Re: [RFC v5 1/2] drm/i915/selftests: Prevent userspace mapping invalidation

[Janusz Krzysztofik <janusz.krzysztofik@linux.intel.com>]

Hi Krzysztof,

On Thu, 2026-04-09 at 08:08 +0000, Krzysztof Karas wrote:
> Migration testing in i915 assumes current task's address space
> to allocate new userspace mapping and uses it without
> registering real user for that address space in mm_struct.
> On single NUMA node setups PCI probe executes in the same
> context as userspace process calling the test (i915_selftest
> from IGT), but when multiple nodes are available, the PCI code
> puts probe into a kernel workqueue. This switches execution in
> a kworker, which does not have its own address space in
> userspace and must borrow such memory from another process, so
> "current->active_mm" is unknown at the start of the test.
> 
> It was observed that mm->mm_users would occasionally be 0
> or drop to 0 during the test due to short delay between
> scheduling and executing work in forked process, which reaped
> userspace mappings, further leading to failures upon reading
> from userland memory.
> 
> Prevent this by adding a PID parameter to a trusted task, so its
> mm struct may be used if needed.
> 
> Closes: https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/14204
> Fixes: 34b1c1c71d37 ("i915/selftest/igt_mmap: let mmap tests run in kthread")
> Signed-off-by: Krzysztof Karas <krzysztof.karas@intel.com>
> ---
> v2 (Janusz):
>  * Reword and shorten commit message to be more precise.
>  * Reorder variable declarations to follow upside down christmas
>  tree style.
> 
> v3 (Andi):
>  * Prevent PID and mm leaks.
>  * Remove a flag and use mm pointer to determine whether to
>  release references to the memory.
> 
> v4:
>  * Revert !current->mm check. (Janusz, Sebastian)
>  * Drop refernce to mm sooner. (Janusz)
>  * Ensure kthread_use_mm did its job. (Janusz)
> 
> v5 (Janusz):
>  * Remove missing PID warning.
> 
>  drivers/gpu/drm/i915/i915_selftest.h          |  1 +
>  .../gpu/drm/i915/selftests/i915_selftest.c    | 43 +++++++++++++++++++
>  2 files changed, 44 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/i915_selftest.h b/drivers/gpu/drm/i915/i915_selftest.h
> index 72922028f4ba..e29ca298e7eb 100644
> --- a/drivers/gpu/drm/i915/i915_selftest.h
> +++ b/drivers/gpu/drm/i915/i915_selftest.h
> @@ -35,6 +35,7 @@ struct i915_selftest {
>  	unsigned long timeout_jiffies;
>  	unsigned int timeout_ms;
>  	unsigned int random_seed;
> +	unsigned int userspace_pid;
>  	char *filter;
>  	int mock;
>  	int live;
> diff --git a/drivers/gpu/drm/i915/selftests/i915_selftest.c b/drivers/gpu/drm/i915/selftests/i915_selftest.c
> index 8460f0a70d04..2c169148c053 100644
> --- a/drivers/gpu/drm/i915/selftests/i915_selftest.c
> +++ b/drivers/gpu/drm/i915/selftests/i915_selftest.c
> @@ -186,6 +186,8 @@ static int __run_selftests(const char *name,
>  			   unsigned int count,
>  			   void *data)
>  {
> +	int u_pid_nr = i915_selftest.userspace_pid;
> +	struct mm_struct *mm = NULL;
>  	int err = 0;
>  
>  	while (!i915_selftest.random_seed)
> @@ -201,6 +203,42 @@ static int __run_selftests(const char *name,
>  	pr_info(DRIVER_NAME ": Performing %s selftests with st_random_seed=0x%x st_timeout=%u\n",
>  		name, i915_selftest.random_seed, i915_selftest.timeout_ms);
>  
> +	/**
> +	 * If we are running in a kthread on a multi NUMA system and the user passed
> +	 * a valid PID of a userspace task, then we may borrow its address space
> +	 * to prepare a safe environment for the mmap selftests.
> +	 */
> +	if (!current->mm) {

Please extend this condition over u_pid_nr actually provided by a user
(not 0?), otherwise we will be trying to get current->mm of a process
with default i915_selftest.userspace_pid and submitting a warning if
not successful.

Thanks,
Janusz

> +		struct pid *u_pid = find_get_pid(u_pid_nr);
> +		struct task_struct *task;
> +
> +		if (!u_pid) {
> +			pr_warn("Could not find PID: %d\n", u_pid_nr);
> +			goto run_tests;
> +		}
> +
> +		task = get_pid_task(u_pid, PIDTYPE_PID);
> +		put_pid(u_pid);
> +		if (!task) {
> +			pr_warn("Could not find userspace task for PID: %d\n", u_pid_nr);
> +			goto run_tests;
> +		}
> +
> +		mm = get_task_mm(task);
> +		put_task_struct(task);
> +		if (!mm) {
> +			pr_warn("Could not find address space of task with PID: %d\n", u_pid_nr);
> +			goto run_tests;
> +		}
> +
> +		kthread_use_mm(mm);
> +		mmput_async(mm);
> +		if (unlikely(!current->mm)) {
> +			pr_warn("Could not set mm as current->mm\n");
> +		}
> +	}
> +
> +run_tests:
>  	/* Tests are listed in order in i915_*_selftests.h */
>  	for (; count--; st++) {
>  		if (!st->enabled)
> @@ -226,6 +264,9 @@ static int __run_selftests(const char *name,
>  		 st->name, err))
>  		err = -1;
>  
> +	if (mm)
> +		kthread_unuse_mm(mm);
> +
>  	return err;
>  }
>  
> @@ -507,6 +548,8 @@ void igt_hexdump(const void *buf, size_t len)
>  module_param_named(st_random_seed, i915_selftest.random_seed, uint, 0400);
>  module_param_named(st_timeout, i915_selftest.timeout_ms, uint, 0400);
>  module_param_named(st_filter, i915_selftest.filter, charp, 0400);
> +module_param_named(st_userspace_pid, i915_selftest.userspace_pid, uint, 0400);
> +MODULE_PARM_DESC(st_userspace_pid, "For usage in tests that map userspace memory and require address space with controllable lifetime.");
>  
>  module_param_named_unsafe(mock_selftests, i915_selftest.mock, int, 0400);
>  MODULE_PARM_DESC(mock_selftests, "Run selftests before loading, using mock hardware (0:disabled [default], 1:run tests then load driver, -1:run tests then leave dummy module)");