[PATCH net] gro: ensure frag0 meets IP header alignment

[PATCH net] gro: ensure frag0 meets IP header alignment

[Eric Dumazet]

From: Eric Dumazet <edumazet@google.com>

After commit 0f6925b3e8da ("virtio_net: Do not pull payload in skb->head")
Guenter Roeck reported one failure in his tests using sh architecture.

After much debugging, we have been able to spot silent unaligned accesses
in inet_gro_receive()

The issue at hand is that upper networking stacks assume their header
is word-aligned. Low level drivers are supposed to reserve NET_IP_ALIGN
bytes before the Ethernet header to make that happen.

This patch hardens skb_gro_reset_offset() to not allow frag0 fast-path
if the fragment is not properly aligned.

Some arches like x86, arm64 and powerpc do not care and define NET_IP_ALIGN
as 0, this extra check will be a NOP for them.

Note that if frag0 is not used, GRO will call pskb_may_pull()
as many times as needed to pull network and transport headers.

Fixes: 0f6925b3e8da ("virtio_net: Do not pull payload in skb->head")
Fixes: 78a478d0efd9 ("gro: Inline skb_gro_header and cache frag0 virtual address")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Guenter Roeck <linux@roeck-us.net>
Cc: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
---
 net/core/dev.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index af8c1ea040b9364b076e2d72f04dc3de2d7e2f11..1f79b9aa9a3f2392fddd1401f95ad098b5e03204 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -5924,7 +5924,8 @@ static void skb_gro_reset_offset(struct sk_buff *skb)
 	NAPI_GRO_CB(skb)->frag0_len = 0;
 
 	if (!skb_headlen(skb) && pinfo->nr_frags &&
-	    !PageHighMem(skb_frag_page(frag0))) {
+	    !PageHighMem(skb_frag_page(frag0)) &&
+	    (!NET_IP_ALIGN || !(skb_frag_off(frag0) & 3))) {
 		NAPI_GRO_CB(skb)->frag0 = skb_frag_address(frag0);
 		NAPI_GRO_CB(skb)->frag0_len = min_t(unsigned int,
 						    skb_frag_size(frag0),
-- 
2.31.1.295.g9ea45b61b8-goog

Re: [PATCH net] gro: ensure frag0 meets IP header alignment

[Michael S. Tsirkin]

On Tue, Apr 13, 2021 at 05:41:35AM -0700, Eric Dumazet wrote:
> From: Eric Dumazet <edumazet@google.com>
> 
> After commit 0f6925b3e8da ("virtio_net: Do not pull payload in skb->head")
> Guenter Roeck reported one failure in his tests using sh architecture.
> 
> After much debugging, we have been able to spot silent unaligned accesses
> in inet_gro_receive()
> 
> The issue at hand is that upper networking stacks assume their header
> is word-aligned. Low level drivers are supposed to reserve NET_IP_ALIGN
> bytes before the Ethernet header to make that happen.
> 
> This patch hardens skb_gro_reset_offset() to not allow frag0 fast-path
> if the fragment is not properly aligned.
> 
> Some arches like x86, arm64 and powerpc do not care and define NET_IP_ALIGN
> as 0, this extra check will be a NOP for them.
> 
> Note that if frag0 is not used, GRO will call pskb_may_pull()
> as many times as needed to pull network and transport headers.
> 
> Fixes: 0f6925b3e8da ("virtio_net: Do not pull payload in skb->head")
> Fixes: 78a478d0efd9 ("gro: Inline skb_gro_header and cache frag0 virtual address")
> Signed-off-by: Eric Dumazet <edumazet@google.com>
> Reported-by: Guenter Roeck <linux@roeck-us.net>
> Cc: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> Cc: "Michael S. Tsirkin" <mst@redhat.com>
> Cc: Jason Wang <jasowang@redhat.com>

Seems to make sense.

