Re: [PATCH net] ipv4: Fix uninit-value access in __ip_make_skb()

[Eric Dumazet]

On Sun, Mar 24, 2024 at 6:06 AM Shigeru Yoshida <syoshida@xxxxxxxxxx> wrote:
>
> KMSAN reported uninit-value access in __ip_make_skb() [1].  __ip_make_skb()
> tests HDRINCL to know if the skb has icmphdr. However, HDRINCL can cause a
> race condition. If calling setsockopt(2) with IP_HDRINCL changes HDRINCL
> while __ip_make_skb() is running, the function will access icmphdr in the
> skb even if it is not included. This causes the issue reported by KMSAN.
>
> Check FLOWI_FLAG_KNOWN_NH on fl4->flowi4_flags instead of testing HDRINCL
> on the socket.
>
> [1]

What is the kernel version for this trace ?

> BUG: KMSAN: uninit-value in __ip_make_skb+0x2b74/0x2d20 net/ipv4/ip_output.c:1481
>  __ip_make_skb+0x2b74/0x2d20 net/ipv4/ip_output.c:1481
>  ip_finish_skb include/net/ip.h:243 [inline]
>  ip_push_pending_frames+0x4c/0x5c0 net/ipv4/ip_output.c:1508
>  raw_sendmsg+0x2381/0x2690 net/ipv4/raw.c:654
>  inet_sendmsg+0x27b/0x2a0 net/ipv4/af_inet.c:851
>  sock_sendmsg_nosec net/socket.c:730 [inline]
>  __sock_sendmsg+0x274/0x3c0 net/socket.c:745
>  __sys_sendto+0x62c/0x7b0 net/socket.c:2191
>  __do_sys_sendto net/socket.c:2203 [inline]
>  __se_sys_sendto net/socket.c:2199 [inline]
>  __x64_sys_sendto+0x130/0x200 net/socket.c:2199
>  do_syscall_64+0xd8/0x1f0 arch/x86/entry/common.c:83
>  entry_SYSCALL_64_after_hwframe+0x6d/0x75
>
> Uninit was created at:
>  slab_post_alloc_hook mm/slub.c:3804 [inline]
>  slab_alloc_node mm/slub.c:3845 [inline]
>  kmem_cache_alloc_node+0x5f6/0xc50 mm/slub.c:3888
>  kmalloc_reserve+0x13c/0x4a0 net/core/skbuff.c:577
>  __alloc_skb+0x35a/0x7c0 net/core/skbuff.c:668
>  alloc_skb include/linux/skbuff.h:1318 [inline]
>  __ip_append_data+0x49ab/0x68c0 net/ipv4/ip_output.c:1128
>  ip_append_data+0x1e7/0x260 net/ipv4/ip_output.c:1365
>  raw_sendmsg+0x22b1/0x2690 net/ipv4/raw.c:648
>  inet_sendmsg+0x27b/0x2a0 net/ipv4/af_inet.c:851
>  sock_sendmsg_nosec net/socket.c:730 [inline]
>  __sock_sendmsg+0x274/0x3c0 net/socket.c:745
>  __sys_sendto+0x62c/0x7b0 net/socket.c:2191
>  __do_sys_sendto net/socket.c:2203 [inline]
>  __se_sys_sendto net/socket.c:2199 [inline]
>  __x64_sys_sendto+0x130/0x200 net/socket.c:2199
>  do_syscall_64+0xd8/0x1f0 arch/x86/entry/common.c:83
>  entry_SYSCALL_64_after_hwframe+0x6d/0x75
>
> Fixes: 99e5acae193e ("ipv4: Fix potential uninit variable access bug in __ip_make_skb()")
> Reported-by: syzkaller <syzkaller@xxxxxxxxxxxxxxxx>
> Signed-off-by: Shigeru Yoshida <syoshida@xxxxxxxxxx>
> ---
> I think IPv6 has a similar issue. If this patch is accepted, I will send
> a patch for IPv6.
> ---
>  net/ipv4/ip_output.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
> index 1fe794967211..39229fd0601a 100644
> --- a/net/ipv4/ip_output.c
> +++ b/net/ipv4/ip_output.c
> @@ -1473,7 +1473,7 @@ struct sk_buff *__ip_make_skb(struct sock *sk,
>                  * by icmp_hdr(skb)->type.
>                  */
>                 if (sk->sk_type == SOCK_RAW &&
> -                   !inet_test_bit(HDRINCL, sk))
> +                   !(fl4->flowi4_flags & FLOWI_FLAG_KNOWN_NH))
>                         icmp_type = fl4->fl4_icmp_type;
>                 else
>                         icmp_type = icmp_hdr(skb)->type;
> --
> 2.44.0
>

Thanks for your patch.

I do not think this is enough, as far as syzkaller is concerned.

raw_probe_proto_opt() can leave garbage in fl4_icmp_type (and fl4_icmp_code)