Re: [PATCH 4.19 1/1] net: prevent mss overflow in skb_segment()

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Fri, Aug 30, 2024 at 10:44:54AM +0200, hsimeliere.opensource@witekio.com wrote:
> From: Eric Dumazet <edumazet@google.com>
> 
> commit 23d05d563b7e7b0314e65c8e882bc27eac2da8e7 upstream.
> 
> Once again syzbot is able to crash the kernel in skb_segment() [1]
> 
> GSO_BY_FRAGS is a forbidden value, but unfortunately the following
> computation in skb_segment() can reach it quite easily :
> 
> 	mss = mss * partial_segs;
> 
> 65535 = 3 * 5 * 17 * 257, so many initial values of mss can lead to
> a bad final result.
> 
> Make sure to limit segmentation so that the new mss value is smaller
> than GSO_BY_FRAGS.
> 
> [1]
> 
> general protection fault, probably for non-canonical address 0xdffffc000000000e: 0000 [#1] PREEMPT SMP KASAN
> KASAN: null-ptr-deref in range [0x0000000000000070-0x0000000000000077]
> CPU: 1 PID: 5079 Comm: syz-executor993 Not tainted 6.7.0-rc4-syzkaller-00141-g1ae4cd3cbdd0 #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 11/10/2023
> RIP: 0010:skb_segment+0x181d/0x3f30 net/core/skbuff.c:4551
> Code: 83 e3 02 e9 fb ed ff ff e8 90 68 1c f9 48 8b 84 24 f8 00 00 00 48 8d 78 70 48 b8 00 00 00 00 00 fc ff df 48 89 fa 48 c1 ea 03 <0f> b6 04 02 84 c0 74 08 3c 03 0f 8e 8a 21 00 00 48 8b 84 24 f8 00
> RSP: 0018:ffffc900043473d0 EFLAGS: 00010202
> RAX: dffffc0000000000 RBX: 0000000000010046 RCX: ffffffff886b1597
> RDX: 000000000000000e RSI: ffffffff886b2520 RDI: 0000000000000070
> RBP: ffffc90004347578 R08: 0000000000000005 R09: 000000000000ffff
> R10: 000000000000ffff R11: 0000000000000002 R12: ffff888063202ac0
> R13: 0000000000010000 R14: 000000000000ffff R15: 0000000000000046
> FS: 0000555556e7e380(0000) GS:ffff8880b9900000(0000) knlGS:0000000000000000
> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000000020010000 CR3: 0000000027ee2000 CR4: 00000000003506f0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
> <TASK>
> udp6_ufo_fragment+0xa0e/0xd00 net/ipv6/udp_offload.c:109
> ipv6_gso_segment+0x534/0x17e0 net/ipv6/ip6_offload.c:120
> skb_mac_gso_segment+0x290/0x610 net/core/gso.c:53
> __skb_gso_segment+0x339/0x710 net/core/gso.c:124
> skb_gso_segment include/net/gso.h:83 [inline]
> validate_xmit_skb+0x36c/0xeb0 net/core/dev.c:3626
> __dev_queue_xmit+0x6f3/0x3d60 net/core/dev.c:4338
> dev_queue_xmit include/linux/netdevice.h:3134 [inline]
> packet_xmit+0x257/0x380 net/packet/af_packet.c:276
> packet_snd net/packet/af_packet.c:3087 [inline]
> packet_sendmsg+0x24c6/0x5220 net/packet/af_packet.c:3119
> sock_sendmsg_nosec net/socket.c:730 [inline]
> __sock_sendmsg+0xd5/0x180 net/socket.c:745
> __sys_sendto+0x255/0x340 net/socket.c:2190
> __do_sys_sendto net/socket.c:2202 [inline]
> __se_sys_sendto net/socket.c:2198 [inline]
> __x64_sys_sendto+0xe0/0x1b0 net/socket.c:2198
> do_syscall_x64 arch/x86/entry/common.c:52 [inline]
> do_syscall_64+0x40/0x110 arch/x86/entry/common.c:83
> entry_SYSCALL_64_after_hwframe+0x63/0x6b
> RIP: 0033:0x7f8692032aa9
> Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 d1 19 00 00 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b8 ff ff ff f7 d8 64 89 01 48
> RSP: 002b:00007fff8d685418 EFLAGS: 00000246 ORIG_RAX: 000000000000002c
> RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 00007f8692032aa9
> RDX: 0000000000010048 RSI: 00000000200000c0 RDI: 0000000000000003
> RBP: 00000000000f4240 R08: 0000000020000540 R09: 0000000000000014
> R10: 0000000000000000 R11: 0000000000000246 R12: 00007fff8d685480
> R13: 0000000000000001 R14: 00007fff8d685480 R15: 0000000000000003
> </TASK>
> Modules linked in:
> ---[ end trace 0000000000000000 ]---
> RIP: 0010:skb_segment+0x181d/0x3f30 net/core/skbuff.c:4551
> Code: 83 e3 02 e9 fb ed ff ff e8 90 68 1c f9 48 8b 84 24 f8 00 00 00 48 8d 78 70 48 b8 00 00 00 00 00 fc ff df 48 89 fa 48 c1 ea 03 <0f> b6 04 02 84 c0 74 08 3c 03 0f 8e 8a 21 00 00 48 8b 84 24 f8 00
> RSP: 0018:ffffc900043473d0 EFLAGS: 00010202
> RAX: dffffc0000000000 RBX: 0000000000010046 RCX: ffffffff886b1597
> RDX: 000000000000000e RSI: ffffffff886b2520 RDI: 0000000000000070
> RBP: ffffc90004347578 R08: 0000000000000005 R09: 000000000000ffff
> R10: 000000000000ffff R11: 0000000000000002 R12: ffff888063202ac0
> R13: 0000000000010000 R14: 000000000000ffff R15: 0000000000000046
> FS: 0000555556e7e380(0000) GS:ffff8880b9900000(0000) knlGS:0000000000000000
> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000000020010000 CR3: 0000000027ee2000 CR4: 00000000003506f0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> 
> Fixes: 3953c46c3ac7 ("sk_buff: allow segmenting based on frag sizes")
> Signed-off-by: Eric Dumazet <edumazet@google.com>
> Cc: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
> Reviewed-by: Willem de Bruijn <willemb@google.com>
> Link: https://lore.kernel.org/r/20231212164621.4131800-1-edumazet@google.com
> Signed-off-by: Jakub Kicinski <kuba@kernel.org>
> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Signed-off-by: Hugo SIMELIERE <hsimeliere.opensource@witekio.com>
> ---
>  net/core/skbuff.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index e03cd719b86b..cb336e79e05f 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -3625,8 +3625,9 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
>  		/* GSO partial only requires that we trim off any excess that
>  		 * doesn't fit into an MSS sized block, so take care of that
>  		 * now.
> +		 * Cap len to not accidentally hit GSO_BY_FRAGS.
>  		 */
> -		partial_segs = len / mss;
> +		partial_segs = min(len, GSO_BY_FRAGS - 1U) / mss;
>  		if (partial_segs > 1)
>  			mss *= partial_segs;
>  		else
> -- 

Note, this backport causes a build warning.  I'll go fix it up by hand,
but next time please be more careful.

thanks,

greg k-h