netdev - Concerns regarding PFMEMALLOC handling in __netdev_alloc

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Message-ID: <20140828232350.70684900@halley>
Date:	Thu, 28 Aug 2014 23:23:50 +0300
From:	Shmulik Ladkani <sladkani@...co.com>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Neil Brown <neilb@...e.de>, Eric Dumazet <eric.dumazet@...il.com>,
	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Concerns regarding PFMEMALLOC handling in __netdev_alloc_skb

Hi,

>From c93bdd0e03 "netvm: allow skb allocation to use PFMEMALLOC reserves":

@@ -366,7 +417,12 @@ struct sk_buff *__netdev_alloc_skb(struct net_device *dev,
 			      SKB_DATA_ALIGN(sizeof(struct skb_shared_info));

 	if (fragsz <= PAGE_SIZE && !(gfp_mask & (__GFP_WAIT | GFP_DMA))) {
-		void *data = netdev_alloc_frag(fragsz);
+		void *data;
+
+		if (sk_memalloc_socks())
+			gfp_mask |= __GFP_MEMALLOC;
+
+		data = __netdev_alloc_frag(fragsz, gfp_mask);

		if (likely(data)) {
			skb = build_skb(data, fragsz);
			if (unlikely(!skb))
				put_page(virt_to_head_page(data));
		}
 	} else {
-		skb = __alloc_skb(length + NET_SKB_PAD, gfp_mask, 0, NUMA_NO_NODE);
+		skb = __alloc_skb(length + NET_SKB_PAD, gfp_mask,
+				  SKB_ALLOC_RX, NUMA_NO_NODE);
 	}

In the 'else' part, SKB_ALLOC_RX is provided to '__alloc_skb()'.
Thus '__alloc_skb()' may attempt using the PFMEMALLOC reserve in case
'sk_memalloc_socks()' is true - and 'skb->pfmemalloc' will be set
accordingly. Good.

However, in the 'if' part, in case 'sk_memalloc_socks()' is true,
__GFP_MEMALLOC is passed to '__netdev_alloc_frag()'.

There are two possible issues here:

1. '__netdev_alloc_frag()' might not honour __GFP_MEMALLOC in case the
   frag fits into current netdev_alloc_cache.frag

2. Even if 'nc->frag.page' gets allocated/refilled, and __GFP_MEMALLOC
   is passed to 'alloc_pages()' - in case the new page is from the
   PFMEMALLOC reserve, that notion is not propagated to back to
   skb->pfmemalloc.

Are these of any concern?

Regards,
Shmulik
--
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html