[PATCH V1] accel/amdxdna: Fix locally exploitable BUG_ON in amdxdna_insert_pages()

[Lizhi Hou]

In amdxdna_insert_pages(), vm_flags_mod() sets VM_MIXEDMAP and clears
VM_PFNMAP. If an unprivileged userspace process mmaps a non-imported GEM
object and then calls madvise(MADV_DONTNEED), the PTEs will be
successfully cleared because VM_MIXEDMAP allows this (unlike VM_PFNMAP).

When userspace subsequently accesses the memory, drm_gem_shmem_fault()
handles the page fault and attempts to map the backing shmem page via
vmf_insert_pfn() which calls vmf_insert_pfn_prot(). Because the backing
shmem page is normal system memory (pfn_valid(pfn) is true) and the VMA
now has VM_MIXEDMAP set, won't this predictably trigger the explicit
assertion BUG_ON((vma->vm_flags & VM_MIXEDMAP) && pfn_valid(pfn))

Fix by removing the vm_flags_mod() call and replacing the vm_insert_pages()
pre-population with the handle_mm_fault() loop that was already used for
the import (dma-buf) path.

Fixes: e486147c912f ("accel/amdxdna: Add BO import and export")
Signed-off-by: Lizhi Hou <lizhi.hou@xxxxxxx>
---
 drivers/accel/amdxdna/amdxdna_gem.c | 27 ++++++++-------------------
 1 file changed, 8 insertions(+), 19 deletions(-)

diff --git a/drivers/accel/amdxdna/amdxdna_gem.c b/drivers/accel/amdxdna/amdxdna_gem.c
index aec69d5f463e..1c63eff0a4a8 100644
--- a/drivers/accel/amdxdna/amdxdna_gem.c
+++ b/drivers/accel/amdxdna/amdxdna_gem.c
@@ -467,25 +467,17 @@ static int amdxdna_insert_pages(struct amdxdna_gem_obj *abo,
 			XDNA_ERR(xdna, "Failed shmem mmap %d", ret);
 			return ret;
 		}
-
-		/* The buffer is based on memory pages. Fix the flag. */
-		vm_flags_mod(vma, VM_MIXEDMAP, VM_PFNMAP);
-		ret = vm_insert_pages(vma, vma->vm_start, abo->base.pages,
-				      &num_pages);
+	} else {
+		vma->vm_private_data = NULL;
+		vma->vm_ops = NULL;
+		ret = dma_buf_mmap(abo->dma_buf, vma, 0);
 		if (ret) {
-			XDNA_ERR(xdna, "Failed insert pages %d", ret);
-			amdxdna_mark_mapp_invalid(abo, vma);
+			XDNA_ERR(xdna, "Failed to mmap dma buf %d", ret);
+			return ret;
 		}
 
-		return 0;
-	}
-
-	vma->vm_private_data = NULL;
-	vma->vm_ops = NULL;
-	ret = dma_buf_mmap(abo->dma_buf, vma, 0);
-	if (ret) {
-		XDNA_ERR(xdna, "Failed to mmap dma buf %d", ret);
-		return ret;
+		/* Drop the reference drm_gem_mmap_obj() acquired.*/
+		drm_gem_object_put(to_gobj(abo));
 	}
 
 	do {
@@ -502,9 +494,6 @@ static int amdxdna_insert_pages(struct amdxdna_gem_obj *abo,
 		offset += PAGE_SIZE;
 	} while (--num_pages);
 
-	/* Drop the reference drm_gem_mmap_obj() acquired.*/
-	drm_gem_object_put(to_gobj(abo));
-
 	return 0;
 }
 
-- 
2.34.1