[PATCH net 0/2] ipv4: fix IP ID reuse for GSO packets

[PATCH net 0/2] ipv4: fix IP ID reuse for GSO packets

[Eric Dumazet]

This series fixes two cases where IPv4 GSO packets can reuse the IP IDs
of previous packets:

- Patch 1 fixes ip_select_ident_segs() for non-TCP sockets, which uses
  the first IP ID of the next packet since commit f866fbc842de
  ("ipv4: fix data-races around inet->inet_id"). SCTP GSO is affected.

- Patch 2 makes __ip_make_skb() reserve one IP ID per segment for
  UDP GSO packets, instead of a single one.

Both issues were found by code inspection.

Eric Dumazet (2):
  ipv4: fix IP ID reuse in ip_select_ident_segs()
  ipv4: reserve one IP ID per segment for UDP GSO packets

 include/net/ip.h     |  2 +-
 net/ipv4/ip_output.c | 15 ++++++++++++++-
 2 files changed, 15 insertions(+), 2 deletions(-)

-- 
2.56.0.rc1.315.gc6ed9934b7-goog

[PATCH net 1/2] ipv4: fix IP ID reuse in ip_select_ident_segs()

[Eric Dumazet]

ip_select_ident_segs() must put the first of the @segs reserved IP IDs
in iph->id, as GSO assigns id, id + 1, ..., id + segs - 1 to segments.

Commit f866fbc842de ("ipv4: fix data-races around inet->inet_id")
used atomic_add_return() for non-TCP sockets, which returns the first
ID of the next packet instead. Consecutive GSO packets can then reuse
IP IDs.

SCTP GSO is affected. Other callers use segs == 1, and only see
a harmless off-by-one (UDP GSO has a separate, older issue, see
following patch in this series).

Use atomic_fetch_add() instead, like the TCP path.

Fixes: f866fbc842de ("ipv4: fix data-races around inet->inet_id")
Signed-off-by: Eric Dumazet <edumazet@kernel.org>
---
 include/net/ip.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/net/ip.h b/include/net/ip.h
index 6f602df72ee621ee4ee45e70beef0a1b5145367f..6a3e8271a73b3669e97c4b389e916dbcbfa9f6ae 100644
--- a/include/net/ip.h
+++ b/include/net/ip.h
@@ -598,7 +598,7 @@ static inline void ip_select_ident_segs(struct net *net, struct sk_buff *skb,
 			val = atomic_read(&inet_sk(sk)->inet_id);
 			atomic_set(&inet_sk(sk)->inet_id, val + segs);
 		} else {
-			val = atomic_add_return(segs, &inet_sk(sk)->inet_id);
+			val = atomic_fetch_add(segs, &inet_sk(sk)->inet_id);
 		}
 		iph->id = htons(val);
 		return;
-- 
2.56.0.rc1.315.gc6ed9934b7-goog

[PATCH net 2/2] ipv4: reserve one IP ID per segment for UDP GSO packets

[Eric Dumazet]

__ip_make_skb() reserves a single IP ID for UDP GSO packets, but GSO
assigns one IP ID per segment. Following packets then reuse these IDs,
either from inet->inet_id or from the shared generator.

Unless IP_PMTUDISC_DO/PROBE is used, DF is not set on UDP GSO packets,
so segments can be fragmented on the path and IP ID reuse can lead to
incorrect reassembly.

Reserve one IP ID per segment, using the same test as udp_send_skb().

Fixes: bec1f6f69736 ("udp: generate gso with UDP_SEGMENT")
Signed-off-by: Eric Dumazet <edumazet@kernel.org>
---
 net/ipv4/ip_output.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
index a24cc8ee11d3ea3069bcc0d4d12e6867c2c475f7..b1cf0c6bfc79c8d234cc81ff32332116c1ee3401 100644
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -1410,6 +1410,7 @@ struct sk_buff *__ip_make_skb(struct sock *sk,
 	struct iphdr *iph;
 	u8 pmtudisc, ttl;
 	__be16 df = 0;
+	int segs;
 
