capability: handle idmapped mounts

In order to determine whether a caller holds privilege over a given
inode the capability framework exposes the two helpers
privileged_wrt_inode_uidgid() and capable_wrt_inode_uidgid(). The former
verifies that the inode has a mapping in the caller's user namespace and
the latter additionally verifies that the caller has the requested
capability in their current user namespace.
If the inode is accessed through an idmapped mount map it into the
mount's user namespace. Afterwards the checks are identical to
non-idmapped inodes. If the initial user namespace is passed all
operations are a nop so non-idmapped mounts will not see a change in
behavior.

Link: https://lore.kernel.org/r/20210121131959.646623-5-christian.brauner@ubuntu.com
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
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: James Morris <jamorris@linux.microsoft.com>
Acked-by: Serge Hallyn <serge@hallyn.com>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

1 files changed, 4 insertions, 4 deletions

diff --git a/fs/attr.c b/fs/attr.c
index b4bbdbd4c8ca..d270f640a192 100644
--- a/fs/attr.c
+++ b/fs/attr.c
@@ -23,7 +23,7 @@ static bool chown_ok(const struct inode *inode, kuid_t uid)
 	if (uid_eq(current_fsuid(), inode->i_uid) &&
 	    uid_eq(uid, inode->i_uid))
 		return true;
-	if (capable_wrt_inode_uidgid(inode, CAP_CHOWN))
+	if (capable_wrt_inode_uidgid(&init_user_ns, inode, CAP_CHOWN))
 		return true;
 	if (uid_eq(inode->i_uid, INVALID_UID) &&
 	    ns_capable(inode->i_sb->s_user_ns, CAP_CHOWN))
@@ -36,7 +36,7 @@ static bool chgrp_ok(const struct inode *inode, kgid_t gid)
 	if (uid_eq(current_fsuid(), inode->i_uid) &&
 	    (in_group_p(gid) || gid_eq(gid, inode->i_gid)))
 		return true;
-	if (capable_wrt_inode_uidgid(inode, CAP_CHOWN))
+	if (capable_wrt_inode_uidgid(&init_user_ns, inode, CAP_CHOWN))
 		return true;
 	if (gid_eq(inode->i_gid, INVALID_GID) &&
 	    ns_capable(inode->i_sb->s_user_ns, CAP_CHOWN))
@@ -92,7 +92,7 @@ int setattr_prepare(struct dentry *dentry, struct iattr *attr)
 		/* Also check the setgid bit! */
 		if (!in_group_p((ia_valid & ATTR_GID) ? attr->ia_gid :
 				inode->i_gid) &&
-		    !capable_wrt_inode_uidgid(inode, CAP_FSETID))
+		    !capable_wrt_inode_uidgid(&init_user_ns, inode, CAP_FSETID))
 			attr->ia_mode &= ~S_ISGID;
 	}
 
@@ -193,7 +193,7 @@ void setattr_copy(struct inode *inode, const struct iattr *attr)
 		umode_t mode = attr->ia_mode;
 
 		if (!in_group_p(inode->i_gid) &&
-		    !capable_wrt_inode_uidgid(inode, CAP_FSETID))
+		    !capable_wrt_inode_uidgid(&init_user_ns, inode, CAP_FSETID))
 			mode &= ~S_ISGID;
 		inode->i_mode = mode;
 	}