[PATCH v2 0/5] x86/mm/pat: CPA fixes

[PATCH v2 0/5] x86/mm/pat: CPA fixes

[Mike Rapoport]

The first three patches are urgent, the third patch fixes BUG() reported
y several people and it depends on the first two.

There were no bug reports that the last two patches fix because bug
manifestations won't yell at users.

TL;DR version:

There are a couple of CPA fixes floating around:

Denis Lunev fixed races between split and collapse of the large mappings:

https://lore.kernel.org/all/20260715183453.2381141-1-den@openvz.org

Lorenzo Stoakes fixed UAF caused by races between CPA and ptdump:

https://lore.kernel.org/all/20260723-series-vmap-race-fix-v6-0-8cc77dcc0018@kernel.org

and an issue with stale page tables in IOMMU:

https://lore.kernel.org/all/20260721-fix-cpa-kernel-pagetables-v2-1-2b255deed710@kernel.org

Mike Rapoport fixed a check of RW attribute in lookup_address_in_pgd_attr()
used for the verification of RWX:

https://lore.kernel.org/all/20260715144519.934289-1-rppt@kernel.org

Pedro Falcato closed a race between text poking and collapse of large
pages:

https://lore.kernel.org/all/anCK3eWFMwZqq5ka@pedro-suse

Some of the fixes got merged into x86 tree, some of them got merged into mm
tree and some are still hanging in the air.

The changes here are collected from all these fixes into a single coherent
set on top of tip/x86/mm:
 
* fix for races between CPA and ptdump causing UAF
* update to the fix of the race between split and collapse of large
  mappings
* fix for races between CPA and vmalloc_to_page() in text poking
* fix for stale page tables in IOMMU
* fix for effective RW computation in lookup_address_in_pgd_attr()

---
v2 changes:
* rebased on the current tip/x86/mm that includes peterz's changes for
  DEBUG_PAGEALLOC
* added fix for CPA vs text poking race

v1: https://patch.msgid.link/20260728-cpa-fixes-v1-0-2ed2352300b3@kernel.org

---
Lorenzo Stoakes (ARM) (3):
      x86/mm/pat: acquire init_mm write lock on collapse to avoid UAF
      x86/mm/pat: acquire init_mm read lock on attribute change to avoid UAF
      x86/mm/pat: allocate split page tables as kernel page tables

Mike Rapoport (Microsoft) (1):
      x86/mm/pat: fix effective RW computation in lookup_address_in_pgd_attr()

Pedro Falcato (1):
      x86/alternative: exclude text poking against change_page_attr()

 arch/x86/kernel/alternative.c | 39 ++++++++++++++++++++++++---
 arch/x86/mm/pat/set_memory.c  | 61 +++++++++++++++++++++++++++++++------------
 include/linux/mmap_lock.h     |  2 ++
 3 files changed, 83 insertions(+), 19 deletions(-)
---
base-commit: 7da514d819a0afb148634aac92b3d190f34947c3
change-id: 20260727-cpa-fixes-d3c73c075672

--
Sincerely yours,
Mike.

[PATCH v2 1/5] x86/mm/pat: acquire init_mm write lock on collapse to avoid UAF

[Mike Rapoport]

From: "Lorenzo Stoakes (ARM)" <ljs@kernel.org>

x86 implements page attribute modification using its Change Page
Attributes (CPA) mechanism.

This tracks properties of ranges such as cache mode through x86 page
attributes, and as part of that logic manipulates kernel page tables.

Since commit 41d88484c71c ("x86/mm/pat: restore large ROX pages after
fragmentation") ranges of kernel page table entries can be collapsed into
huge page table entries as part of this logic.

As part of this collapse, it frees the page tables which the collapsed
entries previously pointed to, and it does so without any relevant locks
being held to preclude concurrent kernel page table walkers.

The only way this code can be reached is if CPA_COLLAPSE is specified, and
this is only set in set_memory_rox() via:

set_memory_rox()
-> change_page_attr_set_clr()
-> cpa_flush()
-> cpa_collapse_large_pages()

Notable users of this are execmem and bpf when manipulating executable
mappings.

However, this is problematic for ptdump as it walks ranges it does not own
and thus runs the risk of a use-after-free on page tables freed underneath
it.

In addition, concurrent CPA collapse operations are possible which can also
cause races.

Resolve the issue by acquiring the mmap write lock on init_mm across the
whole operation.

It is safe to acquire a sleeping lock as all the callers invoke
set_memory_rox() from process context and in any case,
change_page_attr_set_clr() calls vm_unmap_alias() which ultimately takes a
mutex, disallowing atomic context here.

