[PATCH mm-hotfixes v5 0/5] mm: fix UAF caused by race between ptdump and vmap pgtable freeing

[PATCH mm-hotfixes v5 0/5] mm: fix UAF caused by race between ptdump and vmap pgtable freeing

[Lorenzo Stoakes (ARM)]

Kernel page table walkers fall into two broad categories - those ranges
where no exclusion is required via walk_kernel_page_table_range_lockless()
and those where exclusion is required via walk_kernel_page_table_range()
or walk_page_range_debug().

The former category is used only by arm64 arch code operating on ranges it
both wholly owns and does not concurrently write.

The latter category consists of kernel page table walkers operating on
ranges that are wholly owned (but which need exclusion against concurrent
writers).

The lock used for exclusion is the mmap lock, and for kernel ranges this
the mmap lock on init_mm.

ptdump is a special case being both the only user of
walk_page_range_debug(), and the only case in which it walks ranges it does
not own.

This presents a problem, as page tables may be freed under ptdump. And
indeed there is a use-after-free bug in the kernel as a result, which this
series addresses.

vmap promotes page tables to huge leaf entries where possible, freeing the
lower page table when it does. It does this with no meaningful locks held
against concurrent ptdump walks.

As a result, use-after-free can currently occur. This series addresses the
issue by having the vmap huge promotion logic acquire the mmap read lock
while both setting the huge page table entry and freeing the prior leaf
page table.

The ptdump code already acquires the mmap write lock, so by doing so we
ensure that the ptdump walker only ever observes either the huge page table
entry or the existing page table entry, and nothing is freed underneath it.

A mitigation for this issue was already applied for arm64 in commit
fa93b45fd397 ("arm64: Enable vmalloc-huge with ptdump"), which this series
has to deal with carefully.

This mitigation resolves the issue by acquiring the mmap read lock on
init_mm on vmap page table free if a ptdump is in progress.

However the fix in this series would cause a deadlock if we were to simply
apply it for arm64 without also reverting the change.

This is because vmap may acquire the read lock before ptdump attempts to
acquire the write lock, which then gets queued, and rwsem starvation rules
mean that the (unacknowledged) nested mmap read lock in the arm64 code
would also block, meaning the original read lock is never released and thus
deadlock.

This series works around this by #ifndef CONFIG_ARM64'ing the mmap read
lock in vmap logic, then partially reverting commit fa93b45fd397 ("arm64:
Enable vmalloc-huge with ptdump"), keeping the enablement of huge vmap
support, and removing the ifdeffery with the partial revert patch.

There are related issues that we also address in this series:

* x86 page attribute logic, specifically Change Page Attributes (CPA),
  implements a feature whereby huge ranges can be collapsed into huge leaf
  entries. This can similarly cause a UAF when done in parallel with a
  ptdump walk, so similarly acquire the init_mm mmap lock to avoid this.

* The CPA logic allows concurrent page table manipulation and CPA
  collapse, meaning the former risks accessing a page table the latter
  frees. Fix this by acquiring mmap write lock on init_mm across the
  whole CPA collapse operation and read lock on the page table
  manipulation.

* x86 and arm64 permit walks of non-kernel mm's (both allowing efi mm
  walks, and in intel's case arbitrary mm's), so we ensure kernel mappings
  remain stable by locking the init_mm as well as the mm being walked.

The ordering of patches is established for both strict dependencies (the
arm64 partial revert in particular has to be done after the vmap changes)
and logical ones (the non-kernel mm fix only makes sense once the vmap/CPA
fixes are in place).

---
v5:
* Rebased on latest master of Linus's tree.
* Accumulated tags, thanks everybody!
* Added additional commit to fix race between CPA collapse/attribute set.
* Slight commit message tweaks.

v4:
* Rebased on latest master of Linus's tree.
* Accumulated tags, thanks everybody!
* Updated commit messages further as per Kiryl.
* Took the mmap write lock across the whole CPA operation as per Will.
https://patch.msgid.link/20260716-series-vmap-race-fix-v4-0-8c108c4317df@kernel.org

v3:
* Rebased on latest master of Linus's tree.
* Accumulated tags, thanks everybody!
* Reworded commit messages as per Kiryl and Boris.
https://patch.msgid.link/20260714-series-vmap-race-fix-v3-0-b812eccfa0f9@kernel.org

