* [PATCH mm-hotfixes v2 1/4] mm/vmalloc: acquire init_mm lock on huge vmap to avoid ptdump UAF
2026-07-12 10:42 [PATCH mm-hotfixes v2 0/4] mm: fix UAF caused by race between ptdump and vmap pgtable freeing Lorenzo Stoakes
@ 2026-07-12 10:42 ` Lorenzo Stoakes
2026-07-12 13:17 ` Dev Jain
2026-07-12 10:42 ` [PATCH mm-hotfixes v2 2/4] x86/mm/pat: acquire mmap lock on page table free " Lorenzo Stoakes
` (2 subsequent siblings)
3 siblings, 1 reply; 7+ messages in thread
From: Lorenzo Stoakes @ 2026-07-12 10:42 UTC (permalink / raw)
To: Andrew Morton, Suren Baghdasaryan, Liam R. Howlett,
Vlastimil Babka, Shakeel Butt, David Hildenbrand, Mike Rapoport,
Michal Hocko, Uladzislau Rezki, Toshi Kani, Dave Hansen,
Andy Lutomirski, Peter Zijlstra, Thomas Gleixner, Ingo Molnar,
Borislav Petkov, x86, H. Peter Anvin, Kiryl Shutsemau,
Catalin Marinas, Will Deacon, Dev Jain, Ryan Roberts
Cc: David Carlier, ljs, linux-mm, linux-kernel, bpf, linux-arm-kernel,
stable, syzbot+fd95a72470f5a44e464c
Currently there is a nasty race between ptdump and vmap when attempting to
map a huge P4D, PMD or PUD entry.
ptdump is invoked by arch code to walk kernel or EFI page tables, either to
output it for debugging purposes, or to assert that there are no
W+X (i.e. executable writable pages) exposed in these ranges.
The feature is enabled generally via CONFIG_PTDUMP (whose implementation is
in mm/ptdump.c), and expose a debugfs interface for it if
CONFIG_PTDUMP_DEBUGFS is defined.
If CONFIG_PTDUMP is enabled, then /sys/kernel/debug/check_wx_pages is
enabled which checks kernel ranges to perform the W+X check. If
CONFIG_DEBUG_WX is enabled, this is done on boot.
(Note that arm32 implements its own page table walker and uses
CONFIG_ARM_DEBUG_WX and CONFIG_ARM_PTDUMP_DEBUGFS for this.)
The EFI implementations vary by architecture, but are not relevant to the
bug, as the issue is when kernel page ranges are walked.
ptdump_walk_pgd() holds both the mem hotplug lock and the mmap write lock
before invoking walk_page_range_debug(), however this runs into an issue
with vmalloc ranges.
When vmap maps a P4D, PUD or a PMD sized range and encounters an existing
P4d/PUD/PMD entry pointing to a PUD/PMD/PTE page table, it invokes
vmap_try_huge_[p4d,pud,pmd]() to try to convert it to a huge page table
mapping if possible.
However, when it does this, it holds no meaningful locks against other
kernel page table walkers, invoking [p4d,pud,pmd]_free_[pud,pmd,pte]_page()
which calls pagetable_free() and pagetable_free_kernel() in
turn (pte_fragment_free() for powerpc).
This means that a use-after-free becomes possible if the ptdump page table
walker happens to be walking a PUD, PMD or PTE page table after it has been
freed.
Since commit 5ba2f0a15564 ("mm: introduce deferred freeing for kernel page
tables"), if CONFIG_ASYNC_KERNEL_PGTABLE_FREE is set,
pagetable_free_kernel() will batch the page table freeing operation,
otherwise it frees the page table directly.
While the KASAN report that syzbot highlighted indicated that the issue
arose in a workqueue introduced by this change, this is coincidental and
the commit did not alter the race which has existed for quite some time.
This patch resolves the issue by simply having
vmap_try_huge_[p4d,pud,pmd]() hold the mmap read lock on init_mm while
invoking [p4d,pud,pmd]_free_[pud,pmd,pte]_page() and
[p4d,pud,pmd]_set_huge().