Fixes: 41d88484c71c ("x86/mm/pat: restore large ROX pages after fragmentation")
Cc: stable@vger.kernel.org
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Reviewed-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Reviewed-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Will Deacon <will@kernel.org>
Reviewed-by: David Carlier <devnexen@gmail.com>
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
---
 arch/x86/mm/pat/set_memory.c | 15 ++++++++++++++-
 include/linux/mmap_lock.h    |  2 ++
 2 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
index d8d057f44417..c18b887ee4c2 100644
--- a/arch/x86/mm/pat/set_memory.c
+++ b/arch/x86/mm/pat/set_memory.c
@@ -22,6 +22,7 @@
 #include <linux/cc_platform.h>
 #include <linux/set_memory.h>
 #include <linux/memregion.h>
+#include <linux/cleanup.h>
 
 #include <asm/e820/api.h>
 #include <asm/processor.h>
@@ -409,7 +410,7 @@ static void __cpa_flush_tlb(void *data)
 
 static int collapse_large_pages(unsigned long addr, struct list_head *pgtables);
 
-static void cpa_collapse_large_pages(struct cpa_data *cpa)
+static void __cpa_collapse_large_pages(struct cpa_data *cpa)
 {
 	unsigned long start, addr, end;
 	struct ptdesc *ptdesc, *tmp;
@@ -443,6 +444,18 @@ static void cpa_collapse_large_pages(struct cpa_data *cpa)
 	}
 }
 
+static void cpa_collapse_large_pages(struct cpa_data *cpa)
+{
+	/*
+	 * Take the mmap write lock on init_mm to:
+	 * - Avoid a use-after-free if raced by ptdump (which takes its own
+	 *   write lock on init_mm).
+	 * - Serialise concurrent CPA walkers.
+	 */
+	scoped_guard(mmap_write_lock, &init_mm)
+		__cpa_collapse_large_pages(cpa);
+}
+
 static void cpa_flush(struct cpa_data *cpa, int cache)
 {
 	unsigned int i;
diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
index 04b8f61ece5d..f4ceb968aeb3 100644
--- a/include/linux/mmap_lock.h
+++ b/include/linux/mmap_lock.h
@@ -621,6 +621,8 @@ static inline void mmap_read_unlock(struct mm_struct *mm)
 
 DEFINE_GUARD(mmap_read_lock, struct mm_struct *,
 	     mmap_read_lock(_T), mmap_read_unlock(_T))
+DEFINE_GUARD(mmap_write_lock, struct mm_struct *,
+	     mmap_write_lock(_T), mmap_write_unlock(_T))
 
 static inline void mmap_read_unlock_non_owner(struct mm_struct *mm)
 {

-- 
2.53.0

[PATCH v2 2/5] x86/mm/pat: acquire init_mm read lock on attribute change to avoid UAF

[Mike Rapoport]

From: "Lorenzo Stoakes (ARM)" <ljs@kernel.org>

A previous commit protected us against races between ptdump and CPA
collapse, however one still exists between attribute changes and collapse
as reported by Denis V. Lunev (linked).

When an attribute change arises, a lockless page table walker obtains a PTE
entry, which is later written to via set_pte_atomic():

...
-> change_page_attr_set_clr()
-> __change_page_attr_set_clr()
-> __change_page_attr()
	-> _lookup_address_cpa()
	-> lookup_address_in_pgd_attr()
	-> [ lockless page table walker ]
-> set_pte_atomic()

There is nothing preventing a concurrent CPA collapse which can free the
PTE that was retrieved here, resulting in a use-after-free.

With the mmap write lock taken on init_mm over CPA collapse, we can now
resolve this race by acquiring an mmap read lock on init_mm over
__change_page_attr_set_clr().

This locks across the whole operation over which the walk and the PTE entry
write occurs, solving the race.

It is safe to do this here, as no spinlocks are held upon entry to
__change_page_attr_set_clr().

However, the lock must not be held over an allocation, as allocation can
trigger reclaim and shrinkers may call into CPA recursively, making
deadlocks possible (init_mm -> ... -> fs_reclaim -> init_mm).

A page table is allocated when a huge page needs to be split:

-> change_page_attr_set_clr()
-> __change_page_attr_set_clr()
-> __change_page_attr()
-> split_large_page()
[ pagetable_alloc() ]
-> __split_large_page()

Avoid deadlocks by dropping the mmap lock across pagetable_alloc() in
split_large_page() and track whether this is needed by adding a new
'init_mm_read_locked' flag to struct cpa_data.

This is safe as __split_large_page() (called with locks re-established)
revalidates that the page table entry is the same as it was prior to the
locks being dropped and __change_page_attr() repeats the entire page table
walk whenever a split occurs, so concurrent split and collapse are
accounted for.

