Re: [PATCH net v2] net/hsr: fix NULL pointer dereference in skb_clone with hw tag insertion

[Felix Maurer <fmaurer@redhat.com>]

On Thu, Nov 27, 2025 at 10:02:19PM +0530, Shaurya Rane wrote:
> When NETIF_F_HW_HSR_TAG_INS is enabled and frame->skb_std is NULL,
> hsr_create_tagged_frame() and prp_create_tagged_frame() call skb_clone()
> with a NULL pointer.

Have you acually tested this or do you have any other indication that
this can happen? After all, you are suggesting that this kernel crash in
a syzbot VM is caused by a (very uncommon) feature of hardware NICs.

> Similarly, prp_get_untagged_frame() doesn't check
> if __pskb_copy() fails before calling skb_clone().

I suspect that this is really the only condition that can trigger the
crash in question. This would also match that the syzbot reproducer hits
this with a PRP interface (IFLA_HSR_PROTOCOL=0x1).

> This causes a kernel crash reported by Syzbot:
>
> Oops: general protection fault, probably for non-canonical address 0xdffffc000000000f: 0000 [#1] SMP KASAN NOPTI
> KASAN: null-ptr-deref in range [0x0000000000000078-0x000000000000007f]
> CPU: 0 UID: 0 PID: 5625 Comm: syz.1.18 Not tainted syzkaller #0 PREEMPT(full)
> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014
> RIP: 0010:skb_clone+0xd7/0x3a0 net/core/skbuff.c:2041
> Code: 03 42 80 3c 20 00 74 08 4c 89 f7 e8 23 29 05 f9 49 83 3e 00 0f 85 a0 01 00 00 e8 94 dd 9d f8 48 8d 6b 7e 49 89 ee 49 c1 ee 03 <43> 0f b6 04 26 84 c0 0f 85 d1 01 00 00 44 0f b6 7d 00 41 83 e7 0c
> RSP: 0018:ffffc9000d00f200 EFLAGS: 00010207
> RAX: ffffffff892235a1 RBX: 0000000000000000 RCX: ffff88803372a480
> RDX: 0000000000000000 RSI: 0000000000000820 RDI: 0000000000000000
> RBP: 000000000000007e R08: ffffffff8f7d0f77 R09: 1ffffffff1efa1ee
> R10: dffffc0000000000 R11: fffffbfff1efa1ef R12: dffffc0000000000
> R13: 0000000000000820 R14: 000000000000000f R15: ffff88805144cc00
> FS:  0000555557f6d500(0000) GS:ffff88808d72f000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000555581d35808 CR3: 000000005040e000 CR4: 0000000000352ef0
> Call Trace:
>  <TASK>
>  hsr_forward_do net/hsr/hsr_forward.c:-1 [inline]
>  hsr_forward_skb+0x1013/0x2860 net/hsr/hsr_forward.c:741
>  hsr_handle_frame+0x6ce/0xa70 net/hsr/hsr_slave.c:84
>  __netif_receive_skb_core+0x10b9/0x4380 net/core/dev.c:5966
>  __netif_receive_skb_one_core net/core/dev.c:6077 [inline]
>  __netif_receive_skb+0x72/0x380 net/core/dev.c:6192
>  netif_receive_skb_internal net/core/dev.c:6278 [inline]
>  netif_receive_skb+0x1cb/0x790 net/core/dev.c:6337
>  tun_rx_batched+0x1b9/0x730 drivers/net/tun.c:1485
>  tun_get_user+0x2b65/0x3e90 drivers/net/tun.c:1953
>  tun_chr_write_iter+0x113/0x200 drivers/net/tun.c:1999
>  new_sync_write fs/read_write.c:593 [inline]
>  vfs_write+0x5c9/0xb30 fs/read_write.c:686
>  ksys_write+0x145/0x250 fs/read_write.c:738
>  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>  do_syscall_64+0xfa/0xfa0 arch/x86/entry/syscall_64.c:94
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> RIP: 0033:0x7f0449f8e1ff
> Code: 89 54 24 18 48 89 74 24 10 89 7c 24 08 e8 f9 92 02 00 48 8b 54 24 18 48 8b 74 24 10 41 89 c0 8b 7c 24 08 b8 01 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 31 44 89 c7 48 89 44 24 08 e8 4c 93 02 00 48
> RSP: 002b:00007ffd7ad94c90 EFLAGS: 00000293 ORIG_RAX: 0000000000000001
> RAX: ffffffffffffffda RBX: 00007f044a1e5fa0 RCX: 00007f0449f8e1ff
> RDX: 000000000000003e RSI: 0000200000000500 RDI: 00000000000000c8
> RBP: 00007ffd7ad94d20 R08: 0000000000000000 R09: 0000000000000000
> R10: 000000000000003e R11: 0000000000000293 R12: 0000000000000001
> R13: 00007f044a1e5fa0 R14: 00007f044a1e5fa0 R15: 0000000000000003
>  </TASK>
>
> Fix this by adding NULL checks for frame->skb_std before calling
> skb_clone() in the affected functions.
>
> Reported-by: syzbot+2fa344348a579b779e05@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=2fa344348a579b779e05
> Fixes: f266a683a480 ("net/hsr: Better frame dispatch")
> Cc: stable@vger.kernel.org
> Signed-off-by: Shaurya Rane <ssrane_b23@ee.vjti.ac.in>
> ---
>  net/hsr/hsr_forward.c | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/net/hsr/hsr_forward.c b/net/hsr/hsr_forward.c
> index 339f0d220212..8a8559f0880f 100644
> --- a/net/hsr/hsr_forward.c
> +++ b/net/hsr/hsr_forward.c
> @@ -205,6 +205,8 @@ struct sk_buff *prp_get_untagged_frame(struct hsr_frame_info *frame,
>  				__pskb_copy(frame->skb_prp,
>  					    skb_headroom(frame->skb_prp),
>  					    GFP_ATOMIC);
> +			if (!frame->skb_std)
> +				return NULL;
>  		} else {
>  			/* Unexpected */
>  			WARN_ONCE(1, "%s:%d: Unexpected frame received (port_src %s)\n",

This check looks good to me.

> @@ -341,6 +343,8 @@ struct sk_buff *hsr_create_tagged_frame(struct hsr_frame_info *frame,
>  		hsr_set_path_id(frame, hsr_ethhdr, port);
>  		return skb_clone(frame->skb_hsr, GFP_ATOMIC);
>  	} else if (port->dev->features & NETIF_F_HW_HSR_TAG_INS) {
> +		if (!frame->skb_std)
> +			return NULL;
>  		return skb_clone(frame->skb_std, GFP_ATOMIC);
>  	}
>
> @@ -385,6 +389,8 @@ struct sk_buff *prp_create_tagged_frame(struct hsr_frame_info *frame,
>  		}
>  		return skb_clone(frame->skb_prp, GFP_ATOMIC);
>  	} else if (port->dev->features & NETIF_F_HW_HSR_TAG_INS) {
> +		if (!frame->skb_std)
> +			return NULL;
>  		return skb_clone(frame->skb_std, GFP_ATOMIC);
>  	}

If any of these two conditions happen we have a different, serious
problem that needs to be fixed elsewhere.

In hsr_create_tagged_frame(), we first check if we have an skb_hsr (an
skb containing an already tagged message). If we don't, it has to be an
skb_std (an skb without any tag). If !skb_std, we are either 1) handing
around a frame without any skb; or 2) handing a PRP frame to an HSR
function. In both cases, this would need to be fixed where the problem
is introduced.

Thanks,
   Felix