v2:
* Rebased on latest master of Linus's tree.
* Accumulated tags, thanks everybody!
* Fixed cover letter reference to arm64 partial revert as per David C.
* Combined all patches into a 4 patch series for ease of tracking/review
  and updated cover letter to reflect.
* Reordered patches in series logically - fix vmap, fix CPA issue, handle
  mm vs. init_mm then revert arm64 mitigation.
* Reworded first patch to be consistent with x86 wording to clearly
  indicate the intent of the fix is to fix ptdump UAF.
* Reworded arm64 revert patch subject as per Mike, Dev to make clear this
  is only reverting the ptdump mitigation not the vmap huge support.
* Added Fixes: tag to arm64 revert patch so we backport this also for
  neatness. Not strictly necessary, but is a better fix overall applied.
* Added note about stable dependency to arm64 revert patch.
* Updated walk_page_range_debug() to remove pointless !walk.mm check - if
  NULL mm then the mmap lock asserts would NULL pointer deref, and of
  course no caller does this anyway.
* Updated walk_page_range_debug() to always assert init_mm mmap write lock
  is held.
* Updated walk_page_range_debug() to always check for start >= end and that
  it has safe walk ops, and remove the unnecessary
  walk_kernel_page_table_range() invocation which ultimately does the same
  thing.
* Typo fixups as per Mike.
* Some small commit message/comment wording fixups.
https://patch.msgid.link/20260712-series-vmap-race-fix-v2-0-ad134cc3a12a@kernel.org

v1:
* vmap/arm64 partial revert series:
  https://patch.msgid.link/20260710-series-vmap-race-fix-v1-0-5b3794c113fe@kernel.org
* CPA patch:
  https://patch.msgid.link/20260710-fix-cpa-ptdump-race-v1-1-d898699a7417@kernel.org
* non-init_mm patch:
  https://patch.msgid.link/20260710-b4-fix-non-init_mm-ptdump-v1-1-2d40982c98ec@kernel.org

To: Andrew Morton <akpm@linux-foundation.org>
To: Suren Baghdasaryan <surenb@google.com>
To: "Liam R. Howlett" <liam@infradead.org>
To: Vlastimil Babka <vbabka@kernel.org>
To: Shakeel Butt <shakeel.butt@linux.dev>
To: David Hildenbrand <david@kernel.org>
To: Mike Rapoport <rppt@kernel.org>
To: Michal Hocko <mhocko@suse.com>
To: Uladzislau Rezki <urezki@gmail.com>
To: Toshi Kani <toshi.kani@hpe.com>
To: Dave Hansen <dave.hansen@linux.intel.com>
To: Andy Lutomirski <luto@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
To: Thomas Gleixner <tglx@kernel.org>
To: Ingo Molnar <mingo@redhat.com>
To: Borislav Petkov <bp@alien8.de>
To: x86@kernel.org
To: "H. Peter Anvin" <hpa@zytor.com>
To: Kiryl Shutsemau <kas@kernel.org>
To: Catalin Marinas <catalin.marinas@arm.com>
To: Will Deacon <will@kernel.org>
To: Dev Jain <dev.jain@arm.com>
To: Ryan Roberts <ryan.roberts@arm.com>
Cc: David Carlier <devnexen@gmail.com>
Cc: ljs@kernel.org
Cc: linux-mm@kvack.org
Cc: linux-kernel@vger.kernel.org
Cc: bpf@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org
Cc:  "Denis V. Lunev" <den@virtuozzo.com>

---
Lorenzo Stoakes (ARM) (5):
      mm/vmalloc: acquire init_mm lock on huge vmap to avoid ptdump UAF
      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
      mm/ptdump: always stabilise against page table freeing using init_mm
      arm64: remove redundant concurrent ptdump UAF mitigation

 arch/arm64/include/asm/ptdump.h |  2 --
 arch/arm64/mm/mmu.c             | 43 ++++-------------------------------------
 arch/arm64/mm/ptdump.c          | 11 ++---------
 arch/x86/mm/pat/set_memory.c    | 19 ++++++++++++++++--
 include/linux/mmap_lock.h       |  3 +++
 mm/pagewalk.c                   | 36 ++++++++++++++++++++--------------
 mm/ptdump.c                     |  7 +++++++
 mm/vmalloc.c                    | 40 +++++++++++++++++++++++++++++---------
 8 files changed, 85 insertions(+), 76 deletions(-)
---
base-commit: fce2dfa773ced15f27dd27cd0b482a7473cdcf2a
change-id: 20260710-series-vmap-race-fix-2a4cac988938

Cheers,
-- 
Lorenzo Stoakes (ARM) <ljs@kernel.org>

[PATCH mm-hotfixes v5 1/5] mm/vmalloc: acquire init_mm lock on huge vmap to avoid ptdump UAF

[Lorenzo Stoakes (ARM)]

Currently there is a nasty race between ptdump and vmap when attempting to
map a huge P4D, PUD or PMD entry:

* ptdump walks kernel page table ranges it doesn't own.

* When vmap maps ranges it tries to promotes existing ones to huge page
  tables in vmap_try_huge_[p4d,pud,pmd]() at P4D, PUD and PMD level,
  freeing the lower page table in [p4d,pud,pmd]_free_[pud,pmd,pte]_page()
  when it succeeds.

Both of these things can happen at the same time and as a result ptdump can
access a freed page table, resulting in a use-after-free and memory
corruption.

This is possible because while ptdump_walk_pgd() holds both the mem hotplug
lock and the mmap write lock before invoking walk_page_range_debug(), vmap
takes no relevant locks at all.