Concurrent ptdump is also safe as the lock is only dropped over page table
allocation during which time the page table has not yet been modified.

The CPA_COLLAPSE flag is only set by set_memory_rox(), which exclusively
operates upon vmalloc ranges, and on x86 only within the module mapping
space.

This is important, because some callers directly invoke
__change_page_attr_set_clr(), bypassing this lock. However, none of these
operate within the module mapping space.

* cpa_process_alias() - a recursive helper called by
  __change_page_attr_set_clr().
* __set_memory_enc_pgtable() - operates on the direct mapping and (via
  __vmbus_establish_gpadl()) the vmalloc mapping space.
* __set_pages_[n]p() - called by set_direct_map_[invalid, default,
  valid]_noflush(), __kernel_map_pages() - operates on the direct map.
* kernel_[un]map_pages_in_pgd() - operates on EFI ranges.

This work is based upon Denis V. Lunev's excellent analysis of the bug with
gratitude.

Link: https://lore.kernel.org/all/20260626163213.2284080-1-den@openvz.org/
Fixes: 41d88484c71c ("x86/mm/pat: restore large ROX pages after fragmentation")
Cc: stable@vger.kernel.org
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
---
 arch/x86/mm/pat/set_memory.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
index c18b887ee4c2..2d04a4bf34aa 100644
--- a/arch/x86/mm/pat/set_memory.c
+++ b/arch/x86/mm/pat/set_memory.c
@@ -50,7 +50,8 @@ struct cpa_data {
 	unsigned int	flags;
 	unsigned int	force_split		: 1,
 			force_static_prot	: 1,
-			force_flush_all		: 1;
+			force_flush_all		: 1,
+			init_mm_read_locked	: 1;
 	struct page	**pages;
 };
 
@@ -1240,7 +1241,11 @@ static int split_large_page(struct cpa_data *cpa, pte_t *kpte,
 	struct ptdesc *ptdesc;
 
 	spin_unlock(&cpa_lock);
+	if (cpa->init_mm_read_locked)
+		mmap_read_unlock(&init_mm);
 	ptdesc = pagetable_alloc(GFP_KERNEL, 0);
+	if (cpa->init_mm_read_locked)
+		mmap_read_lock(&init_mm);
 	spin_lock(&cpa_lock);
 	if (!ptdesc)
 		return -ENOMEM;
@@ -2134,7 +2139,11 @@ static int change_page_attr_set_clr(unsigned long *addr, int numpages,
 	cpa.curpage = 0;
 	cpa.force_split = force_split;
 
-	ret = __change_page_attr_set_clr(&cpa, 1);
+	/* Avoid race with concurrent CPA collapse. */
+	cpa.init_mm_read_locked = true;
+	scoped_guard(mmap_read_lock, &init_mm)
+		ret = __change_page_attr_set_clr(&cpa, 1);
+	cpa.init_mm_read_locked = false;
 
 	/*
 	 * Check whether we really changed something:

-- 
2.53.0

[PATCH v2 3/5] x86/alternative: exclude text poking against change_page_attr()

[Mike Rapoport]

From: Pedro Falcato <pfalcato@suse.de>

From time to time, the following BUG can be observed[0]:

> kernel BUG at arch/x86/kernel/alternative.c:2576!
> Oops: invalid opcode: 0000 [#1] SMP NOPTI
> CPU: 0 UID: 0 PID: 355 Comm: (udev-worker) Not tainted 7.1.3-1-default #1 PREEMPT(full) openSUSE Tumbleweed  8c1795b03ec64f997e57a8ad38b1161e3b98da64
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS unknown 02/02/2022
> RIP: 0010:__text_poke+0x2aa/0x450
> Call Trace:
>  <TASK>
>  smp_text_poke_batch_finish+0x2a7/0x320
>  __static_call_transform+0xb7/0x220
>  arch_static_call_transform+0x5b/0xb0
>  __static_call_init+0xe9/0x270
>  static_call_module_notify+0x11f/0x150
>  notifier_call_chain+0x61/0xe0
>  blocking_notifier_call_chain_robust+0x63/0xc0
>  load_module+0x1c92/0x20c0
>  init_module_from_file+0xd8/0x140
>  idempotent_init_module+0x100/0x2f0
>  __x64_sys_finit_module+0x71/0xe0
>  do_syscall_64+0xe1/0x610
>  entry_SYSCALL_64_after_hwframe+0x76/0x7e

which matches the following BUG_ON in alternative.c:
	/*
	 * If something went wrong, crash and burn since recovery paths are not
	 * implemented.
	 */
	BUG_ON(!pages[0] || (cross_page_boundary && !pages[1]));

This can happen if vmalloc_to_page() fails, for any reason. Such can happen
if text poking races with CPA, which can possibly result in the collapsing
of page tables (or breaking of PMD hugepages). It is not a problem for most
users of vmalloc_to_page() (they solely own the vmalloc'd range) but, when
CONFIG_ARCH_HAS_EXECMEM_ROX=y, various modules own a single execmem vmalloc
range, and can call set_memory_*() in parallel on it. This can happen to
race against __text_poke and cause havoc in vmalloc_to_page().

