6.6-stable review patch. If anyone has any objections, please let me know.
------------------
From: Jakub Acs acsjakub@amazon.de
commit 7af76e9d18a9fd6f8611b3313c86c190f9b6a5a7 upstream.
Receiving HSR frame with insufficient space to hold HSR tag in the skb can result in a crash (kernel BUG):
[ 45.390915] skbuff: skb_under_panic: text:ffffffff86f32cac len:26 put:14 head:ffff888042418000 data:ffff888042417ff4 tail:0xe end:0x180 dev:bridge_slave_1 [ 45.392559] ------------[ cut here ]------------ [ 45.392912] kernel BUG at net/core/skbuff.c:211! [ 45.393276] Oops: invalid opcode: 0000 [#1] SMP DEBUG_PAGEALLOC KASAN NOPTI [ 45.393809] CPU: 1 UID: 0 PID: 2496 Comm: reproducer Not tainted 6.15.0 #12 PREEMPT(undef) [ 45.394433] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org 04/01/2014 [ 45.395273] RIP: 0010:skb_panic+0x15b/0x1d0
<snip registers, remove unreliable trace>
[ 45.402911] Call Trace: [ 45.403105] <IRQ> [ 45.404470] skb_push+0xcd/0xf0 [ 45.404726] br_dev_queue_push_xmit+0x7c/0x6c0 [ 45.406513] br_forward_finish+0x128/0x260 [ 45.408483] __br_forward+0x42d/0x590 [ 45.409464] maybe_deliver+0x2eb/0x420 [ 45.409763] br_flood+0x174/0x4a0 [ 45.410030] br_handle_frame_finish+0xc7c/0x1bc0 [ 45.411618] br_handle_frame+0xac3/0x1230 [ 45.413674] __netif_receive_skb_core.constprop.0+0x808/0x3df0 [ 45.422966] __netif_receive_skb_one_core+0xb4/0x1f0 [ 45.424478] __netif_receive_skb+0x22/0x170 [ 45.424806] process_backlog+0x242/0x6d0 [ 45.425116] __napi_poll+0xbb/0x630 [ 45.425394] net_rx_action+0x4d1/0xcc0 [ 45.427613] handle_softirqs+0x1a4/0x580 [ 45.427926] do_softirq+0x74/0x90 [ 45.428196] </IRQ>
This issue was found by syzkaller.
The panic happens in br_dev_queue_push_xmit() once it receives a corrupted skb with ETH header already pushed in linear data. When it attempts the skb_push() call, there's not enough headroom and skb_push() panics.
The corrupted skb is put on the queue by HSR layer, which makes a sequence of unintended transformations when it receives a specific corrupted HSR frame (with incomplete TAG).
Fix it by dropping and consuming frames that are not long enough to contain both ethernet and hsr headers.
Alternative fix would be to check for enough headroom before skb_push() in br_dev_queue_push_xmit().
In the reproducer, this is injected via AF_PACKET, but I don't easily see why it couldn't be sent over the wire from adjacent network.
Further Details:
In the reproducer, the following network interface chain is set up:
┌────────────────┐ ┌────────────────┐ │ veth0_to_hsr ├───┤ hsr_slave0 ┼───┐ └────────────────┘ └────────────────┘ │ │ ┌──────┐ ├─┤ hsr0 ├───┐ │ └──────┘ │ ┌────────────────┐ ┌────────────────┐ │ │┌────────┐ │ veth1_to_hsr ┼───┤ hsr_slave1 ├───┘ └┤ │ └────────────────┘ └────────────────┘ ┌┼ bridge │ ││ │ │└────────┘ │ ┌───────┐ │ │ ... ├──────┘ └───────┘
To trigger the events leading up to crash, reproducer sends a corrupted HSR frame with incomplete TAG, via AF_PACKET socket on 'veth0_to_hsr'.
