linux-kernel - [RFC] [PATCH] net: account for possible negative frag_mem

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Date:   Thu,  2 Mar 2017 12:37:52 +0500
From:   Valentine Sinitsyn <valentine.sinitsyn@...il.com>
To:     linux-kernel@...r.kernel.org
Cc:     Valentine Sinitsyn <valentine.sinitsyn@...il.com>
Subject: [RFC] [PATCH] net: account for possible negative frag_mem_limit

I'm also not sure if we'd better disable bottom halves before
calculating the counter sum, as sum_frag_mem_limit() does.

--- 8< ---

On SMP systems, percpu_counter_sum() is permitted to return negative
values. However, inet_frag_exit_net() function is following the
assumption that its return value is either positive or zero.

If a negative value is returned, the code loops forever, possibly
keeping net_mutex locked. This could break many things, including
unshare(CLONE_NEWNET) system call which would hang forever.

Fix this by explicitly asking for a positive return value from the
percpu counter.

Signed-off-by: Valentine Sinitsyn <valentine.sinitsyn@...il.com>
---
 net/ipv4/inet_fragment.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/ipv4/inet_fragment.c b/net/ipv4/inet_fragment.c
index b5e9317..0a63c9e 100644
--- a/net/ipv4/inet_fragment.c
+++ b/net/ipv4/inet_fragment.c
@@ -234,7 +234,7 @@ void inet_frags_exit_net(struct netns_frags *nf, struct inet_frags *f)
 	cond_resched();

 	if (read_seqretry(&f->rnd_seqlock, seq) ||
-	    percpu_counter_sum(&nf->mem))
+	    percpu_counter_sum_positive(&nf->mem))
 		goto evict_again;

 	percpu_counter_destroy(&nf->mem);
-- 
2.7.4