Re: [PATCH v2] mm/mlock: fix potential imbalanced rlimit ucounts adjustment

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

Miaohe Lin <linmiaohe@huawei.com> writes:

> user_shm_lock forgets to set allowed to 0 when get_ucounts fails. So
> the later user_shm_unlock might do the extra dec_rlimit_ucounts. Fix
> this by resetting allowed to 0.

This fix looks correct.  But the ability for people to follow and read
the code seems questionable.  I saw in v1 of this patch Hugh originally
misread the logic.

Could we instead change the code to leave lock_limit at ULONG_MAX aka
RLIM_INFINITY, leave initialized to 0, and not even need a special case
of RLIM_INFINITY as nothing can be greater that ULONG_MAX?

Something like this?

diff --git a/mm/mlock.c b/mm/mlock.c
index 8f584eddd305..e7eabf5193ab 100644
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -827,13 +827,12 @@ int user_shm_lock(size_t size, struct ucounts *ucounts)
 
 	locked = (size + PAGE_SIZE - 1) >> PAGE_SHIFT;
 	lock_limit = rlimit(RLIMIT_MEMLOCK);
-	if (lock_limit == RLIM_INFINITY)
-		allowed = 1;
-	lock_limit >>= PAGE_SHIFT;
+	if (lock_limit != RLIM_INFINITY)
+		lock_limit >>= PAGE_SHIFT;
 	spin_lock(&shmlock_user_lock);
 	memlock = inc_rlimit_ucounts(ucounts, UCOUNT_RLIMIT_MEMLOCK, locked);
 
-	if (!allowed && (memlock == LONG_MAX || memlock > lock_limit) && !capable(CAP_IPC_LOCK)) {
+	if ((memlock == LONG_MAX || memlock > lock_limit) && !capable(CAP_IPC_LOCK)) {
 		dec_rlimit_ucounts(ucounts, UCOUNT_RLIMIT_MEMLOCK, locked);
 		goto out;
 	}

>
> Fixes: d7c9e99aee48 ("Reimplement RLIMIT_MEMLOCK on top of ucounts")
> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
> Acked-by: Hugh Dickins <hughd@google.com>
> ---
> v1->v2:
>   correct Fixes tag and collect Acked-by tag
>   Thanks Hugh for review!
> ---
>  mm/mlock.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/mm/mlock.c b/mm/mlock.c
> index 29372c0eebe5..efd2dd2943de 100644
> --- a/mm/mlock.c
> +++ b/mm/mlock.c
> @@ -733,6 +733,7 @@ int user_shm_lock(size_t size, struct ucounts *ucounts)
>  	}
>  	if (!get_ucounts(ucounts)) {
>  		dec_rlimit_ucounts(ucounts, UCOUNT_RLIMIT_MEMLOCK, locked);
> +		allowed = 0;
>  		goto out;
>  	}
>  	allowed = 1;

Eric