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

[Gabriel Totev]

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.

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;
-- 
2.34.1