Fix this by holding the mmap read lock in vmap_try_huge_*() when freeing
page tables.

The read lock is sufficient: ptdump is the only walker that must be
excluded and it holds the mmap write lock.  Other holders of the read lock
may run concurrently, but each exclusively owns the range it operates on
and cannot reach the page tables freed here.

We also hold the lock while assigning the huge page table entry, which
means page table walkers observe only the huge or non-huge page table
entry.

We use a trylock to prevent ptdump from blocking vmap making forward
progress. This is fine because it's an optimisation in any case, and thus
the vmap can safely proceed regardless.

All other kernel page table walkers that touch vmalloc ranges either
exclusively own the memory walked or acquire the mmap lock, so this
correctly excludes those walkers.

One wrinkle here is commit fa93b45fd397 ("arm64: Enable vmalloc-huge with
ptdump"), which addresses the issue for arm64 only by explicitly acquiring
the mmap read lock on kernel page table freeing should a concurrent ptdump
be in progress.

This is problematic as vmap may acquire the mmap read lock prior to ptdump
attempting to acquire an mmap write lock, leading to a deadlock when the
mmap read lock is slept upon on page table freeing due to rwsem
anti-starvation.

We work around this by predicating the mmap lock being taken on
!CONFIG_ARM64 for the time being.

With this patch applied, a follow up will partially revert commit
fa93b45fd397 ("arm64: Enable vmalloc-huge with ptdump") and at that stage
remove the arm64 ifdeffery.

We also update walk_page_range_debug() to assert the mmap write lock
unconditionally and update the comment here to reflect this change.

The issue has existed as long as ptdump was available and vmap freed page
tables when promoting to a huge leaf entry, that is, since commit
b6bdb7517c3d ("mm/vmalloc: add interfaces to free unmapped page table") for
huge ioremap, and commit 121e6f3258fe ("mm/vmalloc: hugepage vmalloc
mappings") for huge vmalloc.

Since the former is the earlier of the two we choose that for our Fixes
tag.

We also define a guard class for mmap_read_trylock() so we can use
cleanup.h to make the scope handling cleaner in the implementation.

This patch is based on work by David Carlier (linked), with gratitude!

Fixes: b6bdb7517c3d ("mm/vmalloc: add interfaces to free unmapped page table")
Cc: stable@vger.kernel.org
Reported-by: syzbot+fd95a72470f5a44e464c@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/6a287988.39669fcc.33b062.00a0.GAE@google.com/T/
Link: https://lore.kernel.org/linux-mm/20260706203128.162335-1-devnexen@gmail.com/
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Reviewed-by: Dev Jain <dev.jain@arm.com>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Kiryl Shutsemau <kas@kernel.org>
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
---
 include/linux/mmap_lock.h |  1 +
 mm/pagewalk.c             | 22 +++++++++++----------
 mm/vmalloc.c              | 49 ++++++++++++++++++++++++++++++++++++++---------
 3 files changed, 53 insertions(+), 19 deletions(-)

diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
index 04b8f61ece5d..6b5c2390cc30 100644
--- a/include/linux/mmap_lock.h
+++ b/include/linux/mmap_lock.h
@@ -621,6 +621,7 @@ 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_COND(mmap_read_lock, _try, mmap_read_trylock(_T))
 
 static inline void mmap_read_unlock_non_owner(struct mm_struct *mm)
 {
diff --git a/mm/pagewalk.c b/mm/pagewalk.c
index 3ae2586ff45b..bbcfd68d0907 100644
--- a/mm/pagewalk.c
+++ b/mm/pagewalk.c
@@ -678,6 +678,8 @@ int walk_kernel_page_table_range_lockless(unsigned long start, unsigned long end
  * will also not lock the PTEs for the pte_entry() callback.
  *
  * This is for debugging purposes ONLY.
+ *
+ * The mmap write lock must be held.
  */
 int walk_page_range_debug(struct mm_struct *mm, unsigned long start,
 			  unsigned long end, const struct mm_walk_ops *ops,
@@ -691,6 +693,16 @@ int walk_page_range_debug(struct mm_struct *mm, unsigned long start,
 		.no_vma		= true
 	};
 
+	/*
+	 * When walking userland page tables, an mmap write lock must be held to
+	 * account for munmap() downgrading to an mmap read lock when tearing
+	 * down page tables.
+	 *
+	 * When walking kernel page tables, an mmap write lock must also be held
+	 * to account for page table freeing on vmap huge page mapping.
+	 */
+	mmap_assert_write_locked(mm);
+
 	/* For convenience, we allow traversal of kernel mappings. */
 	if (mm == &init_mm)
 		return walk_kernel_page_table_range(start, end, ops,
@@ -700,16 +712,6 @@ int walk_page_range_debug(struct mm_struct *mm, unsigned long start,
 	if (!check_ops_safe(ops))
 		return -EINVAL;
 
