Re: [PATCH 4/4] fixup! drm/gpusvm: use hmm_range_fault_unlocked_timeout() for range faults

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:

Pre-existing issues:
- [High] drm_gpusvm passes uninitialized pfns arrays to hmm_range_fault, causing unintended page faults and write requests.
- [High] Memory leak of `mm_struct` in `drm_gpusvm_range_evict` on allocation failure.
--

--- Patch [4]: [PATCH 4/4] fixup! drm/gpusvm: use hmm_range_fault_unlocked_timeout() for range faults ---
commit afecb2c70fcbb0789fe0b4b948e05bb9f8070c63
Author: Stanislav Kinsburskii <skinsburskii@gmail.com>

fixup! drm/gpusvm: use hmm_range_fault_unlocked_timeout() for range faults

The patch removes the outer deadline tracking from hmm_range_fault loops
and passes msecs_to_jiffies(HMM_RANGE_DEFAULT_TIMEOUT) directly to each
hmm_range_fault_unlocked_timeout() call. This avoids accounting for outer
driver retry logic inside the HMM retry budget.

> diff --git a/drivers/gpu/drm/drm_gpusvm.c b/drivers/gpu/drm/drm_gpusvm.c
> index b8f2dd9982f53..76e8a0028c7fc 100644
> --- a/drivers/gpu/drm/drm_gpusvm.c
> +++ b/drivers/gpu/drm/drm_gpusvm.c
> @@ -867,8 +866,7 @@ enum drm_gpusvm_scan_result drm_gpusvm_scan_mm(struct drm_gpusvm_range *range,
>  	hmm_range.hmm_pfns = pfns;
>  
>  retry:
> -	err = hmm_range_fault_unlocked_timeout(&hmm_range,
> -					       max(timeout - jiffies, 1L));
> +	err = hmm_range_fault_unlocked_timeout(&hmm_range, timeout);

[Severity: High]
This is a pre-existing issue, but does using kvmalloc_array() without
zeroing memory cause unintended page faults?

HMM reads hmm_pfns as input to determine per-page fault requests. The
uninitialized memory can contain random bits like HMM_PFN_REQ_FAULT or
HMM_PFN_REQ_WRITE, potentially triggering unintended faults or violating
read-only mapping constraints. 

Additionally, the retry loop reuses output flags (HMM_PFN_VALID), which
aliases with HMM_PFN_REQ_FAULT (both are the MSB), unintentionally forcing
faults on previously valid pages during a scan.

Should this allocate with kvcalloc() instead?

[ ... ]

> @@ -1495,8 +1489,7 @@ int drm_gpusvm_get_pages(struct drm_gpusvm *gpusvm,
>  	}
>  
>  	hmm_range.hmm_pfns = pfns;
> -	err = hmm_range_fault_unlocked_timeout(&hmm_range,
> -				max_t(long, timeout - jiffies, 1));
> +	err = hmm_range_fault_unlocked_timeout(&hmm_range, timeout);

[Severity: High]
This is also a pre-existing issue, but similar to drm_gpusvm_scan_mm(),
pfns is allocated with kvmalloc_array() earlier in drm_gpusvm_get_pages()
and passed to hmm_range_fault_unlocked_timeout() here.

Could this uninitialized memory cause unintended write requests or page
faults?

[ ... ]

> @@ -1734,8 +1726,7 @@ int drm_gpusvm_range_evict(struct drm_gpusvm *gpusvm,
>  		return -ENOMEM;

[Severity: High]
This isn't a bug introduced by this patch, but does returning -ENOMEM here
leak the mm_struct?

Earlier in drm_gpusvm_range_evict(), mmget_not_zero(mm) increments the
reference count:

	if (!mmget_not_zero(mm))
		return -EFAULT;

	pfns = kvmalloc_array(npages, sizeof(*pfns), GFP_KERNEL);
	if (!pfns)
		return -ENOMEM;

If the kvmalloc_array() allocation for pfns fails, it returns directly
without calling mmput(mm).

>  
>  	hmm_range.hmm_pfns = pfns;
> -	err = hmm_range_fault_unlocked_timeout(&hmm_range,
> -				max_t(long, timeout - jiffies, 1));
> +	err = hmm_range_fault_unlocked_timeout(&hmm_range, timeout);

[Severity: High]
This is a pre-existing issue, but again, pfns is allocated with
kvmalloc_array() and passed here. Does this need to be kvcalloc() to
prevent uninitialized memory from causing unintended page faults?

>  
>  	kvfree(pfns);
>  	mmput(mm);

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/178405975214.1082778.5193079941156341151.stgit@skinsburskii?part=4