CVE-2024-57843: virtio-net: fix overflow inside virtnet_rq_alloc

In the Linux kernel, the following vulnerability has been resolved:

virtio-net: fix overflow inside virtnetrqalloc

When the frag just got a page, then may lead to regression on VM. Specially if the sysctl net.core.highorderallocdisable value is 1, then the frag always get a page when do refill.

Which could see reliable crashes or scp failure (scp a file 100M in size to VM).

The issue is that the virtnetrqdma takes up 16 bytes at the beginning of a new frag. When the frag size is larger than PAGESIZE, everything is fine. However, if the frag is only one page and the total size of the buffer and virtnetrqdma is larger than one page, an overflow may occur.

The commit f9dac92ba908 ("virtioring: enable premapped mode whatever usedmaapi") introduced this problem. And we reverted some commits to fix this in last linux version. Now we try to enable it and fix this bug directly.

Here, when the frag size is not enough, we reduce the buffer len to fix this problem.