[PATCH v2 1/1] mm/memory: constrain generic_access_phys() to page boundary

[Ren Wei <weir@nebusec.ai>]

From: Luxiao Xu <rakukuip@gmail.com>

generic_access_phys() improperly validates the memory access range: it
only validates the start address using follow_pfnmap_start() and passes
PAGE_ALIGN(len + offset) to ioremap_prot().

This poses two problems:
1. In PFNMAP VMAs, consecutive virtual pages are not guaranteed to be
   physically contiguous, and individual PTEs may have different access
   permissions or writability.
2. The mapping may cross VMA boundaries if len extends beyond vma->vm_end.

Since __access_remote_vm() already loops over the requested length and
accesses normal (struct page) memory page-by-page, constrain the ->access()
callback in __access_remote_vm() to at most the current page boundary.
Because VMA boundaries are always page-aligned, this also ensures the
access never exceeds the current VMA.

In generic_access_phys(), defensively clamp len to the page boundary as
well, map only a single PAGE_SIZE via ioremap_prot(), and add a missing
(resource_size_t) cast during PFN re-validation to avoid truncation on
32-bit PAE systems.

Fixes: 9cb12d7b4cca ("mm/memory.c: actually remap enough memory")
Cc: stable@vger.kernel.org
Reported-by: Vega <vega@nebusec.ai>
Assisted-by: LLM
Suggested-by: David Hildenbrand <david@kernel.org>
Signed-off-by: Luxiao Xu <rakukuip@gmail.com>
Signed-off-by: Ren Wei <weir@nebusec.ai>
---
v1 -> v2:
- Drop internal multi-page loop in generic_access_phys(); clamp the chunk
  size in __access_remote_vm() to PAGE_SIZE - offset instead, leveraging
  the existing caller loop (David Hildenbrand).
- Defensively clamp len in generic_access_phys() and only map PAGE_SIZE.
- Add missing (resource_size_t) cast when checking args.pfn (Andrew Morton).
Link: https://lore.kernel.org/r/f618e2f57ad0086a8d7a95cb17dcf5a1f0cf0e45.1784428532.git.rakukuip@gmail.com
---
 mm/memory.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 8b0c2c735d3d..09390e51432f 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -7164,6 +7164,11 @@ int generic_access_phys(struct vm_area_struct *vma, unsigned long addr,
 	bool writable;
 	struct follow_pfnmap_args args = { .vma = vma, .address = addr };
 
+	if (len <= 0)
+		return 0;
+
+	len = min_t(int, len, PAGE_SIZE - offset);
+
 retry:
 	if (follow_pfnmap_start(&args))
 		return -EINVAL;
@@ -7175,7 +7180,7 @@ int generic_access_phys(struct vm_area_struct *vma, unsigned long addr,
 	if ((write & FOLL_WRITE) && !writable)
 		return -EINVAL;
 
-	maddr = ioremap_prot(phys_addr, PAGE_ALIGN(len + offset), prot);
+	maddr = ioremap_prot(phys_addr, PAGE_SIZE, prot);
 	if (!maddr)
 		return -ENOMEM;
 
@@ -7183,7 +7188,7 @@ int generic_access_phys(struct vm_area_struct *vma, unsigned long addr,
 		goto out_unmap;
 
 	if ((pgprot_val(prot) != pgprot_val(args.pgprot)) ||
-	    (phys_addr != (args.pfn << PAGE_SHIFT)) ||
+	    (phys_addr != ((resource_size_t)args.pfn << PAGE_SHIFT)) ||
 	    (writable != args.writable)) {
 		follow_pfnmap_end(&args);
 		iounmap(maddr);
@@ -7254,7 +7259,8 @@ static int __access_remote_vm(struct mm_struct *mm, unsigned long addr,
 #ifdef CONFIG_HAVE_IOREMAP_PROT
 			if (vma->vm_ops && vma->vm_ops->access)
 				bytes = vma->vm_ops->access(vma, addr, buf,
-							    len, write);
+							    min_t(int, len, PAGE_SIZE - offset_in_page(addr)),
+							    write);
 #endif
 			if (bytes <= 0)
 				break;
-- 
2.43.0