Fix it by excluding against CPA using the init_mm mmap read lock.

Fixes: 64f6a4e10c05 ("x86: re-enable EXECMEM_ROX support")
Reported-by: Jiri Slaby <jirislaby@kernel.org>
Link: https://bugzilla.opensuse.org/show_bug.cgi?id=1271202 [0]
Reported-by: Steffen Dirkwinkel <lists@steffen.cc>
Link: https://lore.kernel.org/linux-mm/555ea1d43a12c30a8f1eaf10c899b3790d728f33.camel@dirkwinkel.cc/
Cc: stable@vger.kernel.org
Co-developed-by: "Lorenzo Stoakes (ARM)" <ljs@kernel.org>
Signed-off-by: "Lorenzo Stoakes (ARM)" <ljs@kernel.org>
Signed-off-by: Pedro Falcato <pfalcato@suse.de>
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
---
 arch/x86/kernel/alternative.c | 39 ++++++++++++++++++++++++++++++++++++---
 1 file changed, 36 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 62936a3bde19..f81d6bc90a6f 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -6,6 +6,9 @@
 #include <linux/vmalloc.h>
 #include <linux/memory.h>
 #include <linux/execmem.h>
+#include <linux/cleanup.h>
+#include <linux/kgdb.h>
+#include <linux/mmap_lock.h>
 
 #include <asm/text-patching.h>
 #include <asm/insn.h>
@@ -2543,6 +2546,38 @@ static void text_poke_memset(void *dst, const void *src, size_t len)
 
 typedef void text_poke_f(void *dst, const void *src, size_t len);
 
+static void __poke_vmalloc_pages(struct page **pages, void *addr,
+				 bool cross_page_boundary)
+{
+	pages[0] = vmalloc_to_page(addr);
+	if (cross_page_boundary)
+		pages[1] = vmalloc_to_page(addr + PAGE_SIZE);
+}
+
+static void poke_vmalloc_pages(struct page **pages, void *addr,
+			       bool cross_page_boundary)
+{
+	if (in_dbg_master()) {
+		/*
+		 * If called from kgdb cannot sleep, but all other CPUs stopped
+		 * anyway so safe to proceed without locks
+		 */
+		__poke_vmalloc_pages(pages, addr, cross_page_boundary);
+	} else {
+		/*
+		 * execmem ROX ranges are shared between modules and can be
+		 * collapsed to huge PMD entries, and this collapse can happen
+		 * concurrently with a racing set_memory_rox().
+		 *
+		 * Prevent vmalloc_to_page() from racing by acquiring an
+		 * init_mm read lock which pairs with the init_mm write lock in
+		 * cpa_collapse_large_pages().
+		 */
+		guard(mmap_read_lock)(&init_mm);
+		__poke_vmalloc_pages(pages, addr, cross_page_boundary);
+	}
+}
+
 static void *__text_poke(text_poke_f func, void *addr, const void *src, size_t len)
 {
 	bool cross_page_boundary = offset_in_page(addr) + len > PAGE_SIZE;
@@ -2560,9 +2595,7 @@ static void *__text_poke(text_poke_f func, void *addr, const void *src, size_t l
 	BUG_ON(!after_bootmem);
 
 	if (!core_kernel_text((unsigned long)addr)) {
-		pages[0] = vmalloc_to_page(addr);
-		if (cross_page_boundary)
-			pages[1] = vmalloc_to_page(addr + PAGE_SIZE);
+		poke_vmalloc_pages(pages, addr, cross_page_boundary);
 	} else {
 		pages[0] = virt_to_page(addr);
 		WARN_ON(!PageReserved(pages[0]));

-- 
2.53.0

[PATCH v2 4/5] x86/mm/pat: allocate split page tables as kernel page tables

[Mike Rapoport]

From: "Lorenzo Stoakes (ARM)" <ljs@kernel.org>

When splitting a large page in CPA in __split_large_page() we allocate a
PTE directly without going through the standard page table allocation
routines such as pte_alloc_one_kernel().

This means the page table constructor is never called nor is the page table
marked as a kernel page table.