-	/*
-	 * The mmap lock protects the page walker from changes to the page
-	 * tables during the walk.  However a read lock is insufficient to
-	 * protect those areas which don't have a VMA as munmap() detaches
-	 * the VMAs before downgrading to a read lock and actually tearing
-	 * down PTEs/page tables. In which case, the mmap write lock should
-	 * be held.
-	 */
-	mmap_assert_write_locked(mm);
-
 	return walk_pgd_range(start, end, &walk);
 }
 
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 1afca3568b9b..d5c4d2bb770b 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -43,6 +43,7 @@
 #include <asm/tlbflush.h>
 #include <asm/shmparam.h>
 #include <linux/page_owner.h>
+#include <linux/cleanup.h>
 
 #define CREATE_TRACE_POINTS
 #include <trace/events/vmalloc.h>
@@ -158,10 +159,24 @@ static int vmap_try_huge_pmd(pmd_t *pmd, unsigned long addr, unsigned long end,
 	if (!IS_ALIGNED(phys_addr, PMD_SIZE))
 		return 0;
 
-	if (pmd_present(*pmd) && !pmd_free_pte_page(pmd, addr))
-		return 0;
+	if (!pmd_present(*pmd))
+		return pmd_set_huge(pmd, phys_addr, prot);
 
-	return pmd_set_huge(pmd, phys_addr, prot);
+	/*
+	 * Acquire the mmap read lock to exclude ptdump, which walks
+	 * kernel page tables it does not own under the mmap write lock.
+	 *
+	 * Concurrent read lock holders are safe: each exclusively owns
+	 * the range it operates on and cannot reach this page table.
+	 */
+#ifndef CONFIG_ARM64
+	scoped_cond_guard(mmap_read_lock_try, return 0, &init_mm)
+#endif
+	{
+		if (!pmd_free_pte_page(pmd, addr))
+			return 0;
+		return pmd_set_huge(pmd, phys_addr, prot);
+	}
 }
 
 static int vmap_pmd_range(pud_t *pud, unsigned long addr, unsigned long end,
@@ -210,10 +225,18 @@ static int vmap_try_huge_pud(pud_t *pud, unsigned long addr, unsigned long end,
 	if (!IS_ALIGNED(phys_addr, PUD_SIZE))
 		return 0;
 
-	if (pud_present(*pud) && !pud_free_pmd_page(pud, addr))
-		return 0;
+	if (!pud_present(*pud))
+		return pud_set_huge(pud, phys_addr, prot);
 
-	return pud_set_huge(pud, phys_addr, prot);
+	/* See comment in vmap_try_huge_pmd(). */
+#ifndef CONFIG_ARM64
+	scoped_cond_guard(mmap_read_lock_try, return 0, &init_mm)
+#endif
+	{
+		if (!pud_free_pmd_page(pud, addr))
+			return 0;
+		return pud_set_huge(pud, phys_addr, prot);
+	}
 }
 
 static int vmap_pud_range(p4d_t *p4d, unsigned long addr, unsigned long end,
@@ -262,10 +285,18 @@ static int vmap_try_huge_p4d(p4d_t *p4d, unsigned long addr, unsigned long end,
 	if (!IS_ALIGNED(phys_addr, P4D_SIZE))
 		return 0;
 
-	if (p4d_present(*p4d) && !p4d_free_pud_page(p4d, addr))
-		return 0;
+	if (!p4d_present(*p4d))
+		return p4d_set_huge(p4d, phys_addr, prot);
 
-	return p4d_set_huge(p4d, phys_addr, prot);
+	/* See comment in vmap_try_huge_pmd(). */
+#ifndef CONFIG_ARM64
+	scoped_cond_guard(mmap_read_lock_try, return 0, &init_mm)
+#endif
+	{
+		if (!p4d_free_pud_page(p4d, addr))
+			return 0;
+		return p4d_set_huge(p4d, phys_addr, prot);
+	}
 }
 
 static int vmap_p4d_range(pgd_t *pgd, unsigned long addr, unsigned long end,

-- 
2.55.0

[PATCH mm-hotfixes v5 2/5] x86/mm/pat: acquire init_mm write lock on collapse to avoid UAF

[Lorenzo Stoakes (ARM)]

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>
---
 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 d023a40a1e03..d1e63f7d267f 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>
@@ -410,7 +411,7 @@ 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;
@@ -442,6 +443,18 @@ 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 6b5c2390cc30..047f5f5e2c34 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))
 DEFINE_GUARD_COND(mmap_read_lock, _try, mmap_read_trylock(_T))
 
 static inline void mmap_read_unlock_non_owner(struct mm_struct *mm)

-- 
2.55.0

[PATCH mm-hotfixes v5 3/5] x86/mm/pat: acquire init_mm read lock on attribute change to avoid UAF

[Lorenzo Stoakes (ARM)]

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().

