path link = { .mnt = new_dir->mnt, .dentry = new_dentry };
>   	struct path target = { .mnt = new_dir->mnt, .dentry = old_dentry };
> +	struct inode *inode = d_backing_inode(old_dentry);
> +	vfsuid_t vfsuid = i_uid_into_vfsuid(mnt_idmap(target.mnt), inode);
>   	struct path_cond cond = {
> -		d_backing_inode(old_dentry)->i_uid,
> -		d_backing_inode(old_dentry)->i_mode
> +		.uid = vfsuid_into_kuid(vfsuid),
> +		.mode = inode->i_mode,
>   	};
>   	char *buffer = NULL, *buffer2 = NULL;
>   	struct aa_profile *profile;