In the Linux kernel, the following vulnerability has been resolved:
bnxt: fix head underflow on XDP head-grow
The xdp.py test test_xdp_native_adjst_head_grow_data crashes when run on
a bnxt machine (and also crashes in NIPA).
It seems that the bug is an underflow in bnxt_rx_multi_page_skb, which
builds the skb head:
napi_build_skb(data_ptr – bp->rx_offset, rxr->rx_page_size);
The problem with this expression is that in page mode, rx_offset is:
bp->rx_offset = NET_IP_ALIGN + XDP_PACKET_HEADROOM;
Which evaluates (at least on x86_64) to 258.
The test test_xdp_native_adjst_head_grow_data tests a case where the
head is adjusted by -256.
When this test runs, data_ptr is shifted to frag_start + 2 (where
frag_start = page_address(page) + offset).
Then, bnxt_rx_multi_page_skb is invoked and the napi_build_skb
expression subtracts 258, landing at an address before frag_start. This
could be either the previous fragment or the previous physical page when
the offset is = 64k, but it unintentionally broke the head grow
case.
To fix this, add an offset field to struct bnxt_sw_rx_bd, mirroring the
existing offset field in struct bnxt_sw_rx_agg_bd. Populate it on
allocation and preserve it on reuse.
In bnxt_rx_multi_page_skb, use the newly added offset field to compute
the fragment start and pass that to napi_build_skb. Adjust the layout
with skb_reserve.
There are two cases, the non-adjustment case and the adjustment case.
In both cases, the skb is built at page_address(page) + offset to
account for the case where the native page size >= 64K and skb_reserve
is called with data_ptr – (page_address(page) + offset). That
difference equals bp->rx_offset when data_ptr was not moved, or
bp->rx_offset + xdp_adjust when XDP adjusted the head.
Re-running the failing test with this commit applied causes the test to
run successfully to completion.
The other rx_skb_func implementations don’t have this issue.
