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

[David Laight <David.Laight@ACULAB.COM>]

From: Tadeusz Struk
> Sent: 08 March 2022 00:02
> 
> 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().
> 
> 
> Reported-by: syzbot+e223cf47ec8ae183f2a0@syzkaller.appspotmail.com
> Signed-off-by: Tadeusz Struk <tadeusz.struk@linaro.org>
> ---
>  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;
> +			}
> +

Except that is a valid case, see a few lines higher:

				alloclen = min_t(int, fraglen, MAX_HEADER);
				pagedlen = fraglen - alloclen;

You need to report the input values that cause the problem later on.

	David

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