[PATCH] net: ipv6: fix invalid alloclen in __ip6_append_data

[Tadeusz Struk]

Syzbot found a kernel bug in the ipv6 stack:
LINK: https://syzkaller.appspot.com/bug?id=205d6f11d72329ab8d62a610c44c5e7e25415580

The reproducer triggers it by sending an invalid message via sendmmsg() call,
which triggers skb_over_panic, and crashes the kernel:

skbuff: skb_over_panic: text:ffffffff84647fb4 len:65575 put:65575
head:ffff888109ff0000 data:ffff888109ff0088 tail:0x100af end:0xfec0
dev:<NULL>

------------[ cut here ]------------
kernel BUG at net/core/skbuff.c:113!
PREEMPT SMP KASAN
CPU: 1 PID: 1818 Comm: repro Not tainted 5.17.0-rc7-dirty #9
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1.fc35
RIP: 0010:skb_panic+0x173/0x175
RSP: 0018:ffffc900015bf3b8 EFLAGS: 00010282
RAX: 0000000000000090 RBX: ffff88810e848c80 RCX: 0000000000000000
RDX: ffff88810fd84300 RSI: ffffffff814fa5ef RDI: fffff520002b7e69
RBP: ffffc900015bf420 R08: 0000000000000090 R09: 0000000000000000
R10: ffffffff814f55f4 R11: 203a666675626b73 R12: ffffffff855bff80
R13: ffffffff84647fb4 R14: 0000000000010027 R15: ffffffff855bf420
FS:  0000000000c8b3c0(0000) GS:ffff88811b100000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000020000040 CR3: 0000000106b68000 CR4: 0000000000150ea0
Call Trace:
 <TASK>
 skb_put.cold+0x23/0x23
 __ip6_append_data.isra.0.cold+0x396/0xe3a
 ip6_append_data+0x1e5/0x320
 rawv6_sendmsg.cold+0x1618/0x2ba9
 inet_sendmsg+0x9e/0xe0
 sock_sendmsg+0xd7/0x130
 ____sys_sendmsg+0x381/0x8a0
 ___sys_sendmsg+0x100/0x170
 __sys_sendmmsg+0x26c/0x3b7
 __x64_sys_sendmmsg+0xb2/0xbd
 do_syscall_64+0x35/0xb0
 entry_SYSCALL_64_after_hwframe+0x44/0xae

The reproducer can be found here:
LINK: https://syzkaller.appspot.com/text?tag=ReproC&x=1648c83fb00000
This can be fixed by increasing the alloclen in case it is smaller than
fraglen in __ip6_append_data().

Cc: David S. Miller <davem@xxxxxxxxxxxxx>
Cc: Hideaki YOSHIFUJI <yoshfuji@xxxxxxxxxxxxxx>
Cc: David Ahern <dsahern@xxxxxxxxxx>
Cc: Jakub Kicinski <kuba@xxxxxxxxxx>
Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
Cc: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
Cc: Andrii Nakryiko <andrii@xxxxxxxxxx>
Cc: Martin KaFai Lau <kafai@xxxxxx>
Cc: Song Liu <songliubraving@xxxxxx>
Cc: Yonghong Song <yhs@xxxxxx>
Cc: John Fastabend <john.fastabend@xxxxxxxxx>
Cc: KP Singh <kpsingh@xxxxxxxxxx>
Cc: netdev@xxxxxxxxxxxxxxx
Cc: bpf@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: stable@xxxxxxxxxxxxxxx

Reported-by: syzbot+e223cf47ec8ae183f2a0@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Tadeusz Struk <tadeusz.struk@xxxxxxxxxx>
---
 net/ipv6/ip6_output.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index 4788f6b37053..622345af323e 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -1629,6 +1629,13 @@ static int __ip6_append_data(struct sock *sk,
 				err = -EINVAL;
 				goto error;
 			}
+			if (unlikely(alloclen < fraglen)) {
+				if (printk_ratelimit())
+					pr_warn("%s: wrong alloclen: %d, fraglen: %d",
+						__func__, alloclen, fraglen);
+				alloclen = fraglen;
+			}
+
 			if (transhdrlen) {
 				skb = sock_alloc_send_skb(sk, alloclen,
 						(flags & MSG_DONTWAIT), &err);
-- 
2.35.1