The former results in the folio associated with the page table not being
marked as a page table (__pagetable_ctor() is never called thus neither is
__folio_set_pgtable()) nor are statistics updated to reflect
it (lruvec_stat_add_folio() is never called).

The latter issue of failing to mark the page table as a kernel page
table (ptdesc_set_kernel() is never called) is far more problematic.

Since commit 5ba2f0a15564 ("mm: introduce deferred freeing for kernel page
tables") kernel page table freeing has been batched and since the
subsequent commit e37d5a2d60a3 ("iommu/sva: invalidate stale IOTLB entries
for kernel address space") IOTLB cache entries for kernel page tables have
been invalidated upon being freed.

Since split page tables are freed without this invalidation, the IOTLB can
contain stale entries for them.

Resolve the issue by using the ordinary PTE allocation API at split time.

This results in these kernel page tables invoking a page table constructor,
and thus requires a page table destructor.

Since we cannot assume one is always present (early allocated direct map
page tables are not marked as such), we conditionally call
pagetable_dtor_free() if the PG_table folio flag for the ptdesc is set,
otherwise we free the page table via pagetable_free().

Regardless of which path is taken page tables marked as kernel page tables,
which now includes split page tables, take the correct route through
pagetable_free_kernel().

There is a user-visible side effect in that split page tables will appear
in nr_page_table_pages in /proc/vmstat (as do other kernel page tables
allocated after early boot), however this is a positive change.

This issue started being markedly problematic after commit
5ba2f0a15564 ("mm: introduce deferred freeing for kernel page tables") so
choose this as the Fixes target.

Fixes: 5ba2f0a15564 ("mm: introduce deferred freeing for kernel page tables")
Cc: stable@vger.kernel.org
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Acked-by: Vishal Moola <vishal.moola@gmail.com>
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
---
 arch/x86/mm/pat/set_memory.c | 25 ++++++++++++++++---------
 1 file changed, 16 insertions(+), 9 deletions(-)

diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
index 2d04a4bf34aa..fbc418dfc597 100644
--- a/arch/x86/mm/pat/set_memory.c
+++ b/arch/x86/mm/pat/set_memory.c
@@ -441,7 +441,15 @@ static void __cpa_collapse_large_pages(struct cpa_data *cpa)
 
 	list_for_each_entry_safe(ptdesc, tmp, &pgtables, pt_list) {
 		list_del(&ptdesc->pt_list);
-		pagetable_free(ptdesc);
+		/*
+		 * Only early alloc'd direct map should not be flagged PG_table
+		 * here and those shouldn't be collapsed. However be abundantly
+		 * cautious and handle the !PG_table case too.
+		 */
+		if (PageTable((ptdesc_page(ptdesc))))
+			pagetable_dtor_free(ptdesc);
+		else
+			pagetable_free(ptdesc);
 	}
 }
 
@@ -1134,11 +1142,10 @@ static void split_set_pte(struct cpa_data *cpa, pte_t *pte, unsigned long pfn,
 
 static int
 __split_large_page(struct cpa_data *cpa, pte_t *kpte, unsigned long address,
-		   struct ptdesc *ptdesc)
+		   pte_t *pbase)
 {
 	unsigned long lpaddr, lpinc, ref_pfn, pfn, pfninc = 1;
-	struct page *base = ptdesc_page(ptdesc);
-	pte_t *pbase = (pte_t *)page_address(base);
+	struct page *base = virt_to_page(pbase);
 	unsigned int i, level;
 	pgprot_t ref_prot;
 	bool nx, rw;
@@ -1238,20 +1245,20 @@ __split_large_page(struct cpa_data *cpa, pte_t *kpte, unsigned long address,
 static int split_large_page(struct cpa_data *cpa, pte_t *kpte,
 			    unsigned long address)
 {
-	struct ptdesc *ptdesc;
+	pte_t *pte;
 
 	spin_unlock(&cpa_lock);
 	if (cpa->init_mm_read_locked)
 		mmap_read_unlock(&init_mm);
-	ptdesc = pagetable_alloc(GFP_KERNEL, 0);
+	pte = pte_alloc_one_kernel(&init_mm);
 	if (cpa->init_mm_read_locked)
 		mmap_read_lock(&init_mm);
 	spin_lock(&cpa_lock);
-	if (!ptdesc)
+	if (!pte)
 		return -ENOMEM;
 
-	if (__split_large_page(cpa, kpte, address, ptdesc))
-		pagetable_free(ptdesc);
+	if (__split_large_page(cpa, kpte, address, pte))
+		pte_free_kernel(&init_mm, pte);
 
 	return 0;
 }

-- 
2.53.0

[PATCH v2 5/5] x86/mm/pat: fix effective RW computation in lookup_address_in_pgd_attr()

[Mike Rapoport (Microsoft)]

lookup_address_in_pgd_attr() accumulates the effective NX and RW bits of
the walked page table levels so that verify_rwx() can detect mappings that
are both writable and executable.

