FAILED: patch "[PATCH] mm: disallow ioremap'ing ordinary RAM in generic_access_phys()" failed to apply to 6.12-stable tree

The patch below does not apply to the 6.12-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.12.y
git checkout FETCH_HEAD
git cherry-pick -x 4590752e20aa393cc0d4fe538c0747d14a98c97a
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '[email protected]' --subject-prefix 'PATCH 6.12.y' 'HEAD^..'

Thanks,
Sasha

------------------ original commit in Linus's tree ------------------

From 4590752e20aa393cc0d4fe538c0747d14a98c97a Mon Sep 17 00:00:00 2001
From: "David Hildenbrand (Arm)" <[email protected]>
Date: Tue, 13 Oct 2026 13:41:24 +0200
Subject: [PATCH] mm: disallow ioremap'ing ordinary RAM in
generic_access_phys()

generic_access_phys() can end up getting called on CoW PFNMAP mappings, resulting in it trying to ioremap anonymous folios, which especially the x86 code doesn't enjoy, and which is in general just odd/wrong.

E.g., with a CoW mapping of a PCI BAR:

ioremap on RAM at 0x0000000045623000 - 0x0000000045623fff

  WARNING: arch/x86/mm/ioremap.c:216 at __ioremap_caller.isra.0+0x4c2/0x5f0
  Call Trace:
   generic_access_phys+0x130/0x4d0 mm/memory.c:7178
   kernfs_vma_access+0x1ce/0x280 fs/kernfs/file.c:437
   __access_remote_vm+0x58f/0x890 mm/memory.c:7256
   mem_rw+0x2a1/0x670 fs/proc/base.c:912

But more generically, we should disallow access to anything that looks like ordinary RAM in the system: including allocating memory (e.g., alloc_pages) and mapping them in a VM_PFNMAP.

For pages with an online memmap, allow only pages marked as reserved (PageReserved). This also includes memblock allocations, however, distinguishing them from other MMIO is tricky, and nobody should really be mapping them in a PFNMAP (except someone abusing /dev/mem, which we do not care about).

So only allow access if there is no online memmap, or if the pages are marked reserved. Reject other pages with -EINVAL.

Note that, just like other pfn_to_online_page() users, we won't care about concurrent memory offlining; mapping memory that can concurrently get offlined is highly dubious.

---
 mm/memory.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

index ed64455c73bdb..34e60a5baf7ce 100644
@@ -7129,6 +7129,7 @@ int generic_access_phys(struct vm_area_struct *vma, unsigned long addr,
 	resource_size_t phys_addr;
 	pgprot_t prot = __pgprot(0);
 	void __iomem *maddr;
+	struct page *page;
 	int offset = offset_in_page(addr);
 	int ret = -EINVAL;
 	bool writable;

@@ -7143,6 +7144,19 @@ int generic_access_phys(struct vm_area_struct *vma, unsigned long addr,
 retry:
 	if (follow_pfnmap_start(&args))
 		return -EINVAL;
+
+	/*
+	 * Reject anything that looks like ordinary RAM, including anonymous
+	 * folios in CoW mappings or allocated kernel memory. Note that we
+	 * cannot distinguish memblock allocations from non-RAM reliably, so
+	 * we cannot easily reject them here.
+	 */
+	page = pfn_to_online_page(args.pfn);
+	if (page && !PageReserved(page)) {
+		follow_pfnmap_end(&args);
+		return -EINVAL;
+	}
+
 	prot = args.pgprot;
 	phys_addr = (resource_size_t)args.pfn << PAGE_SHIFT;
 	writable = args.writable;

2.53.0

From: "David Hildenbrand (Arm)" <[email protected]>

[ Upstream commit 4590752e20aa393cc0d4fe538c0747d14a98c97a ]

generic_access_phys() can end up getting called on CoW PFNMAP mappings, resulting in it trying to ioremap anonymous folios, which especially the x86 code doesn't enjoy, and which is in general just odd/wrong.

E.g., with a CoW mapping of a PCI BAR:

ioremap on RAM at 0x0000000045623000 - 0x0000000045623fff

  WARNING: arch/x86/mm/ioremap.c:216 at __ioremap_caller.isra.0+0x4c2/0x5f0
  Call Trace:
   generic_access_phys+0x130/0x4d0 mm/memory.c:7178
   kernfs_vma_access+0x1ce/0x280 fs/kernfs/file.c:437
   __access_remote_vm+0x58f/0x890 mm/memory.c:7256
   mem_rw+0x2a1/0x670 fs/proc/base.c:912

But more generically, we should disallow access to anything that looks like ordinary RAM in the system: including allocating memory (e.g., alloc_pages) and mapping them in a VM_PFNMAP.

For pages with an online memmap, allow only pages marked as reserved (PageReserved). This also includes memblock allocations, however, distinguishing them from other MMIO is tricky, and nobody should really be mapping them in a PFNMAP (except someone abusing /dev/mem, which we do not care about).

So only allow access if there is no online memmap, or if the pages are marked reserved. Reject other pages with -EINVAL.

Note that, just like other pfn_to_online_page() users, we won't care about concurrent memory offlining; mapping memory that can concurrently get offlined is highly dubious.

---
 mm/memory.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

index 564c0ce0aa6d7..379ebbb834565 100644
@@ -6567,6 +6567,7 @@ int generic_access_phys(struct vm_area_struct *vma, unsigned long addr,
 	resource_size_t phys_addr;
 	unsigned long prot = 0;
 	void __iomem *maddr;
+	struct page *page;
 	int offset = offset_in_page(addr);
 	int ret = -EINVAL;
 	bool writable;

@@ -6575,6 +6576,19 @@ int generic_access_phys(struct vm_area_struct *vma, unsigned long addr,
 retry:
 	if (follow_pfnmap_start(&args))
 		return -EINVAL;
+
+	/*
+	 * Reject anything that looks like ordinary RAM, including anonymous
+	 * folios in CoW mappings or allocated kernel memory. Note that we
+	 * cannot distinguish memblock allocations from non-RAM reliably, so
+	 * we cannot easily reject them here.
+	 */
+	page = pfn_to_online_page(args.pfn);
+	if (page && !PageReserved(page)) {
+		follow_pfnmap_end(&args);
+		return -EINVAL;
+	}
+
 	prot = pgprot_val(args.pgprot);
 	phys_addr = (resource_size_t)args.pfn << PAGE_SHIFT;
 	writable = args.writable;

2.53.0