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

[Eric Dumazet]

On Mon, Mar 25, 2024 at 6:46 PM Shigeru Yoshida <syoshida@xxxxxxxxxx> wrote:
>
> On Mon, 25 Mar 2024 11:05:33 +0100, Eric Dumazet wrote:
> > On Mon, Mar 25, 2024 at 10:38 AM Shigeru Yoshida <syoshida@xxxxxxxxxx> wrote:
> >>
> >> 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?
> >
> > There are multiple ways to return early from this function.
> >
> > In all of them, fl4->fl4_icmp_type is left uninitialized, so syzbot
> > will find ways to trigger a related bug,
> > if you assume later that fl4->fl4_icmp_type contains valid (initialized) data.
>
> Thank you for your reply. I see your point.
>
> fl4->fl4_icmp_type is part of flowi_uli union in flowi4 structure, and
> flowi4_init_output() initializes fl4_dport and fl4_sport to zero.
>
> I thought this also initializes fl4_icmp_type and fl4_icmp_code. Do
> you think we should initialize fl4_icmp_type and fl4_icmp_code
> explicitly, otherwise am I misunderstanding?

Yes, I am precisely saying this : do not rely on some union layout,
without mentioning it in the changelog
or even better in a comment.

If you want to avoid clearing these fields, please add a
BUILD_BUG_ON() to make sure
a unrelated future change in include/net/flow.h does not break a
hidden assumption.

(ie : clearing fl4_dport and fl4_sport also clears fl4_icmp_type and
fl4_icmp_code.)