Re: [PATCH net] seg6: keep room for the mac header when growing the headroom

[Justin Iurman]

On 9/17/26 18:28, Andrea Mayer wrote:
> On Thu, 17 Sep 2026 12:12:11 +0200
> Justin Iurman <justin.iurman@xxxxxxxxx> wrote:
>
> > On 9/16/26 23:38, Yuya Kusakabe wrote:
> > > [snip]
> >
> > Overall, LGTM, thanks. However, I think we'd need a v2 with the followings:
> >
> > - use max_t(unsigned int, skb->mac_len, dst_dev_overhead(cache_dst,
> > skb)) instead of max()
> > - apply the same changes to ioam6_iptunnel and rpl_iptunnel (all in one
> > patch is fine)
> >
> > Reviewed-by: Justin Iurman <justin.iurman@xxxxxxxxx>
>
> Hi Justin,
>
> Agreed, rpl and ioam6 inline do trigger. Single VLAN device per side,
> reorder_hdr off on the receiving one, plain ping:
>
>    BUG: KASAN: slab-out-of-bounds in rpl_do_srh_inline.isra.0+0x3d3/0x770
>    Write of size 18 at addr ffff88810deeba7e by task ping/447
>
>    CPU: 0 UID: 0 PID: 447 Comm: ping Not tainted 7.3.0-rc1 #364
>    Call Trace:
>     <IRQ>
>     __asan_memmove+0x38/0x60
>     rpl_do_srh_inline.isra.0+0x3d3/0x770
>     rpl_input+0xd3/0x5e0
>     lwtunnel_input+0x18d/0x420
>     ipv6_rcv+0x452/0x460
>
>    BUG: KASAN: slab-use-after-free in ioam6_do_inline+0x2d8/0x5e0
>    Write of size 18 at addr ffff88811480fa7e by task ping/432
>
>    CPU: 0 UID: 0 PID: 432 Comm: ping Not tainted 7.3.0-rc1 #364
>    Call Trace:
>     <IRQ>
>     __asan_memmove+0x38/0x60
>     ioam6_do_inline+0x2d8/0x5e0
>     ioam6_output+0x335/0x970
>     lwtunnel_output+0x1b0/0x440
>     ip6_forward+0x16a7/0x16f0
>     ipv6_rcv+0x452/0x460
>
> ioam6_do_encap triggers too, with three VLAN tags via tc push:
>
>    BUG: KASAN: use-after-free in ioam6_do_encap+0x202/0x5c0
>    Write of size 26 at addr ffff88810de227fe by task ping/453
>
>    CPU: 0 UID: 0 PID: 453 Comm: ping Not tainted 7.3.0-rc1 #364
>    Call Trace:
>     <IRQ>
>     __asan_memmove+0x38/0x60
>     ioam6_do_encap+0x202/0x5c0
>     ioam6_output+0x3cc/0x970
>     lwtunnel_output+0x1b0/0x440
>     ip6_forward+0x16a7/0x16f0
>     ipv6_rcv+0x452/0x460
>
> I would fix dst_dev_overhead() itself rather than patching every
> caller individually, that covers all callers at once and protects
> any future user of the helper. dst_dev_overhead() already returns
> skb->mac_len when dst is NULL, the fix would make the other branch
> consistent:
>
> --- a/include/net/dst.h
> +++ b/include/net/dst.h
> @@ -455,7 +455,8 @@ static inline unsigned int dst_dev_overhead(struct dst_entry *dst,
>   					    struct sk_buff *skb)
>   {
>   	if (likely(dst))
> -		return LL_RESERVED_SPACE(dst->dev);
> +		return max_t(unsigned int, skb->mac_len,
> +			     LL_RESERVED_SPACE(dst->dev));
>
>   	return skb->mac_len;
>   }

+1. That's even better, thanks!