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

[Shigeru Yoshida]

On Mon, 25 Mar 2024 10:01:25 +0100, Eric Dumazet wrote:
> 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 ?

Sorry, I used the following version:

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

>> 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)

Thank you for your comment. But I don't understand it clearly. What
exactly do you mean by "garbage"?

raw_probe_proto_opt() immediately returns 0 if fl4->flowi4_proto is
not IPPROTO_ICMP:

static int raw_probe_proto_opt(struct raw_frag_vec *rfv, struct flowi4 *fl4)
{
	int err;

	if (fl4->flowi4_proto != IPPROTO_ICMP)
		return 0;

In this case, the function doesn't set fl4_icmp_type. Do you mean this
case?

Thanks,
Shigeru