This way, page table walkers either observe a newly promoted huge
P4D/PUD/PMD leaf entry or the prior PUD/PMD/PTE entry and never get passed
a dangling pointer, whether the page is freed asynchronously or not.
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.
We acquire the mmap read lock as a trylock, as this is an optimisation that
is permitted not to succeed, a race is very unlikely, and doing so
eliminates latency sleeping on the lock would have otherwise caused.
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.
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.
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>
Signed-off-by: Lorenzo Stoakes <ljs@kernel.org>
---
include/linux/mmap_lock.h | 1 +
mm/pagewalk.c | 22 +++++++++++----------
mm/vmalloc.c | 50 ++++++++++++++++++++++++++++++++++++++---------
3 files changed, 54 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..1fa9ac6e43d4 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 +159,25 @@ 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);
+ /*
+ * Kernel page table walkers either walk ranges they own exclusively or
+ * hold the mmap write lock on init_mm (ptdump being the motivating
+ * case).
+ *
+ * Therefore, acquire the mmap read lock to prevent use-after-free when
+ * freeing page tables.
+ */
+#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 +226,18 @@ 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 +286,18 @@ 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
^ permalink raw reply related [flat|nested] 7+ messages in thread* Re: [PATCH mm-hotfixes v2 1/4] mm/vmalloc: acquire init_mm lock on huge vmap to avoid ptdump UAF
2026-07-12 10:42 ` [PATCH mm-hotfixes v2 1/4] mm/vmalloc: acquire init_mm lock on huge vmap to avoid ptdump UAF Lorenzo Stoakes
@ 2026-07-12 13:17 ` Dev Jain
0 siblings, 0 replies; 7+ messages in thread
From: Dev Jain @ 2026-07-12 13:17 UTC (permalink / raw)
To: Lorenzo Stoakes, Andrew Morton, Suren Baghdasaryan,
Liam R. Howlett, Vlastimil Babka, Shakeel Butt, David Hildenbrand,
Mike Rapoport, Michal Hocko, Uladzislau Rezki, Toshi Kani,
Dave Hansen, Andy Lutomirski, Peter Zijlstra, Thomas Gleixner,
Ingo Molnar, Borislav Petkov, x86, H. Peter Anvin,
Kiryl Shutsemau, Catalin Marinas, Will Deacon, Ryan Roberts
Cc: David Carlier, linux-mm, linux-kernel, bpf, linux-arm-kernel,
stable, syzbot+fd95a72470f5a44e464c
On 12/07/26 4:12 pm, Lorenzo Stoakes wrote:
> Currently there is a nasty race between ptdump and vmap when attempting to
> map a huge P4D, PMD or PUD entry.
>
> ptdump is invoked by arch code to walk kernel or EFI page tables, either to
> output it for debugging purposes, or to assert that there are no
> W+X (i.e. executable writable pages) exposed in these ranges.
>
> The feature is enabled generally via CONFIG_PTDUMP (whose implementation is
> in mm/ptdump.c), and expose a debugfs interface for it if
> CONFIG_PTDUMP_DEBUGFS is defined.
>
> If CONFIG_PTDUMP is enabled, then /sys/kernel/debug/check_wx_pages is
> enabled which checks kernel ranges to perform the W+X check. If
> CONFIG_DEBUG_WX is enabled, this is done on boot.
>
> (Note that arm32 implements its own page table walker and uses
> CONFIG_ARM_DEBUG_WX and CONFIG_ARM_PTDUMP_DEBUGFS for this.)
>
> The EFI implementations vary by architecture, but are not relevant to the
> bug, as the issue is when kernel page ranges are walked.
>
> ptdump_walk_pgd() holds both the mem hotplug lock and the mmap write lock
> before invoking walk_page_range_debug(), however this runs into an issue
> with vmalloc ranges.
>
> When vmap maps a P4D, PUD or a PMD sized range and encounters an existing
> P4d/PUD/PMD entry pointing to a PUD/PMD/PTE page table, it invokes
> vmap_try_huge_[p4d,pud,pmd]() to try to convert it to a huge page table
> mapping if possible.
