[PATCH v2 28/39] exec: handle idmapped mounts

Christian Brauner
2020-11-16T02:15:37Z

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.On
regular mounts this is a nop.

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
---
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 */