CVE-2026-90251: Bluetooth: MSFT: validate evt_prefix_len against the response length

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

Bluetooth: MSFT: validate evtprefixlen against the response length

readsupportedfeatures() only checks that the response covers the fixed part of struct msftrpreadsupportedfeatures, which is 11 bytes:

if (skb->len < sizeof(rp)) { btdeverr(hdev, "MSFT supported features length mismatch"); goto failed; }

evtprefix[] is a flexible array member and rp->evtprefixlen is an unvalidated u8 taken straight out of that response, so

msft->evtprefix = kmemdup(rp->evtprefix, rp->evtprefixlen, GFPKERNEL);

copies up to 255 bytes from a reply that may have carried none of them. What is copied is data the controller never sent, and it is then used to match incoming vendor events in msftvendorevt().

This is not an out-of-bounds access. An skb data allocation always has at least SKBDATAALIGN(sizeof(struct skbsharedinfo)) bytes past the payload, which is more than the 255 byte maximum, so the read stays inside the allocation and KASAN does not report it. It is still a read of bytes the host was never given, with the length fully controlled by the controller.

Reject a response that is too short for the prefix it declares.

Verified with an emulated controller over /dev/vhci on a KASAN kernel, with vhci made to advertise an MSFT opcode the way btintel, btqca, btmtk and btrtl do unconditionally. A reply of exactly 11 bytes declaring evtprefixlen = 255 reaches kmemdup and copies 255 bytes ("skb->len=11 evtprefixlen=255", with the copied buffer dumped); since the reply ends at the fixed part, all 255 come from past the end of the response. No KASAN report is produced, as expected from the allocation slack described above. With this patch the response is rejected with "MSFT event prefix length mismatch" and msft->evtprefix is left unset.