[PATCH net] net/packet: check TX_RING fragments before memory accounting

[PATCH net] net/packet: check TX_RING fragments before memory accounting

[Eric Dumazet]

From: Kyle Zeng <kylebot@openai.com>

tpacket_fill_skb() adds the full non-linear payload length to
skb->truesize and sk_wmem_alloc before checking MAX_SKB_FRAGS. With
PACKET_VNET_HDR, userspace can submit a ring-sized frame even when it
greatly exceeds the device MTU.

A near-INT_MAX frame can saturate sk_wmem_alloc before the fragment
loop rejects it. A subsequent refcount_add(0x40000001) can temporarily
turn REFCOUNT_SATURATED into 1. A concurrent TX completion can then
drop that apparent last reference and free the still-live packet
socket before refcount_warn_saturate() restores the counter.

Check that the remaining payload fits in MAX_SKB_FRAGS pages before
accounting it. Include the initial page offset in the calculation,
and move the existing -EFAULT handling out of the fragment loop.
This preserves the accepted packet sizes and error code while
preventing an invalid frame from injecting an oversized byte charge.

Fixes: 69e3c75f4d54 ("net: TX_RING and packet mmap")
Cc: stable@vger.kernel.org
Cc: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Signed-off-by: Kyle Zeng <kylebot@openai.com>
[edumazet: use net_warn_ratelimited() instead of pr_err(), add a Fixes: tag]
Signed-off-by: Eric Dumazet <edumazet@kernel.org>
---
 net/packet/af_packet.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index 823e0911c910fa95d85bdf187eddef802cf65726..cf6429f09477263534b631199aae85c30de2fffe 100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -2655,6 +2655,12 @@ static int tpacket_fill_skb(struct packet_sock *po, struct sk_buff *skb,
 	}
 
 	offset = offset_in_page(data);
+	if (unlikely(to_write > MAX_SKB_FRAGS * PAGE_SIZE - offset)) {
+		net_warn_ratelimited("Packet exceed the number of skb frags(%u)\n",
+				     (unsigned int)MAX_SKB_FRAGS);
+		return -EFAULT;
+	}
+
 	len_max = PAGE_SIZE - offset;
 	len = ((to_write > len_max) ? len_max : to_write);
 
@@ -2666,12 +2672,6 @@ static int tpacket_fill_skb(struct packet_sock *po, struct sk_buff *skb,
 	while (likely(to_write)) {
 		nr_frags = skb_shinfo(skb)->nr_frags;
 
-		if (unlikely(nr_frags >= MAX_SKB_FRAGS)) {
-			pr_err("Packet exceed the number of skb frags(%u)\n",
-			       (unsigned int)MAX_SKB_FRAGS);
-			return -EFAULT;
-		}
-
 		page = pgv_to_page(data);
 		data += len;
 		flush_dcache_page(page);
-- 
2.53.0

Re: [PATCH net] net/packet: check TX_RING fragments before memory accounting

[netdev-bot+sinfo]

Hi!

This is an automated message. This series looks like a fix, but its
commit messages seem to be missing some information:

 - How the issue was discovered, e.g. hit in production, hit during
   development, syzbot report, manual code inspection, LLM or static
   analysis tool scan.

 - Whether the issue was actually triggered, or is only theoretical
   (e.g. found by code inspection). If it was triggered please include
   the symptoms, like the stack trace or error messages.

Please do not repost the series just to address the above. Instead,
reply to this email with the missing information, so that reviewers
can take it into account. If the series needs another revision for
other reasons, please include the information in the commit messages
then.

The evaluation is done by an LLM so it may be wrong, if you think
that is the case please reply and explain.

Re: [PATCH net] net/packet: check TX_RING fragments before memory accounting

[Willem de Bruijn]

Eric Dumazet wrote:
> From: Kyle Zeng <kylebot@openai.com>
> 
> tpacket_fill_skb() adds the full non-linear payload length to
> skb->truesize and sk_wmem_alloc before checking MAX_SKB_FRAGS. With
> PACKET_VNET_HDR, userspace can submit a ring-sized frame even when it
> greatly exceeds the device MTU.
> 
> A near-INT_MAX frame can saturate sk_wmem_alloc before the fragment
> loop rejects it. A subsequent refcount_add(0x40000001) can temporarily
> turn REFCOUNT_SATURATED into 1. A concurrent TX completion can then
> drop that apparent last reference and free the still-live packet
> socket before refcount_warn_saturate() restores the counter.
> 
> Check that the remaining payload fits in MAX_SKB_FRAGS pages before
> accounting it. Include the initial page offset in the calculation,
> and move the existing -EFAULT handling out of the fragment loop.
> This preserves the accepted packet sizes and error code while
> preventing an invalid frame from injecting an oversized byte charge.
> 
> Fixes: 69e3c75f4d54 ("net: TX_RING and packet mmap")
> Cc: stable@vger.kernel.org
> Cc: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
> Signed-off-by: Kyle Zeng <kylebot@openai.com>
> [edumazet: use net_warn_ratelimited() instead of pr_err(), add a Fixes: tag]
> Signed-off-by: Eric Dumazet <edumazet@kernel.org>

Reviewed-by: Willem de Bruijn <willemb@google.com>

Re: [PATCH net] net/packet: check TX_RING fragments before memory accounting

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net.git (main)
by Jakub Kicinski <kuba@kernel.org>:

On Wed, 28 Oct 2026 19:14:50 +0100 you wrote:
> From: Kyle Zeng <kylebot@openai.com>
> 
> tpacket_fill_skb() adds the full non-linear payload length to
> skb->truesize and sk_wmem_alloc before checking MAX_SKB_FRAGS. With
> PACKET_VNET_HDR, userspace can submit a ring-sized frame even when it
> greatly exceeds the device MTU.
> 
> [...]

Here is the summary with links:
  - [net] net/packet: check TX_RING fragments before memory accounting
    https://git.kernel.org/netdev/net/c/dc4dd12f80e4

You are awesome, thank you!
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