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Commit aa6d054e authored by Eric W. Biederman's avatar Eric W. Biederman
Browse files

userns: Add a more complete capability subset test to commit_creds 



When unsharing a user namespace we reduce our credentials to just what
can be done in that user namespace.  This is a subset of the credentials
we previously had.  Teach commit_creds to recognize this is a subset
of the credentials we have had before and don't clear the dumpability flag.

This allows an unprivileged  program to do:
unshare(CLONE_NEWUSER);
fd = open("/proc/self/uid_map", O_RDWR);

Where previously opening the uid_map writable would fail because
the the task had been made non-dumpable.

Acked-by: default avatarSerge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: default avatar"Eric W. Biederman" <ebiederm@xmission.com>
parent 5e4a0847

kernel/cred.c

+26 −1
Original line number Diff line number Diff line
@@ -455,6 +455,31 @@ int copy_creds(struct task_struct *p, unsigned long clone_flags) 
	return ret;

}



static bool cred_cap_issubset(const struct cred *set, const struct cred *subset)

{

	const struct user_namespace *set_ns = set->user_ns;

	const struct user_namespace *subset_ns = subset->user_ns;



	/* If the two credentials are in the same user namespace see if

	 * the capabilities of subset are a subset of set.

	 */

	if (set_ns == subset_ns)

		return cap_issubset(subset->cap_permitted, set->cap_permitted);



	/* The credentials are in a different user namespaces

	 * therefore one is a subset of the other only if a set is an

	 * ancestor of subset and set->euid is owner of subset or one

	 * of subsets ancestors.

	 */

	for (;subset_ns != &init_user_ns; subset_ns = subset_ns->parent) {

		if ((set_ns == subset_ns->parent)  &&

		    uid_eq(subset_ns->owner, set->euid))

			return true;

	}



	return false;

}



/**

 * commit_creds - Install new credentials upon the current task

 * @new: The credentials to be assigned
@@ -493,7 +518,7 @@ int commit_creds(struct cred *new) 
	    !gid_eq(old->egid, new->egid) ||

	    !uid_eq(old->fsuid, new->fsuid) ||

	    !gid_eq(old->fsgid, new->fsgid) ||

	    !cap_issubset(new->cap_permitted, old->cap_permitted)) {

	    !cred_cap_issubset(old, new)) {

		if (task->mm)

			set_dumpable(task->mm, suid_dumpable);

		task->pdeath_signal = 0;