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>
---
 arch/x86/mm/pat/set_memory.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
index d1e63f7d267f..301fb9e77d91 100644
--- a/arch/x86/mm/pat/set_memory.c
+++ b/arch/x86/mm/pat/set_memory.c
@@ -2122,7 +2122,9 @@ 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. */
+	scoped_guard(mmap_read_lock, &init_mm)
+		ret = __change_page_attr_set_clr(&cpa, 1);
 
 	/*
 	 * Check whether we really changed something:

-- 
2.55.0

[PATCH mm-hotfixes v5 4/5] mm/ptdump: always stabilise against page table freeing using init_mm

[Lorenzo Stoakes (ARM)]

Previous commits have established the invariant that kernel page table
freeing is performed while an mmap read lock on init_mm is held, which
fixes races between ptdump and kernel page table freeing over init_mm.

However, x86 and arm64 can perform a ptdump over an mm other than init_mm
via ptdump_walk_pgd() and since kernel memory ranges are shared across
non-kernel mm's, this means that the race still exists for these cases.

Fix this by acquiring a nested mmap write lock for init_mm in
ptdump_walk_pgd().

This is safe as we take this after mmap write locking the mm, and nothing
acquires the init_mm lock first before locking an arbitrary mm, so no
deadlock is possible.

Also update walk_page_range_debug() to assert that init_mm is write locked,
add a comment explaining why and remove some redundant code, and eliminate
the unnecessary and confusing invocation of walk_kernel_page_table_range().

We can safely remove the non-NULL check for walk.mm, as the mmap lock
asserts would NULL pointer deref if it was (and of course no callers do
this).

The first point at which ptdump can race kernel page table freeing is
commit b6bdb7517c3d ("mm/vmalloc: add interfaces to free unmapped page
table"), so we target this in the Fixes tag.

Fixes: b6bdb7517c3d ("mm/vmalloc: add interfaces to free unmapped page table")
Cc: stable@vger.kernel.org
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Kiryl Shutsemau <kas@kernel.org>
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
---
 mm/pagewalk.c | 14 +++++++++-----
 mm/ptdump.c   |  7 +++++++
 2 files changed, 16 insertions(+), 5 deletions(-)

diff --git a/mm/pagewalk.c b/mm/pagewalk.c
index bbcfd68d0907..5d87c632a255 100644
--- a/mm/pagewalk.c
+++ b/mm/pagewalk.c
@@ -702,12 +702,16 @@ int walk_page_range_debug(struct mm_struct *mm, unsigned long start,
 	 * to account for page table freeing on vmap huge page mapping.
 	 */
 	mmap_assert_write_locked(mm);
+	/*
+	 * x86, arm64 ptdump allow walks of efi mm's and x86 ptdump allows walks
+	 * of arbitrary mm's.
+	 *
+	 * However, they both must also hold the init_mm lock to account for
+	 * concurrent kernel page table freeing.
+	 */
+	mmap_assert_write_locked(&init_mm);
 
-	/* For convenience, we allow traversal of kernel mappings. */
-	if (mm == &init_mm)
-		return walk_kernel_page_table_range(start, end, ops,
-						    pgd, private);
-	if (start >= end || !walk.mm)
+	if (start >= end)
 		return -EINVAL;
 	if (!check_ops_safe(ops))
 		return -EINVAL;
diff --git a/mm/ptdump.c b/mm/ptdump.c
index 973020000096..5851096e6f65 100644
--- a/mm/ptdump.c
+++ b/mm/ptdump.c
@@ -178,11 +178,18 @@ void ptdump_walk_pgd(struct ptdump_state *st, struct mm_struct *mm, pgd_t *pgd)
 
 	get_online_mems();
 	mmap_write_lock(mm);
+	/* To stabilise kernel page tables we must hold the init_mm lock too. */
+	if (mm != &init_mm)
+		mmap_write_lock_nested(&init_mm, SINGLE_DEPTH_NESTING);
+
 	while (range->start != range->end) {
 		walk_page_range_debug(mm, range->start, range->end,
 				      &ptdump_ops, pgd, st);
 		range++;
 	}
+
+	if (mm != &init_mm)
+		mmap_write_unlock(&init_mm);
 	mmap_write_unlock(mm);
 	put_online_mems();
 

-- 
2.55.0

[PATCH mm-hotfixes v5 5/5] arm64: remove redundant concurrent ptdump UAF mitigation

[Lorenzo Stoakes (ARM)]

This partially reverts commit fa93b45fd397 ("arm64: Enable vmalloc-huge
with ptdump"), retaining vmalloc-huge support but eliminating the now
redundant mitigation against a race between huge vmap page table freeing
and ptdump, as this issue has now been fixed at core.

We also simultaneously remove the arm64 if-deffery when acquiring the mmap
read lock upon vmap huge page table promotion as it is no longer required.

Note that this patch relies on the preceding vmalloc patch, and should not
be backported alone.

