[PATCH 6.1.y] ipv4: Fix uninit-value access in __ip_make_skb()

[PATCH 6.1.y] ipv4: Fix uninit-value access in __ip_make_skb()

[Zhaoyang Li]

From: Shigeru Yoshida <syoshida@redhat.com>

[ Upstream commit fc1092f51567277509563800a3c56732070b6aa4 ]

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@googlegroups.com>
Signed-off-by: Shigeru Yoshida <syoshida@redhat.com>
Link: https://lore.kernel.org/r/20240430123945.2057348-1-syoshida@redhat.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Zhaoyang Li <lizy04@hust.edu.cn>
---
 net/ipv4/ip_output.c | 3 ++-
 net/ipv4/raw.c       | 3 +++
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
index c82107bbd981..543d029102cf 100644
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -1580,7 +1580,8 @@ struct sk_buff *__ip_make_skb(struct sock *sk,
 		 * so icmphdr does not in skb linear region and can not get icmp_type
 		 * by icmp_hdr(skb)->type.
 		 */
-		if (sk->sk_type == SOCK_RAW && !inet_sk(sk)->hdrincl)
+		if (sk->sk_type == SOCK_RAW &&
+		    !(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 ee0efd0efec4..c109bc376cc5 100644
--- a/net/ipv4/raw.c
+++ b/net/ipv4/raw.c
@@ -608,6 +608,9 @@ 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;
-- 
2.25.1

Re: [PATCH 6.1.y] ipv4: Fix uninit-value access in __ip_make_skb()

[Sasha Levin]

[ Sasha's backport helper bot ]

Hi,

✅ All tests passed successfully. No issues detected.
No action required from the submitter.

The upstream commit SHA1 provided is correct: fc1092f51567277509563800a3c56732070b6aa4

WARNING: Author mismatch between patch and upstream commit:
Backport author: Zhaoyang Li<lizy04@hust.edu.cn>
Commit author: Shigeru Yoshida<syoshida@redhat.com>

Status in newer kernel trees:
6.14.y | Present (exact SHA1)
6.12.y | Present (exact SHA1)
6.6.y | Present (different SHA1: 5db08343ddb1)

Note: The patch differs from the upstream commit:
---
1:  fc1092f515672 ! 1:  d14a3911b2aab ipv4: Fix uninit-value access in __ip_make_skb()
    @@ Metadata
      ## Commit message ##
         ipv4: Fix uninit-value access in __ip_make_skb()
     
    +    [ Upstream commit fc1092f51567277509563800a3c56732070b6aa4 ]
    +
         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
    @@ Commit message
         Signed-off-by: Shigeru Yoshida <syoshida@redhat.com>
         Link: https://lore.kernel.org/r/20240430123945.2057348-1-syoshida@redhat.com
         Signed-off-by: Paolo Abeni <pabeni@redhat.com>
    +    Signed-off-by: Zhaoyang Li <lizy04@hust.edu.cn>
     
      ## net/ipv4/ip_output.c ##
     @@ net/ipv4/ip_output.c: struct sk_buff *__ip_make_skb(struct sock *sk,
    + 		 * so icmphdr does not in skb linear region and can not get icmp_type
      		 * by icmp_hdr(skb)->type.
      		 */
    - 		if (sk->sk_type == SOCK_RAW &&
    --		    !inet_test_bit(HDRINCL, sk))
    +-		if (sk->sk_type == SOCK_RAW && !inet_sk(sk)->hdrincl)
    ++		if (sk->sk_type == SOCK_RAW &&
     +		    !(fl4->flowi4_flags & FLOWI_FLAG_KNOWN_NH))
      			icmp_type = fl4->fl4_icmp_type;
      		else
---

Results of testing on various branches:

| Branch                    | Patch Apply | Build Test |
|---------------------------|-------------|------------|
| stable/linux-6.1.y        |  Success    |  Success   |