>
> However, when it does this, it holds no meaningful locks against other
> kernel page table walkers, invoking [p4d,pud,pmd]_free_[pud,pmd,pte]_page()
> which calls pagetable_free() and pagetable_free_kernel() in
> turn (pte_fragment_free() for powerpc).
>
> This means that a use-after-free becomes possible if the ptdump page table
> walker happens to be walking a PUD, PMD or PTE page table after it has been
> freed.
>
> Since commit 5ba2f0a15564 ("mm: introduce deferred freeing for kernel page
> tables"), if CONFIG_ASYNC_KERNEL_PGTABLE_FREE is set,
> pagetable_free_kernel() will batch the page table freeing operation,
> otherwise it frees the page table directly.
>
> While the KASAN report that syzbot highlighted indicated that the issue
> arose in a workqueue introduced by this change, this is coincidental and
> the commit did not alter the race which has existed for quite some time.
>
> This patch resolves the issue by simply having
> vmap_try_huge_[p4d,pud,pmd]() hold the mmap read lock on init_mm while
> invoking [p4d,pud,pmd]_free_[pud,pmd,pte]_page() and
> [p4d,pud,pmd]_set_huge().
>
> This way, page table walkers either observe a newly promoted huge
> P4D/PUD/PMD leaf entry or the prior PUD/PMD/PTE entry and never get passed
> a dangling pointer, whether the page is freed asynchronously or not.
>
> 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.
>
> We acquire the mmap read lock as a trylock, as this is an optimisation that
> is permitted not to succeed, a race is very unlikely, and doing so
> eliminates latency sleeping on the lock would have otherwise caused.
>
> 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.
>
> 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.
>
> 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>
> Signed-off-by: Lorenzo Stoakes <ljs@kernel.org>
> ---
Reviewed-by: Dev Jain <dev.jain@arm.com>
^ permalink raw reply [flat|nested] 7+ messages in thread
* [PATCH mm-hotfixes v2 2/4] x86/mm/pat: acquire mmap lock on page table free to avoid ptdump UAF
2026-07-12 10:42 [PATCH mm-hotfixes v2 0/4] mm: fix UAF caused by race between ptdump and vmap pgtable freeing Lorenzo Stoakes
2026-07-12 10:42 ` [PATCH mm-hotfixes v2 1/4] mm/vmalloc: acquire init_mm lock on huge vmap to avoid ptdump UAF Lorenzo Stoakes
@ 2026-07-12 10:42 ` Lorenzo Stoakes
2026-07-12 16:53 ` Borislav Petkov
2026-07-12 10:42 ` [PATCH mm-hotfixes v2 3/4] mm/ptdump: always stabilise against page table freeing using init_mm Lorenzo Stoakes
2026-07-12 10:42 ` [PATCH mm-hotfixes v2 4/4] arm64: remove redundant concurrent ptdump UAF mitigation Lorenzo Stoakes
3 siblings, 1 reply; 7+ messages in thread
From: Lorenzo Stoakes @ 2026-07-12 10:42 UTC (permalink / raw)
To: Andrew Morton, Suren Baghdasaryan, Liam R. Howlett,
Vlastimil Babka, Shakeel Butt, David Hildenbrand, Mike Rapoport,
Michal Hocko, Uladzislau Rezki, Toshi Kani, Dave Hansen,
Andy Lutomirski, Peter Zijlstra, Thomas Gleixner, Ingo Molnar,
Borislav Petkov, x86, H. Peter Anvin, Kiryl Shutsemau,
Catalin Marinas, Will Deacon, Dev Jain, Ryan Roberts
Cc: David Carlier, ljs, linux-mm, linux-kernel, bpf, linux-arm-kernel,
stable
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.
This patch resolves the issue by acquiring the mmap read lock on init_mm to
provide mutual exclusion against ptdump, which acquires the init_mm write
lock.
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.
We also include cleanup.h in order to use a scoped_guard() to implement
this cleanly.