The RW bits are folded into a bool with

	rw &= pXd_flags(*pXd) & _PAGE_RW;

but _PAGE_RW is 0x2. So consider the accumulation line:

        rw &= pXd_flags(*pXd) & _PAGE_RW;

where rw=0x1 and the right side evaluates down to 0x2. It'll end up doing:

        rw = 0x1 & 0x2

and rw always ends up 0.

This way rw becomes false at the first level walked, regardless of the
actual permissions, and verify_rwx() treats every mapping as non-writable
and never reports a W^X violation.

Add double negation to the right side to normalize the _PAGE_RW flag to
0 or 1.

Fixes: ceb647b4b529 ("x86/pat: Introduce lookup_address_in_pgd_attr()")
Cc: stable@vger.kernel.org
Assisted-by: Copilot:claude-opus-4.8
Reviewed-by: Juergen Gross <jgross@suse.com>
Tested-by: syzbot@syzkaller.appspotmail.com
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
---
 arch/x86/mm/pat/set_memory.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
index fbc418dfc597..430d0b448371 100644
--- a/arch/x86/mm/pat/set_memory.c
+++ b/arch/x86/mm/pat/set_memory.c
@@ -754,7 +754,7 @@ pte_t *lookup_address_in_pgd_attr(pgd_t *pgd, unsigned long address,
 
 	*level = PG_LEVEL_512G;
 	*nx |= pgd_flags(*pgd) & _PAGE_NX;
-	*rw &= pgd_flags(*pgd) & _PAGE_RW;
+	*rw &= !!(pgd_flags(*pgd) & _PAGE_RW);
 
 	p4d = p4d_offset(pgd, address);
 	if (p4d_none(*p4d))
@@ -765,7 +765,7 @@ pte_t *lookup_address_in_pgd_attr(pgd_t *pgd, unsigned long address,
 
 	*level = PG_LEVEL_1G;
 	*nx |= p4d_flags(*p4d) & _PAGE_NX;
-	*rw &= p4d_flags(*p4d) & _PAGE_RW;
+	*rw &= !!(p4d_flags(*p4d) & _PAGE_RW);
 
 	pud = pud_offset(p4d, address);
 	if (pud_none(*pud))
@@ -776,7 +776,7 @@ pte_t *lookup_address_in_pgd_attr(pgd_t *pgd, unsigned long address,
 
 	*level = PG_LEVEL_2M;
 	*nx |= pud_flags(*pud) & _PAGE_NX;
-	*rw &= pud_flags(*pud) & _PAGE_RW;
+	*rw &= !!(pud_flags(*pud) & _PAGE_RW);
 
 	pmd = pmd_offset(pud, address);
 	if (pmd_none(*pmd))
@@ -787,7 +787,7 @@ pte_t *lookup_address_in_pgd_attr(pgd_t *pgd, unsigned long address,
 
 	*level = PG_LEVEL_4K;
 	*nx |= pmd_flags(*pmd) & _PAGE_NX;
-	*rw &= pmd_flags(*pmd) & _PAGE_RW;
+	*rw &= !!(pmd_flags(*pmd) & _PAGE_RW);
 
 	return pte_offset_kernel(pmd, address);
 }

-- 
2.53.0

Re: [PATCH v2 5/5] x86/mm/pat: fix effective RW computation in lookup_address_in_pgd_attr()

[Lorenzo Stoakes (ARM)]

On Thu, Aug 13, 2026 at 12:01:28PM +0300, Mike Rapoport (Microsoft) wrote:
> lookup_address_in_pgd_attr() accumulates the effective NX and RW bits of
> the walked page table levels so that verify_rwx() can detect mappings that
> are both writable and executable.
>
> The RW bits are folded into a bool with
>
> 	rw &= pXd_flags(*pXd) & _PAGE_RW;
>
> but _PAGE_RW is 0x2. So consider the accumulation line:
>
>         rw &= pXd_flags(*pXd) & _PAGE_RW;
>
> where rw=0x1 and the right side evaluates down to 0x2. It'll end up doing:
>
>         rw = 0x1 & 0x2
>
> and rw always ends up 0.
>
> This way rw becomes false at the first level walked, regardless of the
> actual permissions, and verify_rwx() treats every mapping as non-writable
> and never reports a W^X violation.
>
> Add double negation to the right side to normalize the _PAGE_RW flag to
> 0 or 1.
>
> Fixes: ceb647b4b529 ("x86/pat: Introduce lookup_address_in_pgd_attr()")
> Cc: stable@vger.kernel.org
> Assisted-by: Copilot:claude-opus-4.8
> Reviewed-by: Juergen Gross <jgross@suse.com>
> Tested-by: syzbot@syzkaller.appspotmail.com
> Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

