linux-kernel - Re: [PATCH net] ipv4: Fix uninit-value access in __ip_make

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Date: Tue, 30 Apr 2024 21:38:40 +0900 (JST)
From: Shigeru Yoshida <syoshida@...hat.com>
To: davem@...emloft.net, dsahern@...nel.org, edumazet@...gle.com,
 kuba@...nel.org, pabeni@...hat.com
Cc: netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
 syoshida@...hat.com, syzkaller@...glegroups.com
Subject: Re: [PATCH net] ipv4: Fix uninit-value access in __ip_make_skb()

Sorry, I forgot to add "v2" to the prefix. I will repost this.

On Tue, 30 Apr 2024 00:16:05 +0900, Shigeru Yoshida 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.
> 
> Also, fl4->fl4_icmp_type and fl4->fl4_icmp_code are not initialized. These
> are union in struct flowi4 and are implicitly initialized by
> flowi4_init_output(), but we should not rely on specific union layout.
> 
> Initialize these explicitly in raw_sendmsg().
> 
> [1]
> 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
> 
> CPU: 1 PID: 15709 Comm: syz-executor.7 Not tainted 6.8.0-11567-gb3603fcb79b1 #25
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-1.fc39 04/01/2014
> 
> Fixes: 99e5acae193e ("ipv4: Fix potential uninit variable access bug in __ip_make_skb()")
> Reported-by: syzkaller <syzkaller@...glegroups.com>
> Signed-off-by: Shigeru Yoshida <syoshida@...hat.com>
> ---
> v1->v2: https://lore.kernel.org/all/20240324050554.1609460-1-syoshida@redhat.com/
> - Explicitly initialize fl4->fl4_icmp_type and fl4->fl4_icmp_code because
>   we should not rely on a specific union layout.
> ---
>  net/ipv4/ip_output.c | 2 +-
>  net/ipv4/raw.c       | 3 +++
>  2 files changed, 4 insertions(+), 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;
> diff --git a/net/ipv4/raw.c b/net/ipv4/raw.c
> index dcb11f22cbf2..4cb43401e0e0 100644
> --- a/net/ipv4/raw.c
> +++ b/net/ipv4/raw.c
> @@ -612,6 +612,9 @@ static int raw_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
>  			    (hdrincl ? FLOWI_FLAG_KNOWN_NH : 0),
>  			   daddr, saddr, 0, 0, sk->sk_uid);
>  
> +	fl4.fl4_icmp_type = 0;
> +	fl4.fl4_icmp_code = 0;
> +
>  	if (!hdrincl) {
>  		rfv.msg = msg;
>  		rfv.hlen = 0;
> -- 
> 2.44.0
>