[PATCH v2 0/5] mm: arch/shstk: Common shadow stack mapping helper and VM_NOHUGEPAGE

[PATCH v2 0/5] mm: arch/shstk: Common shadow stack mapping helper and VM_NOHUGEPAGE

[Catalin Marinas]

This is v2 of the series extracting the common shadow stack mmap into a
separate helper for arm64, riscv and x86. Thanks for the review.

Andrew, if you are happy with this, I think taking the patches through
the mm tree makes most sense.

Minor changes since v1:

- Removed #ifdef from the helper function declaration
- Used two tabs for the alignment of multi-line function arguments
- Added "There is no functional change" to the arch patches log
- Collected ack/review/test tags

v1:

https://lore.kernel.org/r/20260224175800.2500729-1-catalin.marinas@arm.com

Catalin Marinas (5):
  mm: Introduce vm_mmap_shadow_stack() as a helper for VM_SHADOW_STACK
    mappings
  arm64: gcs: Use the new common vm_mmap_shadow_stack() helper
  riscv: shstk: Use the new common vm_mmap_shadow_stack() helper
  x86: shstk: Use the new common vm_mmap_shadow_stack() helper
  mm: Do not map the shadow stack as THP

 arch/arm64/mm/gcs.c         | 14 +-------------
 arch/riscv/kernel/usercfi.c | 12 +-----------
 arch/x86/kernel/shstk.c     | 12 ++----------
 include/linux/mm.h          |  2 ++
 mm/util.c                   | 29 +++++++++++++++++++++++++++++
 5 files changed, 35 insertions(+), 34 deletions(-)

[PATCH v2 1/5] mm: Introduce vm_mmap_shadow_stack() as a helper for VM_SHADOW_STACK mappings

[Catalin Marinas]

arm64, riscv and x86 use a similar pattern for mapping the user shadow
stack (cloned from x86). Extract this into a helper to facilitate code
reuse.

The call to do_mmap() from the new helper uses PROT_READ|PROT_WRITE prot
bits instead of the PROT_READ with an explicit VM_WRITE vm_flag. The
x86 intent was to avoid PROT_WRITE implying normal write since the
shadow stack is not writable by normal stores. However, from a kernel
perspective, the vma is writeable. Functionally there is no difference.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Tested-by: Deepak Gupta <debug@rivosinc.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
---
 include/linux/mm.h |  2 ++
 mm/util.c          | 25 +++++++++++++++++++++++++
 2 files changed, 27 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 5be3d8a8f806..61071dd72eb6 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -3908,6 +3908,8 @@ extern int vm_munmap(unsigned long, size_t);
 extern unsigned long __must_check vm_mmap(struct file *, unsigned long,
         unsigned long, unsigned long,
         unsigned long, unsigned long);
+extern unsigned long __must_check vm_mmap_shadow_stack(unsigned long addr,
+		unsigned long len, unsigned long flags);
 
 struct vm_unmapped_area_info {
 #define VM_UNMAPPED_AREA_TOPDOWN 1
diff --git a/mm/util.c b/mm/util.c
index b05ab6f97e11..51f7f417e91f 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -618,6 +618,31 @@ unsigned long vm_mmap(struct file *file, unsigned long addr,
 }
 EXPORT_SYMBOL(vm_mmap);
 
+#ifdef CONFIG_ARCH_HAS_USER_SHADOW_STACK
+/*
+ * Perform a userland memory mapping for a shadow stack into the current
+ * process address space. This is intended to be used by architectures that
+ * support user shadow stacks.
+ */
+unsigned long vm_mmap_shadow_stack(unsigned long addr, unsigned long len,
+		unsigned long flags)
+{
+	struct mm_struct *mm = current->mm;
+	unsigned long ret, unused;
+
+	flags |= MAP_ANONYMOUS | MAP_PRIVATE;
+	if (addr)
+		flags |= MAP_FIXED_NOREPLACE;
+
+	mmap_write_lock(mm);
+	ret = do_mmap(NULL, addr, len, PROT_READ | PROT_WRITE, flags,
+		      VM_SHADOW_STACK, 0, &unused, NULL);
+	mmap_write_unlock(mm);
+
+	return ret;
+}
+#endif /* CONFIG_ARCH_HAS_USER_SHADOW_STACK */
+
 /**
  * __vmalloc_array - allocate memory for a virtually contiguous array.
  * @n: number of elements.

