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.