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 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 Cc: stable@vger.kernel.org Reported-by: Michal Koutný mkoutny@suse.com Reviewed-by: Michal Koutný mkoutny@suse.com Fixes: 21d1c5e386bc ("Reimplement RLIMIT_NPROC on top of ucounts") Signed-off-by: "Eric W. Biederman" ebiederm@xmission.com --- kernel/fork.c | 10 +++++----- 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/kernel/fork.c b/kernel/fork.c index d75a528f7b21..17d8a8c85e3b 100644 --- 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