Fixes: fa93b45fd397 ("arm64: Enable vmalloc-huge with ptdump")
Cc: stable@vger.kernel.org
Reviewed-by: Dev Jain <dev.jain@arm.com>
Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Acked-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Acked-by: Will Deacon <will@kernel.org>
Reviewed-by: David Hildenbrand (Arm) <david@kernel.org>
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
---
 arch/arm64/include/asm/ptdump.h |  2 --
 arch/arm64/mm/mmu.c             | 43 ++++-------------------------------------
 arch/arm64/mm/ptdump.c          | 11 ++---------
 mm/vmalloc.c                    | 15 +++-----------
 4 files changed, 9 insertions(+), 62 deletions(-)

diff --git a/arch/arm64/include/asm/ptdump.h b/arch/arm64/include/asm/ptdump.h
index 5b374a6ab34a..50a195eda8ed 100644
--- a/arch/arm64/include/asm/ptdump.h
+++ b/arch/arm64/include/asm/ptdump.h
@@ -7,8 +7,6 @@
 
 #include <linux/ptdump.h>
 
-DECLARE_STATIC_KEY_FALSE(arm64_ptdump_lock_key);
-
 #ifdef CONFIG_PTDUMP
 
 #include <linux/mm_types.h>
diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index a25d8beacc83..bd52fca6e872 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -49,8 +49,6 @@
 #define NO_CONT_MAPPINGS	BIT(1)
 #define NO_EXEC_MAPPINGS	BIT(2)	/* assumes FEAT_HPDS is not used */
 
-DEFINE_STATIC_KEY_FALSE(arm64_ptdump_lock_key);
-
 u64 kimage_voffset __ro_after_init;
 EXPORT_SYMBOL(kimage_voffset);
 
@@ -1864,8 +1862,7 @@ int pmd_clear_huge(pmd_t *pmdp)
 	return 1;
 }
 
-static int __pmd_free_pte_page(pmd_t *pmdp, unsigned long addr,
-			       bool acquire_mmap_lock)
+int pmd_free_pte_page(pmd_t *pmdp, unsigned long addr)
 {
 	pte_t *table;
 	pmd_t pmd;
@@ -1877,25 +1874,13 @@ static int __pmd_free_pte_page(pmd_t *pmdp, unsigned long addr,
 		return 1;
 	}
 
-	/* See comment in pud_free_pmd_page for static key logic */
 	table = pte_offset_kernel(pmdp, addr);
 	pmd_clear(pmdp);
 	__flush_tlb_kernel_pgtable(addr);
-	if (static_branch_unlikely(&arm64_ptdump_lock_key) && acquire_mmap_lock) {
-		mmap_read_lock(&init_mm);
-		mmap_read_unlock(&init_mm);
-	}
-
 	pte_free_kernel(NULL, table);
 	return 1;
 }
 
-int pmd_free_pte_page(pmd_t *pmdp, unsigned long addr)
-{
-	/* If ptdump is walking the pagetables, acquire init_mm.mmap_lock */
-	return __pmd_free_pte_page(pmdp, addr, /* acquire_mmap_lock = */ true);
-}
-
 int pud_free_pmd_page(pud_t *pudp, unsigned long addr)
 {
 	pmd_t *table;
@@ -1911,36 +1896,16 @@ int pud_free_pmd_page(pud_t *pudp, unsigned long addr)
 	}
 
 	table = pmd_offset(pudp, addr);
-
-	/*
-	 * Our objective is to prevent ptdump from reading a PMD table which has
-	 * been freed. In this race, if pud_free_pmd_page observes the key on
-	 * (which got flipped by ptdump) then the mmap lock sequence here will,
-	 * as a result of the mmap write lock/unlock sequence in ptdump, give
-	 * us the correct synchronization. If not, this means that ptdump has
-	 * yet not started walking the pagetables - the sequence of barriers
-	 * issued by __flush_tlb_kernel_pgtable() guarantees that ptdump will
-	 * observe an empty PUD.
-	 */
-	pud_clear(pudp);
-	__flush_tlb_kernel_pgtable(addr);
-	if (static_branch_unlikely(&arm64_ptdump_lock_key)) {
-		mmap_read_lock(&init_mm);
-		mmap_read_unlock(&init_mm);
-	}
-
 	pmdp = table;
 	next = addr;
 	end = addr + PUD_SIZE;
 	do {
 		if (pmd_present(pmdp_get(pmdp)))
-			/*
-			 * PMD has been isolated, so ptdump won't see it. No
-			 * need to acquire init_mm.mmap_lock.
-			 */
-			__pmd_free_pte_page(pmdp, next, /* acquire_mmap_lock = */ false);
+			pmd_free_pte_page(pmdp, next);
 	} while (pmdp++, next += PMD_SIZE, next != end);
 
+	pud_clear(pudp);
+	__flush_tlb_kernel_pgtable(addr);
 	pmd_free(NULL, table);
 	return 1;
 }
diff --git a/arch/arm64/mm/ptdump.c b/arch/arm64/mm/ptdump.c
index 1c20144700d7..5a76c59b5ada 100644
--- a/arch/arm64/mm/ptdump.c
+++ b/arch/arm64/mm/ptdump.c
@@ -283,13 +283,6 @@ void note_page_flush(struct ptdump_state *pt_st)
 	note_page(pt_st, 0, -1, pte_val(pte_zero));
 }
 
