Syzbot found a kernel bug in the ipv6 stack: LINK: https://syzkaller.appspot.com/bug?id=205d6f11d72329ab8d62a610c44c5e7e2541558... The reproducer triggers it by sending a crafted 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>
Update the check that prevents an invalid packet with MTU equall to the fregment header size to eat up all the space for payload.
The reproducer can be found here: LINK: https://syzkaller.appspot.com/text?tag=ReproC&x=1648c83fb00000
Cc: Willem de Bruijn willemdebruijn.kernel@gmail.com Cc: David S. Miller davem@davemloft.net Cc: Hideaki YOSHIFUJI yoshfuji@linux-ipv6.org Cc: David Ahern dsahern@kernel.org Cc: Jakub Kicinski kuba@kernel.org Cc: Alexei Starovoitov ast@kernel.org Cc: Daniel Borkmann daniel@iogearbox.net Cc: Andrii Nakryiko andrii@kernel.org Cc: Martin KaFai Lau kafai@fb.com Cc: Song Liu songliubraving@fb.com Cc: Yonghong Song yhs@fb.com Cc: John Fastabend john.fastabend@gmail.com Cc: KP Singh kpsingh@kernel.org Cc: netdev@vger.kernel.org Cc: bpf@vger.kernel.org Cc: linux-kernel@vger.kernel.org Cc: stable@vger.kernel.org
Reported-by: syzbot+e223cf47ec8ae183f2a0@syzkaller.appspotmail.com Signed-off-by: Tadeusz Struk tadeusz.struk@linaro.org --- v2: Instead of updating the alloclen add a check that prevents an invalid packet with MTU equall to the fregment header size to eat up all the space for payload. Fix suggested by Willem de Bruijn willemdebruijn.kernel@gmail.com
v3: Update existing check outside of the while loop. --- net/ipv6/ip6_output.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c index 4788f6b37053..194832663d85 100644 --- a/net/ipv6/ip6_output.c +++ b/net/ipv6/ip6_output.c @@ -1476,8 +1476,8 @@ static int __ip6_append_data(struct sock *sk, sizeof(struct frag_hdr) : 0) + rt->rt6i_nfheader_len;
- if (mtu < fragheaderlen || - ((mtu - fragheaderlen) & ~7) + fragheaderlen < sizeof(struct frag_hdr)) + if (mtu <= fragheaderlen || + ((mtu - fragheaderlen) & ~7) + fragheaderlen <= sizeof(struct frag_hdr)) goto emsgsize;
maxfraglen = ((mtu - fragheaderlen) & ~7) + fragheaderlen -