The first HSR-layer function to process this frame is hsr_handle_frame(). It and then checks if the protocol is ETH_P_PRP or ETH_P_HSR. If it is, it calls skb_set_network_header(skb, ETH_HLEN + HSR_HLEN), without checking that the skb is long enough. For the crashing frame it is not, and hence the skb->network_header and skb->mac_len fields are set incorrectly, pointing after the end of the linear buffer.
I will call this a BUG#1 and it is what is addressed by this patch. In the crashing scenario before the fix, the skb continues to go down the hsr path as follows.
hsr_handle_frame() then calls this sequence hsr_forward_skb() fill_frame_info() hsr->proto_ops->fill_frame_info() hsr_fill_frame_info()
hsr_fill_frame_info() contains a check that intends to check whether the skb actually contains the HSR header. But the check relies on the skb->mac_len field which was erroneously setup due to BUG#1, so the check passes and the execution continues back in the hsr_forward_skb():
hsr_forward_skb() hsr_forward_do() hsr->proto_ops->get_untagged_frame() hsr_get_untagged_frame() create_stripped_skb_hsr()
In create_stripped_skb_hsr(), a copy of the skb is created and is further corrupted by operation that attempts to strip the HSR tag in a call to __pskb_copy().
The skb enters create_stripped_skb_hsr() with ethernet header pushed in linear buffer. The skb_pull(skb_in, HSR_HLEN) thus pulls 6 bytes of ethernet header into the headroom, creating skb_in with a headroom of size 8. The subsequent __pskb_copy() then creates an skb with headroom of just 2 and skb->len of just 12, this is how it looks after the copy:
gdb) p skb->len $10 = 12 (gdb) p skb->data $11 = (unsigned char *) 0xffff888041e45382 "\252\252\252\252\252!\210\373", (gdb) p skb->head $12 = (unsigned char *) 0xffff888041e45380 ""
It seems create_stripped_skb_hsr() assumes that ETH header is pulled in the headroom when it's entered, because it just pulls HSR header on top. But that is not the case in our code-path and we end up with the corrupted skb instead. I will call this BUG#2
*I got confused here because it seems that under no conditions can create_stripped_skb_hsr() work well, the assumption it makes is not true during the processing of hsr frames - since the skb_push() in hsr_handle_frame to skb_pull in hsr_deliver_master(). I wonder whether I missed something here.*
Next, the execution arrives in hsr_deliver_master(). It calls skb_pull(ETH_HLEN), which just returns NULL - the SKB does not have enough space for the pull (as it only has 12 bytes in total at this point).
*The skb_pull() here further suggests that ethernet header is meant to be pushed through the whole hsr processing and create_stripped_skb_hsr() should pull it before doing the HSR header pull.*
hsr_deliver_master() then puts the corrupted skb on the queue, it is then picked up from there by bridge frame handling layer and finally lands in br_dev_queue_push_xmit where it panics.
Cc: stable@kernel.org Fixes: 48b491a5cc74 ("net: hsr: fix mac_len checks") Reported-by: syzbot+a81f2759d022496b40ab@syzkaller.appspotmail.com Signed-off-by: Jakub Acs acsjakub@amazon.de Reviewed-by: Eric Dumazet edumazet@google.com Link: https://patch.msgid.link/20250819082842.94378-1-acsjakub@amazon.de Signed-off-by: Jakub Kicinski kuba@kernel.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- net/hsr/hsr_slave.c | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-)
--- a/net/hsr/hsr_slave.c +++ b/net/hsr/hsr_slave.c @@ -62,8 +62,14 @@ static rx_handler_result_t hsr_handle_fr skb_push(skb, ETH_HLEN); skb_reset_mac_header(skb); if ((!hsr->prot_version && protocol == htons(ETH_P_PRP)) || - protocol == htons(ETH_P_HSR)) + protocol == htons(ETH_P_HSR)) { + if (!pskb_may_pull(skb, ETH_HLEN + HSR_HLEN)) { + kfree_skb(skb); + goto finish_consume; + } + skb_set_network_header(skb, ETH_HLEN + HSR_HLEN); + } skb_reset_mac_len(skb);
hsr_forward_skb(skb, port);