ucounts: Enforce RLIMIT_NPROC not RLIMIT_NPROC+1

Michal Koutný <mkoutny@suse.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 96a6f980d813 ("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@vger.kernel.org
Reported-by: Michal Koutný <mkoutny@suse.com>
Reviewed-by: Michal Koutný <mkoutny@suse.com>
Fixes: 96a6f980d813 ("Reimplement RLIMIT_NPROC on top of ucounts")
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

diff --git a/kernel/fork.c b/kernel/fork.c
index d75a528f7b2196675c35a4c3ea8484eff88bddd2..17d8a8c85e3b930097d458f47f93daf24310a4a1 100644 (file)
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2021,18 +2021,18 @@ static __latent_entropy struct task_struct *copy_process(
 #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