linux-kernel - Re: [PATCH] mm/mlock: fix potential imbalanced rlimit ucounts adjustment

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Message-ID: <1e758c8f-c87b-50ef-52db-942c8a86a32a@google.com>
Date:   Sun, 13 Mar 2022 20:11:23 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     Miaohe Lin <linmiaohe@...wei.com>
cc:     Hugh Dickins <hughd@...gle.com>, akpm@...ux-foundation.org,
        Herbert.van.den.Bergh@...cle.com,
        Alexey Gladkov <legion@...nel.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/mlock: fix potential imbalanced rlimit ucounts
 adjustment

On Mon, 14 Mar 2022, Miaohe Lin wrote:
> On 2022/3/14 10:40, Hugh Dickins wrote:
> > On Thu, 10 Mar 2022, Miaohe Lin wrote:
> > 
> >> 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.
> >>
> >> Fixes: 5ed44a401ddf ("do not limit locked memory when RLIMIT_MEMLOCK is RLIM_INFINITY")
> >> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
> > 
> > NAK.  user_shm_lock() remembers to declare "int allowed = 0" on entry.
> > 
> 
> If lock_limit is RLIM_INFINITY, "allowed" will be set to 1. And if get_ucounts fails
> in some corner cases, "allowed" will remain to be 1 while the user_shm_lock ops indeed
> fails. Or am I miss something?

You are right, I am wrong: sorry.
Thanks for pointing now to that RLIM_INFINITY case.

But then the Fixes tag is wrong: it should be
Fixes: d7c9e99aee48 ("Reimplement RLIMIT_MEMLOCK on top of ucounts")
       which introduced the possibility of error down there.

With that,
Acked-by: Hugh Dickins <hughd@...gle.com>

> 
> Many thanks for comment.
> 
> >> ---
> >>  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;
> >> -- 
> >> 2.23.0