linux-kernel - [PATCH 6/8] ucounts: Handle inc_rlimit

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Message-Id: <20220211021324.4116773-6-ebiederm@xmission.com>
Date:   Thu, 10 Feb 2022 20:13:22 -0600
From:   "Eric W. Biederman" <ebiederm@...ssion.com>
To:     linux-kernel@...r.kernel.org
Cc:     Alexey Gladkov <legion@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Shuah Khan <shuah@...nel.org>,
        Christian Brauner <brauner@...nel.org>,
        Solar Designer <solar@...nwall.com>,
        Ran Xiaokai <ran.xiaokai@....com.cn>,
        containers@...ts.linux-foundation.org,
        Michal Koutný <mkoutny@...e.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>
Subject: [PATCH 6/8] ucounts: Handle inc_rlimit_ucounts wrapping in fork

Move inc_rlimit_ucounts from copy_creds into copy_process immediately
after copy_creds where it can be called exactly once.  Test for and
handle it when inc_rlimit_ucounts returns LONG_MAX indicating the
count has wrapped.

This is good hygenine and fixes a theoretical bug.  In practice
PID_MAX_LIMIT is at most 2^22 so there is not a chance the number of
processes would ever wrap even on an architecture with a 32bit long.

Fixes: 21d1c5e386bc ("Reimplement RLIMIT_NPROC on top of ucounts")
Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
---
 kernel/cred.c | 2 --
 kernel/fork.c | 2 ++
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/cred.c b/kernel/cred.c
index 229cff081167..96d5fd6ff26f 100644
--- a/kernel/cred.c
+++ b/kernel/cred.c
@@ -358,7 +358,6 @@ int copy_creds(struct task_struct *p, unsigned long clone_flags)
 		kdebug("share_creds(%p{%d,%d})",
 		       p->cred, atomic_read(&p->cred->usage),
 		       read_cred_subscribers(p->cred));
-		inc_rlimit_ucounts(task_ucounts(p), UCOUNT_RLIMIT_NPROC, 1);
 		return 0;
 	}
 
@@ -395,7 +394,6 @@ int copy_creds(struct task_struct *p, unsigned long clone_flags)
 #endif
 
 	p->cred = p->real_cred = get_cred(new);
-	inc_rlimit_ucounts(task_ucounts(p), UCOUNT_RLIMIT_NPROC, 1);
 	alter_cred_subscribers(new, 2);
 	validate_creds(new);
 	return 0;
diff --git a/kernel/fork.c b/kernel/fork.c
index 6f62d37f3650..69333078259c 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2026,6 +2026,8 @@ static __latent_entropy struct task_struct *copy_process(
 		goto bad_fork_free;
 
 	retval = -EAGAIN;
+	if (inc_rlimit_ucounts(task_ucounts(p), UCOUNT_RLIMIT_NPROC, 1) == LONG_MAX)
+		goto bad_fork_cleanup_count;
 	if (is_ucounts_overlimit(task_ucounts(p), UCOUNT_RLIMIT_NPROC, rlimit(RLIMIT_NPROC))) {
 		if ((task_ucounts(p) != &init_ucounts) &&
 		    !capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN))
-- 
2.29.2