 	skb = __skb_dequeue(queue);
 	if (!skb)
@@ -1464,7 +1465,19 @@ struct sk_buff *__ip_make_skb(struct sock *sk,
 	iph->ttl = ttl;
 	iph->protocol = sk->sk_protocol;
 	ip_copy_addrs(iph, fl4);
-	ip_select_ident(net, skb, sk);
+
+	/* UDP GSO packets are segmented later (see udp_send_skb()):
+	 * reserve one IP ID per segment.
+	 */
+	segs = 1;
+	if (cork->gso_size) {
+		int datalen = skb->len - skb_transport_offset(skb) -
+			      sizeof(struct udphdr);
+
+		if (datalen > cork->gso_size)
+			segs = DIV_ROUND_UP(datalen, cork->gso_size);
+	}
+	ip_select_ident_segs(net, skb, sk, segs);
 
 	if (opt) {
 		iph->ihl += opt->optlen >> 2;
-- 
2.56.0.rc1.315.gc6ed9934b7-goog

Re: [PATCH net 2/2] ipv4: reserve one IP ID per segment for UDP GSO packets

[Jiayuan Chen]

On 9/29/26 9:12 PM, Eric Dumazet wrote:
> __ip_make_skb() reserves a single IP ID for UDP GSO packets, but GSO
> assigns one IP ID per segment. Following packets then reuse these IDs,
> either from inet->inet_id or from the shared generator.
>
> Unless IP_PMTUDISC_DO/PROBE is used, DF is not set on UDP GSO packets,
> so segments can be fragmented on the path and IP ID reuse can lead to
> incorrect reassembly.
>
> Reserve one IP ID per segment, using the same test as udp_send_skb().
>
> Fixes: bec1f6f69736 ("udp: generate gso with UDP_SEGMENT")
> Signed-off-by: Eric Dumazet <edumazet@kernel.org>


Reviewed-by: Jiayuan Chen <jiayuan.chen@linux.dev>


> ---
>   net/ipv4/ip_output.c | 15 ++++++++++++++-
>   1 file changed, 14 insertions(+), 1 deletion(-)
>
> diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
> index a24cc8ee11d3ea3069bcc0d4d12e6867c2c475f7..b1cf0c6bfc79c8d234cc81ff32332116c1ee3401 100644
> --- a/net/ipv4/ip_output.c
> +++ b/net/ipv4/ip_output.c
> @@ -1410,6 +1410,7 @@ struct sk_buff *__ip_make_skb(struct sock *sk,
>   	struct iphdr *iph;
>   	u8 pmtudisc, ttl;
>   	__be16 df = 0;
> +	int segs;
>   
>   	skb = __skb_dequeue(queue);
>   	if (!skb)
> @@ -1464,7 +1465,19 @@ struct sk_buff *__ip_make_skb(struct sock *sk,
>   	iph->ttl = ttl;
>   	iph->protocol = sk->sk_protocol;
>   	ip_copy_addrs(iph, fl4);
> -	ip_select_ident(net, skb, sk);
> +
> +	/* UDP GSO packets are segmented later (see udp_send_skb()):
> +	 * reserve one IP ID per segment.
> +	 */
> +	segs = 1;
> +	if (cork->gso_size) {
> +		int datalen = skb->len - skb_transport_offset(skb) -
> +			      sizeof(struct udphdr);
> +
> +		if (datalen > cork->gso_size)
> +			segs = DIV_ROUND_UP(datalen, cork->gso_size);
> +	}
> +	ip_select_ident_segs(net, skb, sk, segs);
>   
>   	if (opt) {
>   		iph->ihl += opt->optlen >> 2;

Re: [PATCH net 1/2] ipv4: fix IP ID reuse in ip_select_ident_segs()

[Jiayuan Chen]

On 9/29/26 9:12 PM, Eric Dumazet wrote:
> ip_select_ident_segs() must put the first of the @segs reserved IP IDs
> in iph->id, as GSO assigns id, id + 1, ..., id + segs - 1 to segments.
>
> Commit f866fbc842de ("ipv4: fix data-races around inet->inet_id")
> used atomic_add_return() for non-TCP sockets, which returns the first
> ID of the next packet instead. Consecutive GSO packets can then reuse
> IP IDs.
>
> SCTP GSO is affected. Other callers use segs == 1, and only see
> a harmless off-by-one (UDP GSO has a separate, older issue, see
> following patch in this series).
>
> Use atomic_fetch_add() instead, like the TCP path.
>
> Fixes: f866fbc842de ("ipv4: fix data-races around inet->inet_id")
> Signed-off-by: Eric Dumazet <edumazet@kernel.org>



