Re: [PATCH] kunit: fix use-after-free in debugfs when using kunit.filter

[Martin Kaiser]

Thus wrote Florian Schmaus (florian.schmaus@xxxxxxxxxxx):

> When the kernel is booted with a kunit filter (e.g.,
> kunit.filter="speed!=slow"), the kunit executor dynamically allocates
> copies of the filtered test suites using kmalloc/kmemdup.

> During the initial boot execution, kunit_debugfs_create_suite() creates
> debugfs files (such as /sys/kernel/debug/kunit/<suite>/run) and
> permanently stores a pointer to the dynamically allocated suite in the
> inode's i_private field.

> Previously, the executor freed this dynamically allocated suite_set
> immediately after executing the boot-time tests. Because the debugfs
> nodes were not destroyed, any subsequent interaction with the debugfs
> `run` file from userspace triggered a use-after-free (UAF). On systems
> with architectural capabilities, like CHERI RISC-V, this resulted in
> an immediate fatal hardware exception due to the invalidation of the
> capability tags on the reclaimed memory. On other architectures, it
> resulted in silent memory corruption.

> Fix this UAF by properly coupling the lifetime of the filtered suite
> memory allocation to the lifetime of the kunit subsystem and its
> associated VFS nodes. Ownership of the boot-time suite_set is now
> transferred to a global tracker ('kunit_boot_suites'), and the memory
> is cleanly released in kunit_exit() during module teardown.

> Signed-off-by: Florian Schmaus <florian.schmaus@xxxxxxxxxxx>

Reviewed-by: Martin Kaiser <martin@xxxxxxxxx>