[PATCH 4/8] ucounts: Only except the root user in init_user_ns from RLIMIT_NPROC

["Eric W. Biederman" <ebiederm@xmission.com>]

In [1] Michal Koutný <mkoutny@suse.com> reported that it does not make
sense to unconditionally exempt the INIT_USER during fork and exec
from RLIMIT_NPROC and then to impose a limit on that same user with
is_ucounts_overlimit.  So I looked into why the exeception was added.

commit 909cc4ae86f3 ("[PATCH] Fix two bugs with process limits (RLIMIT_NPROC)") says:
>  If a setuid process swaps it's real and effective uids and then
>  forks, the fork fails if the new realuid has more processes than
>  the original process was limited to.  This is particularly a
>  problem if a user with a process limit (e.g. 256) runs a
>  setuid-root program which does setuid() + fork() (e.g. lprng) while
>  root already has more than 256 process (which is quite possible).
>
>  The root problem here is that a limit which should be a per-user
>  limit is being implemented as a per-process limit with per-process
>  (e.g. CAP_SYS_RESOURCE) controls.  Being a per-user limit, it
>  should be that the root-user can over-ride it, not just some
>  process with CAP_SYS_RESOURCE.
>
>  This patch adds a test to ignore process limits if the real user is root.

With the moving of the limits from per-user to per-user-per-user_ns it
is clear that testing a user_struct is no longer the proper test and
the test should be a test against the ucounts struct to match the
rest of the logic change that was made.

With RLIMIT_NPROC not being enforced for the global root user anywhere
else should it be enforced in is_ucounts_overlimit for a user
namespace created by the global root user?

Since this is limited to just the global root user, and RLIMIT_NPROC
is already ignored for that user I am going to vote no.

This change does imply that it becomes possible to limit all users in
a user namespace but to not enforce the rlimits on the root user or
anyone with CAP_SYS_ADMIN and CAP_SYS_RESOURCE in the user namespace.
It is not clear to me why any of those exceptions exist so I figure
we should until this is actually a problem for someone before
we relax the permission checks here.

Cc: stable@vger.kernel.org
Reported-by: Michal Koutný <mkoutny@suse.com>
[1] https://lkml.kernel.org/r/20220207121800.5079-5-mkoutny@suse.com
History-Tree: https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git
Fixes: 21d1c5e386bc ("Reimplement RLIMIT_NPROC on top of ucounts")
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
---
 fs/exec.c               | 2 +-
 kernel/fork.c           | 2 +-
 kernel/user_namespace.c | 2 ++
 3 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index 1e7f757cbc2c..01c8c7bae9b4 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1881,7 +1881,7 @@ static int do_execveat_common(int fd, struct filename *filename,
 	 */
 	if ((current->flags & PF_NPROC_CHECK) &&
 	    is_ucounts_overlimit(current_ucounts(), UCOUNT_RLIMIT_NPROC, rlimit(RLIMIT_NPROC)) &&
-	    (current_user() != INIT_USER) &&
+	    (current_ucounts() != &init_ucounts) &&
 	    !capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN)) {
 		retval = -EAGAIN;
 		goto out_ret;
diff --git a/kernel/fork.c b/kernel/fork.c
index 2b6a28a86325..6f62d37f3650 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2027,7 +2027,7 @@ static __latent_entropy struct task_struct *copy_process(
 
 	retval = -EAGAIN;
 	if (is_ucounts_overlimit(task_ucounts(p), UCOUNT_RLIMIT_NPROC, rlimit(RLIMIT_NPROC))) {
-		if (p->real_cred->user != INIT_USER &&
+		if ((task_ucounts(p) != &init_ucounts) &&
 		    !capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN))
 			goto bad_fork_cleanup_count;
 	}
diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index 6b2e3ca7ee99..f0c04073403d 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -123,6 +123,8 @@ int create_user_ns(struct cred *new)
 		ns->ucount_max[i] = INT_MAX;
 	}
 	set_rlimit_ucount_max(ns, UCOUNT_RLIMIT_NPROC, rlimit(RLIMIT_NPROC));
+	if (new->ucounts == &init_ucounts)
+		set_rlimit_ucount_max(ns, UCOUNT_RLIMIT_NPROC, RLIMIT_INFINITY);
 	set_rlimit_ucount_max(ns, UCOUNT_RLIMIT_MSGQUEUE, rlimit(RLIMIT_MSGQUEUE));
 	set_rlimit_ucount_max(ns, UCOUNT_RLIMIT_SIGPENDING, rlimit(RLIMIT_SIGPENDING));
 	set_rlimit_ucount_max(ns, UCOUNT_RLIMIT_MEMLOCK, rlimit(RLIMIT_MEMLOCK));
-- 
2.29.2