Reviewed-by: Jiayuan Chen <jiayuan.chen@linux.dev>


> ---
>   include/net/ip.h | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/include/net/ip.h b/include/net/ip.h
> index 6f602df72ee621ee4ee45e70beef0a1b5145367f..6a3e8271a73b3669e97c4b389e916dbcbfa9f6ae 100644
> --- a/include/net/ip.h
> +++ b/include/net/ip.h
> @@ -598,7 +598,7 @@ static inline void ip_select_ident_segs(struct net *net, struct sk_buff *skb,
>   			val = atomic_read(&inet_sk(sk)->inet_id);
>   			atomic_set(&inet_sk(sk)->inet_id, val + segs);
>   		} else {
> -			val = atomic_add_return(segs, &inet_sk(sk)->inet_id);
> +			val = atomic_fetch_add(segs, &inet_sk(sk)->inet_id);
>   		}
>   		iph->id = htons(val);
>   		return;

Re: [PATCH net 2/2] ipv4: reserve one IP ID per segment for UDP GSO packets

[David Ahern]

On 9/29/26 8:12 AM, Eric Dumazet wrote:
> @@ -1464,7 +1465,19 @@ struct sk_buff *__ip_make_skb(struct sock *sk,
>  	iph->ttl = ttl;
>  	iph->protocol = sk->sk_protocol;
>  	ip_copy_addrs(iph, fl4);
> -	ip_select_ident(net, skb, sk);
> +
> +	/* UDP GSO packets are segmented later (see udp_send_skb()):
> +	 * reserve one IP ID per segment.
> +	 */
> +	segs = 1;
> +	if (cork->gso_size) {
> +		int datalen = skb->len - skb_transport_offset(skb) -
> +			      sizeof(struct udphdr);

hard coding a specific transport header in L3 code is crossing layers.


> +
> +		if (datalen > cork->gso_size)
> +			segs = DIV_ROUND_UP(datalen, cork->gso_size);
> +	}
> +	ip_select_ident_segs(net, skb, sk, segs);
>  
>  	if (opt) {
>  		iph->ihl += opt->optlen >> 2;

Re: [PATCH net 1/2] ipv4: fix IP ID reuse in ip_select_ident_segs()

[David Ahern]

On 9/29/26 8:12 AM, Eric Dumazet wrote:
> ip_select_ident_segs() must put the first of the @segs reserved IP IDs
> in iph->id, as GSO assigns id, id + 1, ..., id + segs - 1 to segments.
> 
> Commit f866fbc842de ("ipv4: fix data-races around inet->inet_id")
> used atomic_add_return() for non-TCP sockets, which returns the first
> ID of the next packet instead. Consecutive GSO packets can then reuse
> IP IDs.
> 
> SCTP GSO is affected. Other callers use segs == 1, and only see
> a harmless off-by-one (UDP GSO has a separate, older issue, see
> following patch in this series).
> 
> Use atomic_fetch_add() instead, like the TCP path.
> 
> Fixes: f866fbc842de ("ipv4: fix data-races around inet->inet_id")
> Signed-off-by: Eric Dumazet <edumazet@kernel.org>
> ---
>  include/net/ip.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 

Reviewed-by: David Ahern <dsahern@kernel.org>

Re: [PATCH net 2/2] ipv4: reserve one IP ID per segment for UDP GSO packets

[Eric Dumazet]

On Wed, Sep 30, 2026 at 2:29 PM David Ahern <dsahern@kernel.org> wrote:
>
> On 9/29/26 8:12 AM, Eric Dumazet wrote:
> > @@ -1464,7 +1465,19 @@ struct sk_buff *__ip_make_skb(struct sock *sk,
> >       iph->ttl = ttl;
> >       iph->protocol = sk->sk_protocol;
> >       ip_copy_addrs(iph, fl4);
> > -     ip_select_ident(net, skb, sk);
> > +
> > +     /* UDP GSO packets are segmented later (see udp_send_skb()):
> > +      * reserve one IP ID per segment.
> > +      */
> > +     segs = 1;
> > +     if (cork->gso_size) {
> > +             int datalen = skb->len - skb_transport_offset(skb) -
> > +                           sizeof(struct udphdr);
>
> hard coding a specific transport header in L3 code is crossing layers.
>

Only UDP sets cork->gso_size, so this is UDP specific code in
__ip_make_skb(), like the existing sk_is_tcp() case in the same
function.

