[PATCH ipsec] xfrm: xfrm_alloc_spi shouldn't use 0 as SPI

[PATCH ipsec] xfrm: xfrm_alloc_spi shouldn't use 0 as SPI

[Sabrina Dubroca]

x->id.spi == 0 means "no SPI assigned", but since commit
94f39804d891 ("xfrm: Duplicate SPI Handling"), we now create states
and add them to the byspi list with this value.

__xfrm_state_delete doesn't remove those states from the byspi list,
since they shouldn't be there, and this shows up as a UAF the next
time we go through the byspi list.

Reported-by: syzbot+a25ee9d20d31e483ba7b@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=a25ee9d20d31e483ba7b
Fixes: 94f39804d891 ("xfrm: Duplicate SPI Handling")
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
---
 net/xfrm/xfrm_state.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/xfrm/xfrm_state.c b/net/xfrm/xfrm_state.c
index 78fcbb89cf32..d213ca3653a8 100644
--- a/net/xfrm/xfrm_state.c
+++ b/net/xfrm/xfrm_state.c
@@ -2583,6 +2583,8 @@ int xfrm_alloc_spi(struct xfrm_state *x, u32 low, u32 high,
 
 	for (h = 0; h < range; h++) {
 		u32 spi = (low == high) ? low : get_random_u32_inclusive(low, high);
+		if (spi == 0)
+			goto next;
 		newspi = htonl(spi);
 
 		spin_lock_bh(&net->xfrm.xfrm_state_lock);
@@ -2598,6 +2600,7 @@ int xfrm_alloc_spi(struct xfrm_state *x, u32 low, u32 high,
 		xfrm_state_put(x0);
 		spin_unlock_bh(&net->xfrm.xfrm_state_lock);
 
+next:
 		if (signal_pending(current)) {
 			err = -ERESTARTSYS;
 			goto unlock;
-- 
2.51.0

Re: [PATCH ipsec] xfrm: xfrm_alloc_spi shouldn't use 0 as SPI

[Simon Horman]

On Fri, Aug 29, 2025 at 10:54:15AM +0200, Sabrina Dubroca wrote:
> x->id.spi == 0 means "no SPI assigned", but since commit
> 94f39804d891 ("xfrm: Duplicate SPI Handling"), we now create states
> and add them to the byspi list with this value.
> 
> __xfrm_state_delete doesn't remove those states from the byspi list,
> since they shouldn't be there, and this shows up as a UAF the next
> time we go through the byspi list.
> 
> Reported-by: syzbot+a25ee9d20d31e483ba7b@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=a25ee9d20d31e483ba7b
> Fixes: 94f39804d891 ("xfrm: Duplicate SPI Handling")
> Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>

Thanks, I see that prior to the cited commit an error would
be returned if newspi was 0. Where newspi was
assigned the value of get_random_u32_inclusive(low, high).

Reviewed-by: Simon Horman <horms@kernel.org>

...

Re: [PATCH ipsec] xfrm: xfrm_alloc_spi shouldn't use 0 as SPI

[Steffen Klassert]

On Fri, Aug 29, 2025 at 10:54:15AM +0200, Sabrina Dubroca wrote:
> x->id.spi == 0 means "no SPI assigned", but since commit
> 94f39804d891 ("xfrm: Duplicate SPI Handling"), we now create states
> and add them to the byspi list with this value.
> 
> __xfrm_state_delete doesn't remove those states from the byspi list,
> since they shouldn't be there, and this shows up as a UAF the next
> time we go through the byspi list.
> 
> Reported-by: syzbot+a25ee9d20d31e483ba7b@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=a25ee9d20d31e483ba7b
> Fixes: 94f39804d891 ("xfrm: Duplicate SPI Handling")
> Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>

Applied, thanks a lot Sabrina!