CVE-2025-39790

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

bus: mhi: host: Detect events pointing to unexpected TREs

When a remote device sends a completion event to the host, it contains a

pointer to the consumed TRE. The host uses this pointer to process all of

the TREs between it and the host's local copy of the ring's read pointer.

This works when processing completion for chained transactions, but can

lead to nasty results if the device sends an event for a single-element

transaction with a read pointer that is multiple elements ahead of the

host's read pointer.

For instance, if the host accesses an event ring while the device is

updating it, the pointer inside of the event might still point to an old

TRE. If the host uses the channel's xfer_cb() to directly free the buffer

pointed to by the TRE, the buffer will be double-freed.

This behavior was observed on an ep that used upstream EP stack without

'commit 6f18d174b73d ("bus: mhi: ep: Update read pointer only after buffer

is written")'. Where the device updated the events ring pointer before

updating the event contents, so it left a window where the host was able to

access the stale data the event pointed to, before the device had the

chance to update them. The usual pattern was that the host received an

event pointing to a TRE that is not immediately after the last processed

one, so it got treated as if it was a chained transaction, processing all

of the TREs in between the two read pointers.

This commit aims to harden the host by ensuring transactions where the

event points to a TRE that isn't local_rp + 1 are chained.

[mani: added stable tag and reworded commit message]