Acked-by: Michael S. Tsirkin <mst@redhat.com>

> ---
>  net/core/dev.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/net/core/dev.c b/net/core/dev.c
> index af8c1ea040b9364b076e2d72f04dc3de2d7e2f11..1f79b9aa9a3f2392fddd1401f95ad098b5e03204 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -5924,7 +5924,8 @@ static void skb_gro_reset_offset(struct sk_buff *skb)
>  	NAPI_GRO_CB(skb)->frag0_len = 0;
>  
>  	if (!skb_headlen(skb) && pinfo->nr_frags &&
> -	    !PageHighMem(skb_frag_page(frag0))) {
> +	    !PageHighMem(skb_frag_page(frag0)) &&
> +	    (!NET_IP_ALIGN || !(skb_frag_off(frag0) & 3))) {
>  		NAPI_GRO_CB(skb)->frag0 = skb_frag_address(frag0);
>  		NAPI_GRO_CB(skb)->frag0_len = min_t(unsigned int,
>  						    skb_frag_size(frag0),
> -- 
> 2.31.1.295.g9ea45b61b8-goog

Re: [PATCH net] gro: ensure frag0 meets IP header alignment

[Guenter Roeck]

On 4/13/21 5:41 AM, Eric Dumazet wrote:
> From: Eric Dumazet <edumazet@google.com>
> 
> After commit 0f6925b3e8da ("virtio_net: Do not pull payload in skb->head")
> Guenter Roeck reported one failure in his tests using sh architecture.
> 
> After much debugging, we have been able to spot silent unaligned accesses
> in inet_gro_receive()
> 
> The issue at hand is that upper networking stacks assume their header
> is word-aligned. Low level drivers are supposed to reserve NET_IP_ALIGN
> bytes before the Ethernet header to make that happen.
> 
> This patch hardens skb_gro_reset_offset() to not allow frag0 fast-path
> if the fragment is not properly aligned.
> 
> Some arches like x86, arm64 and powerpc do not care and define NET_IP_ALIGN
> as 0, this extra check will be a NOP for them.
> 
> Note that if frag0 is not used, GRO will call pskb_may_pull()
> as many times as needed to pull network and transport headers.
> 
> Fixes: 0f6925b3e8da ("virtio_net: Do not pull payload in skb->head")
> Fixes: 78a478d0efd9 ("gro: Inline skb_gro_header and cache frag0 virtual address")
> Signed-off-by: Eric Dumazet <edumazet@google.com>
> Reported-by: Guenter Roeck <linux@roeck-us.net>
> Cc: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> Cc: "Michael S. Tsirkin" <mst@redhat.com>
> Cc: Jason Wang <jasowang@redhat.com>

Nice catch.

Tested-by: Guenter Roeck <linux@roeck-us.net>

.... and thanks a lot for tracking this down!

Guenter

> ---
>  net/core/dev.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/net/core/dev.c b/net/core/dev.c
> index af8c1ea040b9364b076e2d72f04dc3de2d7e2f11..1f79b9aa9a3f2392fddd1401f95ad098b5e03204 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -5924,7 +5924,8 @@ static void skb_gro_reset_offset(struct sk_buff *skb)
>  	NAPI_GRO_CB(skb)->frag0_len = 0;
>  
>  	if (!skb_headlen(skb) && pinfo->nr_frags &&
> -	    !PageHighMem(skb_frag_page(frag0))) {
> +	    !PageHighMem(skb_frag_page(frag0)) &&
> +	    (!NET_IP_ALIGN || !(skb_frag_off(frag0) & 3))) {
>  		NAPI_GRO_CB(skb)->frag0 = skb_frag_address(frag0);
>  		NAPI_GRO_CB(skb)->frag0_len = min_t(unsigned int,
>  						    skb_frag_size(frag0),
> 

Re: [PATCH net] gro: ensure frag0 meets IP header alignment

[patchwork-bot+netdevbpf]

Hello:

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

On Tue, 13 Apr 2021 05:41:35 -0700 you wrote:
> From: Eric Dumazet <edumazet@google.com>
> 
> After commit 0f6925b3e8da ("virtio_net: Do not pull payload in skb->head")
> Guenter Roeck reported one failure in his tests using sh architecture.
> 
> After much debugging, we have been able to spot silent unaligned accesses
> in inet_gro_receive()
> 
> [...]

Here is the summary with links:
  - [net] gro: ensure frag0 meets IP header alignment
    https://git.kernel.org/netdev/net/c/38ec4944b593

You are awesome, thank you!
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