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Even if we send an IPv6 packet without options, MAX_HEADER might not be enough to account for the additional headroom required by alignment of hardware headers. On a configuration without HYPERV_NET, WLAN, AX25, and with IPV6_TUNNEL, sending short SCTP packets over IPv4 over L2TP over IPv6, we start with 100 bytes of allocated headroom in sctp_packet_transmit(), end up with 54 bytes after l2tp_xmit_skb(), and 14 bytes in ip6_finish_output2(). Those would be enough to append our 14 bytes header, but we're going to align that to 16 bytes, and write 2 bytes out of the allocated slab in neigh_hh_output(). KASan says: [ 264.967848] ================================================================== [ 264.967861] BUG: KASAN: slab-out-of-bounds in ip6_finish_output2+0x1aec/0x1c70 [ 264.967866] Write of size 16 at addr 000000006af1c7fe by task netperf/6201 [ 264.967870] [ 264.967876] CPU: 0 PID: 6201 Comm: netperf Not tainted 4.20.0-rc4+ #1 [ 264.967881] Hardware name: IBM 2827 H43 400 (z/VM 6.4.0) [ 264.967887] Call Trace: [ 264.967896] ([<00000000001347d6>] show_stack+0x56/0xa0) [ 264.967903] [<00000000017e379c>] dump_stack+0x23c/0x290 [ 264.967912] [<00000000007bc594>] print_address_description+0xf4/0x290 [ 264.967919] [<00000000007bc8fc>] kasan_report+0x13c/0x240 [ 264.967927] [<000000000162f5e4>] ip6_finish_output2+0x1aec/0x1c70 [ 264.967935] [<000000000163f890>] ip6_finish_output+0x430/0x7f0 [ 264.967943] [<000000000163fe44>] ip6_output+0x1f4/0x580 [ 264.967953] [<000000000163882a>] ip6_xmit+0xfea/0x1ce8 [ 264.967963] [<00000000017396e2>] inet6_csk_xmit+0x282/0x3f8 [ 264.968033] [<000003ff805fb0ba>] l2tp_xmit_skb+0xe02/0x13e0 [l2tp_core] [ 264.968037] [<000003ff80631192>] l2tp_eth_dev_xmit+0xda/0x150 [l2tp_eth] [ 264.968041] [<0000000001220020>] dev_hard_start_xmit+0x268/0x928 [ 264.968069] [<0000000001330e8e>] sch_direct_xmit+0x7ae/0x1350 [ 264.968071] [<000000000122359c>] __dev_queue_xmit+0x2b7c/0x3478 [ 264.968075] [<00000000013d2862>] ip_finish_output2+0xce2/0x11a0 [ 264.968078] [<00000000013d9b14>] ip_finish_output+0x56c/0x8c8 [ 264.968081] [<00000000013ddd1e>] ip_output+0x226/0x4c0 [ 264.968083] [<00000000013dbd6c>] __ip_queue_xmit+0x894/0x1938 [ 264.968100] [<000003ff80bc3a5c>] sctp_packet_transmit+0x29d4/0x3648 [sctp] [ 264.968116] [<000003ff80b7bf68>] sctp_outq_flush_ctrl.constprop.5+0x8d0/0xe50 [sctp] [ 264.968131] [<000003ff80b7c716>] sctp_outq_flush+0x22e/0x7d8 [sctp] [ 264.968146] [<000003ff80b35c68>] sctp_cmd_interpreter.isra.16+0x530/0x6800 [sctp] [ 264.968161] [<000003ff80b3410a>] sctp_do_sm+0x222/0x648 [sctp] [ 264.968177] [<000003ff80bbddac>] sctp_primitive_ASSOCIATE+0xbc/0xf8 [sctp] [ 264.968192] [<000003ff80b93328>] __sctp_connect+0x830/0xc20 [sctp] [ 264.968208] [<000003ff80bb11ce>] sctp_inet_connect+0x2e6/0x378 [sctp] [ 264.968212] [<0000000001197942>] __sys_connect+0x21a/0x450 [ 264.968215] [<000000000119aff8>] sys_socketcall+0x3d0/0xb08 [ 264.968218] [<000000000184ea7a>] system_call+0x2a2/0x2c0 [...] Just like ip_finish_output2() does for IPv4, check that we have enough headroom in ip6_xmit(), and reallocate it if we don't. This issue is older than git history. Reported-by: Jianlin Shi <jishi@redhat.com> Signed-off-by: Stefano Brivio <sbrivio@redhat.com> Signed-off-by: David S. Miller <davem@davemloft.net>
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