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>
Signed-off-by: Lorenzo Stoakes <ljs@kernel.org>
---
arch/x86/mm/pat/set_memory.c | 14 +++++++++++---
1 file changed, 11 insertions(+), 3 deletions(-)
diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
index d023a40a1e03..4c4b8244502f 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>
@@ -436,9 +437,16 @@ static void cpa_collapse_large_pages(struct cpa_data *cpa)
flush_tlb_all();
- list_for_each_entry_safe(ptdesc, tmp, &pgtables, pt_list) {
- list_del(&ptdesc->pt_list);
- pagetable_free(ptdesc);
+ /*
+ * ptdump might read these page tables, so avoid a use-after-free by
+ * acquiring the mmap read lock on init_mm (ptdump acquires the mmap
+ * write lock).
+ */
+ scoped_guard(mmap_read_lock, &init_mm) {
+ list_for_each_entry_safe(ptdesc, tmp, &pgtables, pt_list) {
+ list_del(&ptdesc->pt_list);
+ pagetable_free(ptdesc);
+ }
}
}
--
2.55.0
^ permalink raw reply related [flat|nested] 7+ messages in thread* Re: [PATCH mm-hotfixes v2 2/4] x86/mm/pat: acquire mmap lock on page table free to avoid ptdump UAF
2026-07-12 10:42 ` [PATCH mm-hotfixes v2 2/4] x86/mm/pat: acquire mmap lock on page table free " Lorenzo Stoakes
@ 2026-07-12 16:53 ` Borislav Petkov
0 siblings, 0 replies; 7+ messages in thread
From: Borislav Petkov @ 2026-07-12 16:53 UTC (permalink / raw)
To: Lorenzo Stoakes
Cc: Andrew Morton, Suren Baghdasaryan, Liam R. Howlett,
Vlastimil Babka, Shakeel Butt, David Hildenbrand, Mike Rapoport,
Michal Hocko, Uladzislau Rezki, Toshi Kani, Dave Hansen,
Andy Lutomirski, Peter Zijlstra, Thomas Gleixner, Ingo Molnar,
x86, H. Peter Anvin, Kiryl Shutsemau, Catalin Marinas,
Will Deacon, Dev Jain, Ryan Roberts, David Carlier, linux-mm,
linux-kernel, bpf, linux-arm-kernel, stable
On Sun, Jul 12, 2026 at 11:42:25AM +0100, Lorenzo Stoakes wrote:
> This patch resolves the issue by acquiring the mmap read lock on init_mm to
s/This patch resolves/Resolve/
> provide mutual exclusion against ptdump, which acquires the init_mm write
> lock.
...
> We also include cleanup.h in order to use a scoped_guard() to implement
> this cleanly.
You don't need to explain that.
Thx.
--
Regards/Gruss,
Boris.
https://people.kernel.org/tglx/notes-about-netiquette
^ permalink raw reply [flat|nested] 7+ messages in thread
* [PATCH mm-hotfixes v2 3/4] mm/ptdump: always stabilise against page table freeing using init_mm
2026-07-12 10:42 [PATCH mm-hotfixes v2 0/4] mm: fix UAF caused by race between ptdump and vmap pgtable freeing Lorenzo Stoakes
2026-07-12 10:42 ` [PATCH mm-hotfixes v2 1/4] mm/vmalloc: acquire init_mm lock on huge vmap to avoid ptdump UAF Lorenzo Stoakes
2026-07-12 10:42 ` [PATCH mm-hotfixes v2 2/4] x86/mm/pat: acquire mmap lock on page table free " Lorenzo Stoakes
@ 2026-07-12 10:42 ` Lorenzo Stoakes
2026-07-12 10:42 ` [PATCH mm-hotfixes v2 4/4] arm64: remove redundant concurrent ptdump UAF mitigation Lorenzo Stoakes
3 siblings, 0 replies; 7+ messages in thread
From: Lorenzo Stoakes @ 2026-07-12 10:42 UTC (permalink / raw)
To: Andrew Morton, Suren Baghdasaryan, Liam R. Howlett,
Vlastimil Babka, Shakeel Butt, David Hildenbrand, Mike Rapoport,
Michal Hocko, Uladzislau Rezki, Toshi Kani, Dave Hansen,
Andy Lutomirski, Peter Zijlstra, Thomas Gleixner, Ingo Molnar,
Borislav Petkov, x86, H. Peter Anvin, Kiryl Shutsemau,
Catalin Marinas, Will Deacon, Dev Jain, Ryan Roberts
Cc: David Carlier, ljs, linux-mm, linux-kernel, bpf, linux-arm-kernel,
stable
x86 and arm64 invoke ptdump_walk_pgd() with non-init_mm mm whilst still
walking kernel page table ranges.
