This is a note to let you know that I've just added the patch titled capabilities: Don't allow writing ambiguous v3 file capabilities to the 5.11-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: capabilities-don-t-allow-writing-ambiguous-v3-file-c.patch and it can be found in the queue-5.11 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. commit 57e1e3b95c3e964b9e52fc9846764825cb1db1ee Author: Eric W. Biederman <ebiederm@xxxxxxxxxxxx> Date: Thu Dec 17 09:42:00 2020 -0600 capabilities: Don't allow writing ambiguous v3 file capabilities [ Upstream commit 95ebabde382c371572297915b104e55403674e73 ] The v3 file capabilities have a uid field that records the filesystem uid of the root user of the user namespace the file capabilities are valid in. When someone is silly enough to have the same underlying uid as the root uid of multiple nested containers a v3 filesystem capability can be ambiguous. In the spirit of don't do that then, forbid writing a v3 filesystem capability if it is ambiguous. Fixes: 8db6c34f1dbc ("Introduce v3 namespaced file capabilities") Reviewed-by: Andrew G. Morgan <morgan@xxxxxxxxxx> Reviewed-by: Serge Hallyn <serge@xxxxxxxxxx> Signed-off-by: Eric W. Biederman <ebiederm@xxxxxxxxxxxx> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx> diff --git a/security/commoncap.c b/security/commoncap.c index 26c1cb725dcbe..78598be45f101 100644 --- a/security/commoncap.c +++ b/security/commoncap.c @@ -500,7 +500,8 @@ int cap_convert_nscap(struct dentry *dentry, const void **ivalue, size_t size) __u32 magic, nsmagic; struct inode *inode = d_backing_inode(dentry); struct user_namespace *task_ns = current_user_ns(), - *fs_ns = inode->i_sb->s_user_ns; + *fs_ns = inode->i_sb->s_user_ns, + *ancestor; kuid_t rootid; size_t newsize; @@ -523,6 +524,15 @@ int cap_convert_nscap(struct dentry *dentry, const void **ivalue, size_t size) if (nsrootid == -1) return -EINVAL; + /* + * Do not allow allow adding a v3 filesystem capability xattr + * if the rootid field is ambiguous. + */ + for (ancestor = task_ns->parent; ancestor; ancestor = ancestor->parent) { + if (from_kuid(ancestor, rootid) == 0) + return -EINVAL; + } + newsize = sizeof(struct vfs_ns_cap_data); nscap = kmalloc(newsize, GFP_ATOMIC); if (!nscap)