[PATCH v3 28/38] exec: handle idmapped mounts

[Christian Brauner <christian.brauner@ubuntu.com>]

When executing a setuid binary the kernel will verify in bprm_fill_uid() that
the inode has a mapping in the caller's user namespace before setting the
callers uid and gid. Let bprm_fill_uid() handle idmapped mounts. If the inode
is accessed through an idmapped mount it is mapped according to the mount's
user namespace. Afterwards the checks are identical to non-idmapped mounts. If
the initial user namespace is passed nothing changes so non-idmapped mounts will
see identical behavior as before.

Cc: Christoph Hellwig <hch@lst.de>
Cc: David Howells <dhowells@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
---
/* v2 */
unchanged

/* v3 */
unchanged
---
 fs/exec.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index 10c06fdf78a7..7d6d3dd17e84 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1567,6 +1567,7 @@ static void check_unsafe_exec(struct linux_binprm *bprm)
 static void bprm_fill_uid(struct linux_binprm *bprm, struct file *file)
 {
 	/* Handle suid and sgid on files */
+	struct user_namespace *user_ns;
 	struct inode *inode;
 	unsigned int mode;
 	kuid_t uid;
@@ -1583,13 +1584,15 @@ static void bprm_fill_uid(struct linux_binprm *bprm, struct file *file)
 	if (!(mode & (S_ISUID|S_ISGID)))
 		return;
 
+	user_ns = mnt_user_ns(file->f_path.mnt);
+
 	/* Be careful if suid/sgid is set */
 	inode_lock(inode);
 
 	/* reload atomically mode/uid/gid now that lock held */
 	mode = inode->i_mode;
-	uid = inode->i_uid;
-	gid = inode->i_gid;
+	uid = i_uid_into_mnt(user_ns, inode);
+	gid = i_gid_into_mnt(user_ns, inode);
 	inode_unlock(inode);
 
 	/* We ignore suid/sgid if there are no mappings for them in the ns */
-- 
2.29.2