-static void arm64_ptdump_walk_pgd(struct ptdump_state *st, struct mm_struct *mm)
-{
-	static_branch_inc(&arm64_ptdump_lock_key);
-	ptdump_walk_pgd(st, mm, NULL);
-	static_branch_dec(&arm64_ptdump_lock_key);
-}
-
 void ptdump_walk(struct seq_file *s, struct ptdump_info *info)
 {
 	unsigned long end = ~0UL;
@@ -318,7 +311,7 @@ void ptdump_walk(struct seq_file *s, struct ptdump_info *info)
 		}
 	};
 
-	arm64_ptdump_walk_pgd(&st.ptdump, info->mm);
+	ptdump_walk_pgd(&st.ptdump, info->mm, NULL);
 }
 
 static void __init ptdump_initialize(void)
@@ -360,7 +353,7 @@ bool ptdump_check_wx(void)
 		}
 	};
 
-	arm64_ptdump_walk_pgd(&st.ptdump, &init_mm);
+	ptdump_walk_pgd(&st.ptdump, &init_mm, NULL);
 
 	if (st.wx_pages || st.uxn_pages) {
 		pr_warn("Checked W+X mappings: FAILED, %lu W+X pages found, %lu non-UXN pages found\n",
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index d5c4d2bb770b..f4fa227a8d7f 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -169,10 +169,7 @@ static int vmap_try_huge_pmd(pmd_t *pmd, unsigned long addr, unsigned long end,
 	 * Concurrent read lock holders are safe: each exclusively owns
 	 * the range it operates on and cannot reach this page table.
 	 */
-#ifndef CONFIG_ARM64
-	scoped_cond_guard(mmap_read_lock_try, return 0, &init_mm)
-#endif
-	{
+	scoped_cond_guard(mmap_read_lock_try, return 0, &init_mm) {
 		if (!pmd_free_pte_page(pmd, addr))
 			return 0;
 		return pmd_set_huge(pmd, phys_addr, prot);
@@ -229,10 +226,7 @@ static int vmap_try_huge_pud(pud_t *pud, unsigned long addr, unsigned long end,
 		return pud_set_huge(pud, phys_addr, prot);
 
 	/* See comment in vmap_try_huge_pmd(). */
-#ifndef CONFIG_ARM64
-	scoped_cond_guard(mmap_read_lock_try, return 0, &init_mm)
-#endif
-	{
+	scoped_cond_guard(mmap_read_lock_try, return 0, &init_mm) {
 		if (!pud_free_pmd_page(pud, addr))
 			return 0;
 		return pud_set_huge(pud, phys_addr, prot);
@@ -289,10 +283,7 @@ static int vmap_try_huge_p4d(p4d_t *p4d, unsigned long addr, unsigned long end,
 		return p4d_set_huge(p4d, phys_addr, prot);
 
 	/* See comment in vmap_try_huge_pmd(). */
-#ifndef CONFIG_ARM64
-	scoped_cond_guard(mmap_read_lock_try, return 0, &init_mm)
-#endif
-	{
+	scoped_cond_guard(mmap_read_lock_try, return 0, &init_mm) {
 		if (!p4d_free_pud_page(p4d, addr))
 			return 0;
 		return p4d_set_huge(p4d, phys_addr, prot);

-- 
2.55.0

Re: [PATCH mm-hotfixes v5 2/5] x86/mm/pat: acquire init_mm write lock on collapse to avoid UAF

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 3 potential issue(s) to consider:

Pre-existing issues:
- [Critical] TOCTOU race in `_lookup_address_cpa` leading to kernel memory corruption.
- [High] Incomplete Use-After-Free fix against ptdump due to mmap_lock mismatch.
- [High] Use-after-free in `populate_pgd` rollback path due to premature page freeing.
--

--- Patch [2]: [PATCH mm-hotfixes v5 2/5] x86/mm/pat: acquire init_mm write lock on collapse to avoid UAF ---
commit b25bc68391fb2bdee5625f9af3d5344a0d10d51f
Author: Lorenzo Stoakes (ARM) <ljs@kernel.org>

x86/mm/pat: acquire init_mm write lock on collapse to avoid UAF
    
This commit adds scoped_guard(mmap_write_lock, &init_mm) to
cpa_collapse_large_pages() to avoid use-after-free issues with ptdump
and to serialize concurrent CPA walkers.

> diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
> index d023a40a1e034..d1e63f7d267ff 100644
> --- a/arch/x86/mm/pat/set_memory.c
> +++ b/arch/x86/mm/pat/set_memory.c
[ ... ]
> @@ -442,6 +443,18 @@ 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);

[Severity: Critical]
This is a pre-existing issue, but does this lock actually serialize
concurrent CPA walkers like set_memory_rw() that might evaluate pmd_leaf()?