Yikes, good find! LGTM so:

Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>

> ---
>  arch/x86/mm/pat/set_memory.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
> index fbc418dfc597..430d0b448371 100644
> --- a/arch/x86/mm/pat/set_memory.c
> +++ b/arch/x86/mm/pat/set_memory.c
> @@ -754,7 +754,7 @@ pte_t *lookup_address_in_pgd_attr(pgd_t *pgd, unsigned long address,
>
>  	*level = PG_LEVEL_512G;
>  	*nx |= pgd_flags(*pgd) & _PAGE_NX;
> -	*rw &= pgd_flags(*pgd) & _PAGE_RW;
> +	*rw &= !!(pgd_flags(*pgd) & _PAGE_RW);
>
>  	p4d = p4d_offset(pgd, address);
>  	if (p4d_none(*p4d))
> @@ -765,7 +765,7 @@ pte_t *lookup_address_in_pgd_attr(pgd_t *pgd, unsigned long address,
>
>  	*level = PG_LEVEL_1G;
>  	*nx |= p4d_flags(*p4d) & _PAGE_NX;
> -	*rw &= p4d_flags(*p4d) & _PAGE_RW;
> +	*rw &= !!(p4d_flags(*p4d) & _PAGE_RW);
>
>  	pud = pud_offset(p4d, address);
>  	if (pud_none(*pud))
> @@ -776,7 +776,7 @@ pte_t *lookup_address_in_pgd_attr(pgd_t *pgd, unsigned long address,
>
>  	*level = PG_LEVEL_2M;
>  	*nx |= pud_flags(*pud) & _PAGE_NX;
> -	*rw &= pud_flags(*pud) & _PAGE_RW;
> +	*rw &= !!(pud_flags(*pud) & _PAGE_RW);
>
>  	pmd = pmd_offset(pud, address);
>  	if (pmd_none(*pmd))
> @@ -787,7 +787,7 @@ pte_t *lookup_address_in_pgd_attr(pgd_t *pgd, unsigned long address,
>
>  	*level = PG_LEVEL_4K;
>  	*nx |= pmd_flags(*pmd) & _PAGE_NX;
> -	*rw &= pmd_flags(*pmd) & _PAGE_RW;
> +	*rw &= !!(pmd_flags(*pmd) & _PAGE_RW);
>
>  	return pte_offset_kernel(pmd, address);
>  }
>
> --
> 2.53.0
>

--
Cheers, Lorenzo

Re: [PATCH v2 0/5] x86/mm/pat: CPA fixes

[Nikunj A. Dadhania]

On 8/13/2026 2:31 PM, Mike Rapoport wrote:

> The first three patches are urgent, the third patch fixes BUG() reported
> y several people and it depends on the first two.
> 
> There were no bug reports that the last two patches fix because bug
> manifestations won't yell at users.
> 
> TL;DR version:
> 
> There are a couple of CPA fixes floating around:
> 
> Denis Lunev fixed races between split and collapse of the large mappings:
> 
> https://lore.kernel.org/all/20260715183453.2381141-1-den@openvz.org
> 
> Lorenzo Stoakes fixed UAF caused by races between CPA and ptdump:
> 
> https://lore.kernel.org/all/20260723-series-vmap-race-fix-v6-0-8cc77dcc0018@kernel.org
> 
> and an issue with stale page tables in IOMMU:
> 
> https://lore.kernel.org/all/20260721-fix-cpa-kernel-pagetables-v2-1-2b255deed710@kernel.org
> 
> Mike Rapoport fixed a check of RW attribute in lookup_address_in_pgd_attr()
> used for the verification of RWX:
> 
> https://lore.kernel.org/all/20260715144519.934289-1-rppt@kernel.org
> 
> Pedro Falcato closed a race between text poking and collapse of large
> pages:
> 
> https://lore.kernel.org/all/anCK3eWFMwZqq5ka@pedro-suse
> 
> Some of the fixes got merged into x86 tree, some of them got merged into mm
> tree and some are still hanging in the air.
> 
> The changes here are collected from all these fixes into a single coherent
> set on top of tip/x86/mm:
> 
> * fix for races between CPA and ptdump causing UAF
> * update to the fix of the race between split and collapse of large
>   mappings
> * fix for races between CPA and vmalloc_to_page() in text poking
> * fix for stale page tables in IOMMU
> * fix for effective RW computation in lookup_address_in_pgd_attr()
> 
> ---
> v2 changes:
> * rebased on the current tip/x86/mm that includes peterz's changes for
>   DEBUG_PAGEALLOC
> * added fix for CPA vs text poking race

Tested v2 on a 512-guest concurrent boot harness across 200 iterations
with no splats. Both crash signatures reported in [1] are no longer seen.

