+ mm-memory-constrain-generic_access_phys-to-page-boundary.patch added to mm-unstable branch

+ mm-memory-constrain-generic_access_phys-to-page-boundary.patch added to mm-unstable branch

[Andrew Morton]

The patch titled
     Subject: mm/memory: constrain generic_access_phys() to page boundary
has been added to the -mm mm-unstable branch.  Its filename is
     mm-memory-constrain-generic_access_phys-to-page-boundary.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-memory-constrain-generic_access_phys-to-page-boundary.patch

This patch will later appear in the mm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Luxiao Xu <rakukuip@gmail.com>
Subject: mm/memory: constrain generic_access_phys() to page boundary
Date: Thu, 10 Sep 2026 11:42:50 +0800

This patch addresses an issue in generic_access_phys() where accessing
memory across page boundaries in PFNMAP VMAs assumes physical pages are
contiguous, which can lead to accessing unintended physical memory or
exceeding VMA boundaries.

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.

Constrain the access in generic_access_phys() to at most the current page
boundary (PAGE_SIZE - offset) and map only a single PAGE_SIZE via
ioremap_prot().  Since the caller __access_remote_vm() already loops over
the requested length and handles partial transfers, it will naturally
iterate over the remaining pages.

Also add a missing (resource_size_t) cast during PFN re-validation to
avoid truncation on 32-bit PAE systems.

Link: https://lore.kernel.org/e06e28a46c2a176238f03b5740df0913e57c2861.1788842306.git.rakukuip@gmail.com
Fixes: 9cb12d7b4cca ("mm/memory.c: actually remap enough memory")
Signed-off-by: Luxiao Xu <rakukuip@gmail.com>
Signed-off-by: Ren Wei <weir@nebusec.ai>
Reported-by: Vega <vega@nebusec.ai>
Assisted-by: LLM
Suggested-by: David Hildenbrand <david@kernel.org>
Cc: Grazvydas Ignotas <notasas@gmail.com>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/memory.c |   10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

--- a/mm/memory.c~mm-memory-constrain-generic_access_phys-to-page-boundary
+++ a/mm/memory.c
@@ -7131,6 +7131,12 @@ int generic_access_phys(struct vm_area_s
 	bool writable;
 	struct follow_pfnmap_args args = { .vma = vma, .address = addr };
 
+	/*
+	 * Limit access to one page at a time, as that's what follow_pfnmap_start()
+	 * guarantees; expect the caller to retry to read larger ranges.
+	 */
+	len = min_t(int, len, PAGE_SIZE - offset);
+
 retry:
 	if (follow_pfnmap_start(&args))
 		return -EINVAL;
@@ -7142,7 +7148,7 @@ retry:
 	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;
 
@@ -7150,7 +7156,7 @@ retry:
 		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);
_

Patches currently in -mm which might be from rakukuip@gmail.com are

mm-memory-constrain-generic_access_phys-to-page-boundary.patch