CVE-2025-21709

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

kernel: be more careful about dup_mmap() failures and uprobe registering

If a memory allocation fails during dup_mmap(), the maple tree can be left in an unsafe state for other iterators besides the exit path. All the locks are dropped before the exit_mmap() call (in mm/mmap.c), but the incomplete mm_struct can be reached through (at least) the rmap finding the vmas which have a pointer back to the mm_struct.

Up to this point, there have been no issues with being able to find an mm_struct that was only partially initialised. Syzbot was able to make the incomplete mm_struct fail with recent forking changes, so it has been proven unsafe to use the mm_struct that hasn't been initialised, as referenced in the link below.

Although 8ac662f5da19f ("fork: avoid inappropriate uprobe access to invalid mm") fixed the uprobe access, it does not completely remove the race.

This patch sets the MMF_OOM_SKIP to avoid the iteration of the vmas on the oom side (even though this is extremely unlikely to be selected as an oom victim in the race window), and sets MMF_UNSTABLE to avoid other potential users from using a partially initialised mm_struct.

When registering vmas for uprobe, skip the vmas in an mm that is marked unstable. Modifying a vma in an unstable mm may cause issues if the mm isn't fully initialised.

This vulnerability is rated as Moderate severity rather than Important because exploitation requires local access and depends on winning a narrow race condition during process cloning under severe memory pressure. A local user would need to induce memory allocation failures during process creation while concurrently interacting with partially initialized memory structures through tracing subsystems, which primarily results in a kernel crash and denial of service. Standard Red Hat Enterprise Linux deployments do not expose this behavior to remote actors, and earlier releases such as Red Hat Enterprise Linux 8 are unaffected as the problematic subsystem code is not present.

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.