Tested-by: Nikunj A Dadhania <nikunj@amd.com>

Regards,
Nikunj

[1] https://lore.kernel.org/all/20260812063316.21371-1-nikunj@amd.com/
 
> v1: https://patch.msgid.link/20260728-cpa-fixes-v1-0-2ed2352300b3@kernel.org
> 
> ---
> Lorenzo Stoakes (ARM) (3):
>       x86/mm/pat: acquire init_mm write lock on collapse to avoid UAF
>       x86/mm/pat: acquire init_mm read lock on attribute change to avoid UAF
>       x86/mm/pat: allocate split page tables as kernel page tables
> 
> Mike Rapoport (Microsoft) (1):
>       x86/mm/pat: fix effective RW computation in lookup_address_in_pgd_attr()
> 
> Pedro Falcato (1):
>       x86/alternative: exclude text poking against change_page_attr()
> 
>  arch/x86/kernel/alternative.c | 39 ++++++++++++++++++++++++---
>  arch/x86/mm/pat/set_memory.c  | 61 +++++++++++++++++++++++++++++++------------
>  include/linux/mmap_lock.h     |  2 ++
>  3 files changed, 83 insertions(+), 19 deletions(-)
> ---
> base-commit: 7da514d819a0afb148634aac92b3d190f34947c3
> change-id: 20260727-cpa-fixes-d3c73c075672
> 
> --
> Sincerely yours,
> Mike.
> 

Re: [PATCH v2 0/5] x86/mm/pat: CPA fixes

[Lorenzo Stoakes (ARM)]

On Thu, Aug 13, 2026 at 08:35:21PM +0530, Nikunj A. Dadhania wrote:
>
> Tested v2 on a 512-guest concurrent boot harness across 200 iterations
> with no splats. Both crash signatures reported in [1] are no longer seen.
>
> Tested-by: Nikunj A Dadhania <nikunj@amd.com>

Thanks so much for the testing!

And all credit to Pedro for reporting and contributing the key commit that
resolves this issue :)

>
> Regards,
> Nikunj
>
> [1] https://lore.kernel.org/all/20260812063316.21371-1-nikunj@amd.com/
>
> > v1: https://patch.msgid.link/20260728-cpa-fixes-v1-0-2ed2352300b3@kernel.org
> >
> > ---
> > Lorenzo Stoakes (ARM) (3):
> >       x86/mm/pat: acquire init_mm write lock on collapse to avoid UAF
> >       x86/mm/pat: acquire init_mm read lock on attribute change to avoid UAF
> >       x86/mm/pat: allocate split page tables as kernel page tables
> >
> > Mike Rapoport (Microsoft) (1):
> >       x86/mm/pat: fix effective RW computation in lookup_address_in_pgd_attr()
> >
> > Pedro Falcato (1):
> >       x86/alternative: exclude text poking against change_page_attr()
> >
> >  arch/x86/kernel/alternative.c | 39 ++++++++++++++++++++++++---
> >  arch/x86/mm/pat/set_memory.c  | 61 +++++++++++++++++++++++++++++++------------
> >  include/linux/mmap_lock.h     |  2 ++
> >  3 files changed, 83 insertions(+), 19 deletions(-)
> > ---
> > base-commit: 7da514d819a0afb148634aac92b3d190f34947c3
> > change-id: 20260727-cpa-fixes-d3c73c075672
> >
> > --
> > Sincerely yours,
> > Mike.
> >
>

--
Cheers, Lorenzo

Re: [PATCH v2 0/5] x86/mm/pat: CPA fixes

[Pedro Falcato]

On Thu, Aug 13, 2026 at 04:07:38PM +0100, Lorenzo Stoakes (ARM) wrote:
> On Thu, Aug 13, 2026 at 08:35:21PM +0530, Nikunj A. Dadhania wrote:
> >
> > Tested v2 on a 512-guest concurrent boot harness across 200 iterations
> > with no splats. Both crash signatures reported in [1] are no longer seen.
> >
> > Tested-by: Nikunj A Dadhania <nikunj@amd.com>

Nice!

> 
> Thanks so much for the testing!
> 
> And all credit to Pedro for reporting and contributing the key commit that
> resolves this issue :)

And all credit to you for starting this flurry of fixes and patches that
solve Critical Kernel Issues(tm)! My fix would be nothing without yours!

-- 
Pedro