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]

This vulnerability is rated as Moderate because exploitation requires local access and the presence of specialized Modem Host Interface (MHI) hardware, such as PCIe cellular modems, or a compromised endpoint device. The flaw cannot be exploited remotely across the network, and standard Red Hat Enterprise Linux deployments rarely interface with MHI peripherals. Consequently, the risk is limited to configurations actively operating MHI host drivers with untrusted or malfunctioning attached devices.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.