Other CPA walkers rely on cpa_lock and do not take init_mm's mmap lock.
If CPU1 executes __change_page_attr() via set_memory_rw(), it takes
cpa_lock, calls _lookup_address_cpa() -> lookup_address_in_pgd_attr(),
and evaluates pmd_leaf(*pmd) as false.

If CPU2 concurrently executes cpa_collapse_large_pages(), taking
mmap_write_lock(&init_mm) and pgd_lock, it collapses the PMD into a large
page (leaf).

CPU1 then continues and calls pte_offset_kernel(pmd), which reads the new
leaf PMD and incorrectly returns a pointer to the 2M data page itself.
Then set_pte_atomic() writes to this data page, corrupting physical memory.

Are concurrent standard CPA walkers able to race with this collapse since
they do not share the same lock?


[Severity: High]
This is a pre-existing issue, but does taking the mmap lock on init_mm
fully prevent races with ptdump?

When ptdump is triggered via debugfs, it calls
ptdump_walk_pgd_level_debugfs() on current->mm:

    ptdump_walk_pgd_level_debugfs(m, current->mm, false);

This takes mmap_write_lock(current->mm) in ptdump_walk_pgd(), which provides
no mutual exclusion against mmap_write_lock(&init_mm) taken here.

Since current->mm can be different from init_mm, won't ptdump concurrently
walk the shared kernel page tables while this function frees the PTE pages,
resulting in the same use-after-free this patch attempts to fix?

> +}
> +
>  static void cpa_flush(struct cpa_data *cpa, int cache)
>  {

[ ... ]

As a related note on pre-existing issues in this file:

[Severity: High]
This is a pre-existing issue, but is there a use-after-free in the
populate_pgd() rollback path due to premature page freeing?

In unmap_pte_range() during rollback (e.g., if populate_pgd() fails due to
OOM during dynamic encryption attribute changes via set_memory_encrypted()):

	if (try_to_free_pte_page((pte_t *)pmd_page_vaddr(*pmd))) {
		pmd_clear(pmd);

The PTE page is returned to the allocator by try_to_free_pte_page() before
pmd_clear() clears the PMD entry.

Could a concurrent ptdump or lockless walker read the PMD and access the
freed memory during this window?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260717-series-vmap-race-fix-v5-0-606a0ac6d3e5@kernel.org?part=2

Re: [PATCH mm-hotfixes v5 0/5] mm: fix UAF caused by race between ptdump and vmap pgtable freeing

[Andrew Morton]

On Fri, 17 Jul 2026 18:30:06 +0100 "Lorenzo Stoakes (ARM)" <ljs@kernel.org> wrote:

> Kernel page table walkers fall into two broad categories - those ranges
> where no exclusion is required via walk_kernel_page_table_range_lockless()
> and those where exclusion is required via walk_kernel_page_table_range()
> or walk_page_range_debug().
> 
> The former category is used only by arm64 arch code operating on ranges it
> both wholly owns and does not concurrently write.
> 
> The latter category consists of kernel page table walkers operating on
> ranges that are wholly owned (but which need exclusion against concurrent
> writers).
> 
> The lock used for exclusion is the mmap lock, and for kernel ranges this
> the mmap lock on init_mm.
> 
> ptdump is a special case being both the only user of
> walk_page_range_debug(), and the only case in which it walks ranges it does
> not own.
> 
> This presents a problem, as page tables may be freed under ptdump. And
> indeed there is a use-after-free bug in the kernel as a result, which this
> series addresses.
> 
> ...
>
> This series works around this by #ifndef CONFIG_ARM64'ing the mmap read
> lock in vmap logic, then partially reverting commit fa93b45fd397 ("arm64:
> Enable vmalloc-huge with ptdump"), keeping the enablement of huge vmap
> support, and removing the ifdeffery with the partial revert patch.
> 

Thanks, I updated mm-hotfixes-unstable to this version.

> v5:
> * Rebased on latest master of Linus's tree.
> * Accumulated tags, thanks everybody!
> * Added additional commit to fix race between CPA collapse/attribute set.
> * Slight commit message tweaks.

Here's how v5 altered mm.git.  This is from the addition of [3/5] - no
other patches changed.


 arch/x86/mm/pat/set_memory.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

--- a/arch/x86/mm/pat/set_memory.c~b
+++ a/arch/x86/mm/pat/set_memory.c
@@ -2122,7 +2122,9 @@ static int change_page_attr_set_clr(unsi
 	cpa.curpage = 0;
 	cpa.force_split = force_split;
 
-	ret = __change_page_attr_set_clr(&cpa, 1);
+	/* Avoid race with concurrent CPA collapse. */
+	scoped_guard(mmap_read_lock, &init_mm)
+		ret = __change_page_attr_set_clr(&cpa, 1);
 
 	/*
 	 * Check whether we really changed something:
_