* Re: [PATCH] drm/shmem_helper: Check VMA boundaries for PMD mappings
2026-06-22 21:57 [PATCH] drm/shmem_helper: Check VMA boundaries for PMD mappings Christian A. Ehrhardt
@ 2026-06-22 22:16 ` sashiko-bot
0 siblings, 0 replies; 2+ messages in thread
From: sashiko-bot @ 2026-06-22 22:16 UTC (permalink / raw)
To: Christian A. Ehrhardt; +Cc: dri-devel
Thank you for your contribution! Sashiko AI review found 3 potential issue(s) to consider:
Pre-existing issues:
- [Critical] Missing VMA boundary check in panthor PMD mapping insertion causes kernel panic or memory overlap.
- [High] Incorrect GEM object page offset calculation causes mapping corruption on VMA split or partial unmap.
- [High] Normal refcounted shmem folios are mapped using raw PFN helpers, improperly marking them as special.
--
commit 07cb1bde7a0031ea636a380e71bef2ca1d93de8c
Author: Christian A. Ehrhardt <lk@c--e.de>
drm/shmem_helper: Check VMA boundaries for PMD mappings
This commit adds a boundary check in the try_insert_pfn() function to ensure
that a PMD huge page mapping is not installed if it exceeds the boundaries
of the VMA.
> In the ->huge_fault handler do not install a PMD huge page
> mapping if the huge page exceeds the boundaries of the VMA.
>
> All other ->huge_fault handlers have similar checks and the
> resulting mapping will trigger a VM_BUG_ON_VMA() if it ever
> reaches copy_pmd_range().
[Severity: Critical]
This is a pre-existing issue, but does panthor_gem.c also need this fix?
Looking at insert_page() in drivers/gpu/drm/panthor/panthor_gem.c at line 802,
it inserts a PMD mapping if the address is aligned, but it lacks the
vma->vm_start <= start && end <= vma->vm_end bounds check:
} else if (order == PMD_ORDER) {
unsigned long pfn = page_to_pfn(page);
unsigned long paddr = pfn << PAGE_SHIFT;
bool aligned = (vmf->address & ~PMD_MASK) == (paddr & ~PMD_MASK);
if (aligned &&
folio_test_pmd_mappable(page_folio(page))) {
pfn &= PMD_MASK >> PAGE_SHIFT;
return vmf_insert_pfn_pmd(vmf, pfn, vmf->flags & FAULT_FLAG_WRITE);
}
If userspace mmaps the panthor GEM object, shrinks the VMA to a
non-PMD-aligned size using munmap(), and triggers a page fault at a
PMD-aligned address near the end before calling fork(), couldn't this cause a
VM_BUG_ON_VMA() in copy_pmd_range() or allow memory overlap?
> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
> index c989459eb2159..00807039f8d7d 100644
> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> @@ -597,9 +597,13 @@ static vm_fault_t try_insert_pfn(struct vm_fault *vmf, unsigned int order,
> #ifdef CONFIG_ARCH_SUPPORTS_PMD_PFNMAP
> } else if (order == PMD_ORDER) {
> unsigned long paddr = pfn << PAGE_SHIFT;
> + struct vm_area_struct *vma = vmf->vma;
> + unsigned long start = ALIGN_DOWN(vmf->address, PMD_SIZE);
> + unsigned long end = start + PMD_SIZE;
> + bool in_range = vma->vm_start <= start && end <= vma->vm_end;
> bool aligned = (vmf->address & ~PMD_MASK) == (paddr & ~PMD_MASK);
>
> - if (aligned &&
> + if (aligned && in_range &&
> folio_test_pmd_mappable(page_folio(pfn_to_page(pfn)))) {
> vm_fault_t ret;
>
> @@ -608,8 +612,10 @@ static vm_fault_t try_insert_pfn(struct vm_fault *vmf, unsigned int order,
> * .huge_fault(). Make sure we pass the "write" info
> * along in that case.
> * This also means we have to record the write fault
> * here, instead of in .pfn_mkwrite().
> */
[Severity: High]
This isn't a bug introduced by this patch, but are we improperly marking
normal refcounted shmem folios as special?
The try_insert_pfn() function uses raw PFN insertion helpers like
vmf_insert_pfn_pmd() and vmf_insert_pfn() to map these folios. According to
MM guidelines, this marks the folios with VM_PFNMAP and pte_special(), which
hides them from vm_normal_page() and breaks GUP (get_user_pages) and refcount
tracking.
Should this code use vmf_insert_folio() or vmf_insert_page() instead?
> - ret = vmf_insert_pfn_pmd(vmf, pfn,
> - vmf->flags & FAULT_FLAG_WRITE);
> + ret = vmf_insert_pfn_pmd(vmf, pfn,
> + vmf->flags & FAULT_FLAG_WRITE);
> + if (ret == VM_FAULT_NOPAGE && (vmf->flags & FAULT_FLAG_WRITE))
> + drm_gem_shmem_record_mkwrite(vmf);
[Severity: High]
This is a pre-existing issue, but does drm_gem_shmem_record_mkwrite() calculate
the page offset incorrectly when a VMA is split?
In drm_gem_shmem_record_mkwrite() at line 576 (and also in
drm_gem_shmem_any_fault() at line 640), the page offset is calculated relative
to the current VMA rather than the GEM object:
pgoff_t page_offset = vmf->pgoff - vma->vm_pgoff;
...
folio_mark_dirty(page_folio(shmem->pages[page_offset]));
If userspace partially unmaps the beginning of the VMA via
munmap(vma->vm_start, PAGE_SIZE), the VMA split increments vma->vm_pgoff.
Because vmf->pgoff - vma->vm_pgoff evaluates exactly to
(vmf->address - vma->vm_start) >> PAGE_SHIFT, this completely ignores the
split offset and accesses pages[0] for the new VMA instead of the correct
shifted page.
Could this cause mapping corruption by marking the wrong page dirty or
exposing the wrong memory pages?
>
> return ret;
> }
--
Sashiko AI review · https://sashiko.dev/#/patchset/20260622215718.1532689-1-lk@c--e.de?part=1
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