[Bridge] [PATCH net] net: bridge: fix stale eth hdr pointer in br_dev_xmit

Nikolay Aleksandrov nikolay at cumulusnetworks.com
Mon Feb 24 15:33:22 UTC 2020

On 24/02/2020 15:07, Nikolay Aleksandrov wrote:
> In br_dev_xmit() we perform vlan filtering in br_allowed_ingress() but
> if the packet has the vlan header inside (e.g. bridge with disabled
> tx-vlan-offload) then the vlan filtering code will use skb_vlan_untag()
> to extract the vid before filtering which in turn calls pskb_may_pull()
> and we may end up with a stale eth pointer. Moreover the cached eth header
> pointer will generally be wrong after that operation. Remove the eth header
> caching and just use eth_hdr() directly, the compiler does the right thing
> and calculates it only once so we don't lose anything.
> Reported-by: syzbot+18c8b623c66fc198c493 at syzkaller.appspotmail.com
> Fixes: 057658cb33fb ("bridge: suppress arp pkts on BR_NEIGH_SUPPRESS ports")
> Signed-off-by: Nikolay Aleksandrov <nikolay at cumulusnetworks.com>
> ---
>  net/bridge/br_device.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)

Funny but this might turn out to be a totally unrelated bug to the one reported by syzbot.
I just noticed it's saying uninit instead of use-after-free. I'm now building the
whole syz environment mentioned in the report to check it out.
That being said - the bug that I'm fixing here exists, we just might have to drop the
Reported-by: tag if it turns out the uninit bug is a different one. :)

I'll report back when I'm able to reproduce the syz uninit bug.

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