[PATCH v2 2/5] arm64: gcs: Use the new common vm_mmap_shadow_stack() helper

[Catalin Marinas]

Replace the arm64 map_shadow_stack() content with a call to
vm_mmap_shadow_stack(). There is no functional change.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Mark Brown <broonie@kernel.org>
Cc: Will Deacon <will@kernel.org>
---
 arch/arm64/mm/gcs.c | 14 +-------------
 1 file changed, 1 insertion(+), 13 deletions(-)

diff --git a/arch/arm64/mm/gcs.c b/arch/arm64/mm/gcs.c
index 04a23a497f20..680749611a9a 100644
--- a/arch/arm64/mm/gcs.c
+++ b/arch/arm64/mm/gcs.c
@@ -12,19 +12,7 @@
 
 static unsigned long alloc_gcs(unsigned long addr, unsigned long size)
 {
-	int flags = MAP_ANONYMOUS | MAP_PRIVATE;
-	struct mm_struct *mm = current->mm;
-	unsigned long mapped_addr, unused;
-
-	if (addr)
-		flags |= MAP_FIXED_NOREPLACE;
-
-	mmap_write_lock(mm);
-	mapped_addr = do_mmap(NULL, addr, size, PROT_READ, flags,
-			      VM_SHADOW_STACK | VM_WRITE, 0, &unused, NULL);
-	mmap_write_unlock(mm);
-
-	return mapped_addr;
+	return vm_mmap_shadow_stack(addr, size, 0);
 }
 
 static unsigned long gcs_size(unsigned long size)

[PATCH v2 3/5] riscv: shstk: Use the new common vm_mmap_shadow_stack() helper

[Catalin Marinas]

Replace part of the allocate_shadow_stack() content with a call to
vm_mmap_shadow_stack(). There is no functional change.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Tested-by: Deepak Gupta <debug@rivosinc.com>
Reviewed-by: David Hildenbrand (Arm) <david@kernel.org>
Cc: Paul Walmsley <pjw@kernel.org>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: Alexandre Ghiti <alex@ghiti.fr>
---
 arch/riscv/kernel/usercfi.c | 12 +-----------
 1 file changed, 1 insertion(+), 11 deletions(-)

diff --git a/arch/riscv/kernel/usercfi.c b/arch/riscv/kernel/usercfi.c
index 1adba746f164..7e57f54dc5b2 100644
--- a/arch/riscv/kernel/usercfi.c
+++ b/arch/riscv/kernel/usercfi.c
@@ -230,17 +230,7 @@ int restore_user_shstk(struct task_struct *tsk, unsigned long shstk_ptr)
 static unsigned long allocate_shadow_stack(unsigned long addr, unsigned long size,
 					   unsigned long token_offset, bool set_tok)
 {
-	int flags = MAP_ANONYMOUS | MAP_PRIVATE;
-	struct mm_struct *mm = current->mm;
-	unsigned long populate;
-
-	if (addr)
-		flags |= MAP_FIXED_NOREPLACE;
-
-	mmap_write_lock(mm);
-	addr = do_mmap(NULL, addr, size, PROT_READ, flags,
-		       VM_SHADOW_STACK | VM_WRITE, 0, &populate, NULL);
-	mmap_write_unlock(mm);
+	addr = vm_mmap_shadow_stack(addr, size, 0);
 
 	if (!set_tok || IS_ERR_VALUE(addr))
 		goto out;

