linux-kernel - [PATCH 5.15 181/196] ucounts: Enforce RLIMIT_NPROC not RLIMIT

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Date:   Mon, 21 Feb 2022 09:50:13 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        Michal Koutný <mkoutny@...e.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>
Subject: [PATCH 5.15 181/196] ucounts: Enforce RLIMIT_NPROC not RLIMIT_NPROC+1

From: Eric W. Biederman <ebiederm@...ssion.com>

commit 8f2f9c4d82f24f172ae439e5035fc1e0e4c229dd upstream.

Michal Koutný <mkoutny@...e.com> wrote:

> It was reported that v5.14 behaves differently when enforcing
> RLIMIT_NPROC limit, namely, it allows one more task than previously.
> This is consequence of the commit 21d1c5e386bc ("Reimplement
> RLIMIT_NPROC on top of ucounts") that missed the sharpness of
> equality in the forking path.

This can be fixed either by fixing the test or by moving the increment
to be before the test.  Fix it my moving copy_creds which contains
the increment before is_ucounts_overlimit.

In the case of CLONE_NEWUSER the ucounts in the task_cred changes.
The function is_ucounts_overlimit needs to use the final version of
the ucounts for the new process.  Which means moving the
is_ucounts_overlimit test after copy_creds is necessary.

Both the test in fork and the test in set_user were semantically
changed when the code moved to ucounts.  The change of the test in
fork was bad because it was before the increment.  The test in
set_user was wrong and the change to ucounts fixed it.  So this
fix only restores the old behavior in one lcation not two.

Link: https://lkml.kernel.org/r/20220204181144.24462-1-mkoutny@suse.com
Link: https://lkml.kernel.org/r/20220216155832.680775-2-ebiederm@xmission.com
Cc: stable@...r.kernel.org
Reported-by: Michal Koutný <mkoutny@...e.com>
Reviewed-by: Michal Koutný <mkoutny@...e.com>
Fixes: 21d1c5e386bc ("Reimplement RLIMIT_NPROC on top of ucounts")
Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 kernel/fork.c |   10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2055,18 +2055,18 @@ static __latent_entropy struct task_stru
 #ifdef CONFIG_PROVE_LOCKING
 	DEBUG_LOCKS_WARN_ON(!p->softirqs_enabled);
 #endif
+	retval = copy_creds(p, clone_flags);
+	if (retval < 0)
+		goto bad_fork_free;
+
 	retval = -EAGAIN;
 	if (is_ucounts_overlimit(task_ucounts(p), UCOUNT_RLIMIT_NPROC, rlimit(RLIMIT_NPROC))) {
 		if (p->real_cred->user != INIT_USER &&
 		    !capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN))
-			goto bad_fork_free;
+			goto bad_fork_cleanup_count;
 	}
 	current->flags &= ~PF_NPROC_EXCEEDED;
 
-	retval = copy_creds(p, clone_flags);
-	if (retval < 0)
-		goto bad_fork_free;
-
 	/*
 	 * If multiple threads are within copy_process(), then this check
 	 * triggers too late. This doesn't hurt, the check is only there