[PATCH] drm/ttm: Do not put non-struct page memory into PUD/PMDs

[Jason Gunthorpe]

PUD and PMD entries do not have a special bit.

get_user_pages_fast() considers any page that passed pmd_huge() as
usable:

		if (unlikely(pmd_trans_huge(pmd) || pmd_huge(pmd) ||
			     pmd_devmap(pmd))) {

And vmf_insert_pfn_pmd_prot() unconditionally sets

	entry = pmd_mkhuge(pfn_t_pmd(pfn, prot));

eg on x86 the page will be _PAGE_PRESENT | PAGE_PSE.

As such gup_huge_pmd() will try to deref a struct page:

	head = try_grab_compound_head(pmd_page(orig), refs, flags);

and thus crash.

Prevent this by never using IO memory with vmf_insert_pfn_pud/pmd_prot().

Fixes: 314b6580adc5 ("drm/ttm, drm/vmwgfx: Support huge TTM pagefaults")
Signed-off-by: Jason Gunthorpe <jgg at nvidia.com>
---
 drivers/gpu/drm/ttm/ttm_bo_vm.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

AFAICT only the vmwgfx driver makes use of this, and I can't tell which path
it is taking.

This function will also try to install a PUD - does vmwgfx have a case where
it has obtained a 1GB high order page - or is that dead code now?

diff --git a/drivers/gpu/drm/ttm/ttm_bo_vm.c b/drivers/gpu/drm/ttm/ttm_bo_vm.c
index f56be5bc0861ec..91d02c345fbba8 100644
--- a/drivers/gpu/drm/ttm/ttm_bo_vm.c
+++ b/drivers/gpu/drm/ttm/ttm_bo_vm.c
@@ -203,10 +203,13 @@ static vm_fault_t ttm_bo_vm_insert_huge(struct vm_fault *vmf,
 	if (page_offset + fault_page_size > bo->resource->num_pages)
 		goto out_fallback;
 
+	/*
+	 * vmf_insert_pfn_pud/pmd_prot() can only be called with struct page
+	 * backed memory
+	 */
 	if (bo->resource->bus.is_iomem)
-		pfn = ttm_bo_io_mem_pfn(bo, page_offset);
-	else
-		pfn = page_to_pfn(ttm->pages[page_offset]);
+		goto out_fallback;
+	pfn = page_to_pfn(ttm->pages[page_offset]);
 
 	/* pfn must be fault_page_size aligned. */
 	if ((pfn & (fault_page_size - 1)) != 0)

base-commit: 519d81956ee277b4419c723adfb154603c2565ba
-- 
2.33.0