[PATCH] net: Fix zero-copy head len calculation.

From: Pravin B Shelar <redacted>

In some cases skb head could be locked and entire header
data is pulled from skb. When skb_zerocopy() called in such cases,
following BUG is triggered. This patch fixes it by copying entire
skb in such cases.
This could be optimized incase this is performance bottleneck.

---8<---
kernel BUG at net/core/skbuff.c:2961!
invalid opcode: 0000 [#1] SMP PTI
CPU: 2 PID: 0 Comm: swapper/2 Tainted: G           OE     5.4.0-77-generic #86-Ubuntu
Hardware name: OpenStack Foundation OpenStack Nova, BIOS 1.13.0-1ubuntu1.1 04/01/2014
RIP: 0010:skb_zerocopy+0x37a/0x3a0
RSP: 0018:ffffbcc70013ca38 EFLAGS: 00010246
Call Trace:
 <IRQ>
 queue_userspace_packet+0x2af/0x5e0 [openvswitch]
 ovs_dp_upcall+0x3d/0x60 [openvswitch]
 ovs_dp_process_packet+0x125/0x150 [openvswitch]
 ovs_vport_receive+0x77/0xd0 [openvswitch]
 netdev_port_receive+0x87/0x130 [openvswitch]
 netdev_frame_hook+0x4b/0x60 [openvswitch]
 __netif_receive_skb_core+0x2b4/0xc90
 __netif_receive_skb_one_core+0x3f/0xa0
 __netif_receive_skb+0x18/0x60
 process_backlog+0xa9/0x160
 net_rx_action+0x142/0x390
 __do_softirq+0xe1/0x2d6
 irq_exit+0xae/0xb0
 do_IRQ+0x5a/0xf0
 common_interrupt+0xf/0xf

Code that triggered BUG:
int
skb_zerocopy(struct sk_buff *to, struct sk_buff *from, int len, int hlen)
{
        int i, j = 0;
        int plen = 0; /* length of skb->head fragment */
        int ret;
        struct page *page;
        unsigned int offset;

        BUG_ON(!from->head_frag && !hlen);

Signed-off-by: Pravin B Shelar <redacted>
---
 net/core/skbuff.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index f63de967ac25..eaa264984535 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c

@@ -3011,8 +3011,11 @@ skb_zerocopy_headlen(const struct sk_buff *from)
 
 	if (!from->head_frag ||
 	    skb_headlen(from) < L1_CACHE_BYTES ||
-	    skb_shinfo(from)->nr_frags >= MAX_SKB_FRAGS)
+	    skb_shinfo(from)->nr_frags >= MAX_SKB_FRAGS) {
 		hlen = skb_headlen(from);
+		if (!hlen)
+			hlen = from->len;
+	}
 
 	if (skb_has_frag_list(from))
 		hlen = from->len;

-- 
2.25.1

Hello:

This patch was applied to netdev/net.git (refs/heads/master):

On Thu, 15 Jul 2021 16:59:00 -0700 you wrote:

From: Pravin B Shelar <redacted>

In some cases skb head could be locked and entire header
data is pulled from skb. When skb_zerocopy() called in such cases,
following BUG is triggered. This patch fixes it by copying entire
skb in such cases.
This could be optimized incase this is performance bottleneck.

[...]

Here is the summary with links:
  - net: Fix zero-copy head len calculation.
    https://git.kernel.org/netdev/net/c/a17ad0961706

You are awesome, thank you!
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