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

Eric W. Biederman
2022-03-14T15:21:40Z

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