I can make this explicit in v2 with:

if (cork->gso_size && sk_is_udp(sk)) {

Selecting the IP ID from udp_send_skb() instead would either burn
one extra IP ID per packet (50 % more for 2-segment GSO packets,
making wraparound faster), or require __ip_make_skb() to leave
iph->id unset for UDP GSO packets, with a simlar kayer violation concern.

Would this be acceptable, or do you have another suggestion?

Re: [PATCH net 2/2] ipv4: reserve one IP ID per segment for UDP GSO packets

[David Ahern]

On 9/30/26 7:58 AM, Eric Dumazet wrote:
> On Wed, Sep 30, 2026 at 2:29 PM David Ahern <dsahern@kernel.org> wrote:
>>
>> On 9/29/26 8:12 AM, Eric Dumazet wrote:
>>> @@ -1464,7 +1465,19 @@ struct sk_buff *__ip_make_skb(struct sock *sk,
>>>       iph->ttl = ttl;
>>>       iph->protocol = sk->sk_protocol;
>>>       ip_copy_addrs(iph, fl4);
>>> -     ip_select_ident(net, skb, sk);
>>> +
>>> +     /* UDP GSO packets are segmented later (see udp_send_skb()):
>>> +      * reserve one IP ID per segment.
>>> +      */
>>> +     segs = 1;
>>> +     if (cork->gso_size) {
>>> +             int datalen = skb->len - skb_transport_offset(skb) -
>>> +                           sizeof(struct udphdr);
>>
>> hard coding a specific transport header in L3 code is crossing layers.
>>
> 
> Only UDP sets cork->gso_size, so this is UDP specific code in
> __ip_make_skb(), like the existing sk_is_tcp() case in the same
> function.

I went looking for other transport references in L3 code and missed this
one.

> 
> I can make this explicit in v2 with:
> 
> if (cork->gso_size && sk_is_udp(sk)) {
> 
> Selecting the IP ID from udp_send_skb() instead would either burn
> one extra IP ID per packet (50 % more for 2-segment GSO packets,
> making wraparound faster), or require __ip_make_skb() to leave
> iph->id unset for UDP GSO packets, with a simlar kayer violation concern.
> 
> Would this be acceptable, or do you have another suggestion?

sure. I went down the path of adding an input arg to __ip_make_skb, but
this is simpler.

Re: [PATCH net 2/2] ipv4: reserve one IP ID per segment for UDP GSO packets

[Eric Dumazet]

On Wed, Sep 30, 2026 at 3:50 PM David Ahern <dsahern@kernel.org> wrote:
>
> On 9/30/26 7:58 AM, Eric Dumazet wrote:
> > On Wed, Sep 30, 2026 at 2:29 PM David Ahern <dsahern@kernel.org> wrote:
> >>
> >> On 9/29/26 8:12 AM, Eric Dumazet wrote:
> >>> @@ -1464,7 +1465,19 @@ struct sk_buff *__ip_make_skb(struct sock *sk,
> >>>       iph->ttl = ttl;
> >>>       iph->protocol = sk->sk_protocol;
> >>>       ip_copy_addrs(iph, fl4);
> >>> -     ip_select_ident(net, skb, sk);
> >>> +
> >>> +     /* UDP GSO packets are segmented later (see udp_send_skb()):
> >>> +      * reserve one IP ID per segment.
> >>> +      */
> >>> +     segs = 1;
> >>> +     if (cork->gso_size) {
> >>> +             int datalen = skb->len - skb_transport_offset(skb) -
> >>> +                           sizeof(struct udphdr);
> >>
> >> hard coding a specific transport header in L3 code is crossing layers.
> >>
> >
> > Only UDP sets cork->gso_size, so this is UDP specific code in
> > __ip_make_skb(), like the existing sk_is_tcp() case in the same
> > function.
>
> I went looking for other transport references in L3 code and missed this
> one.
>
> >
> > I can make this explicit in v2 with:
> >
> > if (cork->gso_size && sk_is_udp(sk)) {
> >
> > Selecting the IP ID from udp_send_skb() instead would either burn
> > one extra IP ID per packet (50 % more for 2-segment GSO packets,
> > making wraparound faster), or require __ip_make_skb() to leave
> > iph->id unset for UDP GSO packets, with a simlar kayer violation concern.
> >
> > Would this be acceptable, or do you have another suggestion?
>
> sure. I went down the path of adding an input arg to __ip_make_skb, but
> this is simpler.

Great, thanks, I will send V2 shortly.

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