For x86 this is done in ptdump_curknl_show() and ptdump_efi_show(), the
first passing current->mm, and the second passing efi_mm (we reach kernel
mappings that init_mm protects for current->mm due to x86 cloning shared
kernel page tables for arbitrary mm's).
arm64 does so via ptdump_debugfs_register(), configured by efi_ptdump_info
for efi ranges against efi_mm.
The init_mm mmap lock is used to stabilise page table freeing against
ptdump, so take a nested lock on init_mm to ensure that we are correctly
stabilised.
We take this after mmap write locking the non-init_mm mm. Nothing acquires
the init_mm lock first before locking an arbitrary mm, so no deadlock is
possible.
The preceding patches in this series updated the two cases which can cause
races with ptdump in init_mm ranges - vmap and x86 CPA huge page promotion.
For arm64, commit fa93b45fd397 ("arm64: Enable vmalloc-huge with ptdump")
already provides exclusion against init_mm for the vmap case, which this
patch also pairs with.
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.
Also update walk_page_range_debug() to assert that init_mm is write locked,
add a comment explaining why and remove some redundant code.
We do not need to check for walk.mm being non-NULL, as the mmap lock
asserts would NULL pointer deref if it was (and of course no callers do
this).
The rest of this code is exactly as it would be if invoking
walk_kernel_page_table_range(), only now it's far clearer that that's the
case.
Fixes: b6bdb7517c3d ("mm/vmalloc: add interfaces to free unmapped page table")
Cc: stable@vger.kernel.org
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Signed-off-by: Lorenzo Stoakes <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
^ permalink raw reply related [flat|nested] 7+ messages in thread* [PATCH mm-hotfixes v2 4/4] arm64: remove redundant concurrent ptdump UAF mitigation
2026-07-12 10:42 [PATCH mm-hotfixes v2 0/4] mm: fix UAF caused by race between ptdump and vmap pgtable freeing Lorenzo Stoakes
` (2 preceding siblings ...)
2026-07-12 10:42 ` [PATCH mm-hotfixes v2 3/4] mm/ptdump: always stabilise against page table freeing using init_mm Lorenzo Stoakes
@ 2026-07-12 10:42 ` Lorenzo Stoakes
3 siblings, 0 replies; 7+ messages in thread
From: Lorenzo Stoakes @ 2026-07-12 10:42 UTC (permalink / raw)
To: Andrew Morton, Suren Baghdasaryan, Liam R. Howlett,
Vlastimil Babka, Shakeel Butt, David Hildenbrand, Mike Rapoport,
Michal Hocko, Uladzislau Rezki, Toshi Kani, Dave Hansen,
Andy Lutomirski, Peter Zijlstra, Thomas Gleixner, Ingo Molnar,
Borislav Petkov, x86, H. Peter Anvin, Kiryl Shutsemau,
Catalin Marinas, Will Deacon, Dev Jain, Ryan Roberts
Cc: David Carlier, ljs, linux-mm, linux-kernel, bpf, linux-arm-kernel,
stable
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>
Signed-off-by: Lorenzo Stoakes <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 1fa9ac6e43d4..400563ac6d5d 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -170,10 +170,7 @@ static int vmap_try_huge_pmd(pmd_t *pmd, unsigned long addr, unsigned long end,
* Therefore, acquire the mmap read lock to prevent use-after-free when
* freeing page tables.
*/
-#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);
@@ -230,10 +227,7 @@ 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);
@@ -290,10 +284,7 @@ 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
^ permalink raw reply related [flat|nested] 7+ messages in thread