CVE-2026-89507: RDMA/ucma: Lock the handler in ucma_write_cm_event()

CVE-2026-89507: RDMA/ucma: Lock the handler in ucma_write_cm_event()

[Greg Kroah-Hartman]

From: Greg Kroah-Hartman <gregkh@kernel.org>

Description
===========

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

RDMA/ucma: Lock the handler in ucma_write_cm_event()

ctx->file may only be changed under the handler lock and the xa_lock, which
is what stops uevents being queued for a ctx while ucma_migrate_id() moves
it to another file.  The CM core takes that lock before invoking
ucma_event_handler(), but the write() paths that queue uevents themselves
do not.

ucma_write_cm_event() re-reads ctx->file for each of its four dereferences,
so ucma_migrate_id() can swap it mid-sequence:

	mutex_lock(&ctx->file->mut);			/* file A */
	list_add_tail(&uevent->list, &ctx->file->event_list);	/* file B */
	mutex_unlock(&ctx->file->mut);			/* file B */
	wake_up_interruptible(&ctx->file->poll_wait);	/* file B */

The window is the mutex_lock() itself: the writer sleeps in it while the
migration reassigns ctx->file.  The list_add_tail() then runs on file B's
event_list holding only file A's mutex:

  list_add corruption. prev->next should be next (ffff888101320f30),
    but was ffff88814a08c418. (prev=ffff88814a075c18).
  kernel BUG at lib/list_debug.c:32!
  Call Trace:
   ucma_write_cm_event+0x36e/0x5e0

and file A's mut is left held forever, wedging its next writer in D state.
The uevent is also stranded on a list ucma_cleanup_ctx_events() will not
walk, so it outlives its context.  /dev/infiniband/rdma_cm is 0666 and no
RDMA device is involved, so an unprivileged user reaches all of this.

Take the handler lock, as ucma_cleanup_mc_events() does; ctx->cm_id is
pinned by the ucma_get_ctx() reference.

The Linux kernel CVE team has assigned CVE-2026-89507 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 6.18 with commit a3c9d0fcd3715541bbf97da2ddde9d032e2fe6d5 and fixed in 6.18.50 with commit 4f8bb11dd2ff365e7cff1c9964ab4607292d364e
	Issue introduced in 6.18 with commit a3c9d0fcd3715541bbf97da2ddde9d032e2fe6d5 and fixed in 7.2.4 with commit 0be1955040a2eceed0ecfc387fdc92305411d273
	Issue introduced in 6.18 with commit a3c9d0fcd3715541bbf97da2ddde9d032e2fe6d5 and fixed in 7.3-rc1 with commit f4cc21c6a8e9d392871477f9fd98d68e5ad80272

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-89507
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	drivers/infiniband/core/ucma.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/4f8bb11dd2ff365e7cff1c9964ab4607292d364e
	https://git.kernel.org/stable/c/0be1955040a2eceed0ecfc387fdc92305411d273
	https://git.kernel.org/stable/c/f4cc21c6a8e9d392871477f9fd98d68e5ad80272