[PATCH v2 4/5] x86: shstk: Use the new common vm_mmap_shadow_stack() helper

[Catalin Marinas]

Replace part of the x86 alloc_shstk() content with a call to
vm_mmap_shadow_stack(). There is no functional change.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Tested-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Reviewed-by: David Hildenbrand (Arm) <david@kernel.org>
Cc: Thomas Gleixner <tglx@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
---
 arch/x86/kernel/shstk.c | 12 ++----------
 1 file changed, 2 insertions(+), 10 deletions(-)

diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c
index 978232b6d48d..9725e7d89b1e 100644
--- a/arch/x86/kernel/shstk.c
+++ b/arch/x86/kernel/shstk.c
@@ -100,17 +100,9 @@ static int create_rstor_token(unsigned long ssp, unsigned long *token_addr)
 static unsigned long alloc_shstk(unsigned long addr, unsigned long size,
 				 unsigned long token_offset, bool set_res_tok)
 {
-	int flags = MAP_ANONYMOUS | MAP_PRIVATE | MAP_ABOVE4G;
-	struct mm_struct *mm = current->mm;
-	unsigned long mapped_addr, unused;
+	unsigned long mapped_addr;
 
-	if (addr)
-		flags |= MAP_FIXED_NOREPLACE;
-
-	mmap_write_lock(mm);
-	mapped_addr = do_mmap(NULL, addr, size, PROT_READ, flags,
-			      VM_SHADOW_STACK | VM_WRITE, 0, &unused, NULL);
-	mmap_write_unlock(mm);
+	mapped_addr = vm_mmap_shadow_stack(addr, size, MAP_ABOVE4G);
 
 	if (!set_res_tok || IS_ERR_VALUE(mapped_addr))
 		goto out;

[PATCH v2 5/5] mm: Do not map the shadow stack as THP

[Catalin Marinas]

The default shadow stack size allocated on first prctl() for the main
thread or subsequently on clone() is either half of RLIMIT_STACK or half
of a thread's stack size (for arm64). Both of these are likely to be
suitable for a THP allocation and the kernel is more aggressive in
creating such mappings. However, it does not make much sense to use a
huge page. It didn't make sense for the normal stacks either, see commit
c4608d1bf7c6 ("mm: mmap: map MAP_STACK to VM_NOHUGEPAGE").

Force VM_NOHUGEPAGE when allocating/mapping the shadow stack. As per
commit 7190b3c8bd2b ("mm: mmap: map MAP_STACK to VM_NOHUGEPAGE only if
THP is enabled"), only pass this flag if TRANSPARENT_HUGEPAGE is enabled
as not to confuse CRIU tools.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Tested-by: Deepak Gupta <debug@rivosinc.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
---
 mm/util.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/mm/util.c b/mm/util.c
index 51f7f417e91f..419cb81ab353 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -629,14 +629,18 @@ unsigned long vm_mmap_shadow_stack(unsigned long addr, unsigned long len,
 {
 	struct mm_struct *mm = current->mm;
 	unsigned long ret, unused;
+	vm_flags_t vm_flags = VM_SHADOW_STACK;
 
 	flags |= MAP_ANONYMOUS | MAP_PRIVATE;
 	if (addr)
 		flags |= MAP_FIXED_NOREPLACE;
 
+	if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE))
+		vm_flags |= VM_NOHUGEPAGE;
+
 	mmap_write_lock(mm);
 	ret = do_mmap(NULL, addr, len, PROT_READ | PROT_WRITE, flags,
-		      VM_SHADOW_STACK, 0, &unused, NULL);
+		      vm_flags, 0, &unused, NULL);
 	mmap_write_unlock(mm);
 
 	return ret;

Re: [PATCH v2 0/5] mm: arch/shstk: Common shadow stack mapping helper and VM_NOHUGEPAGE

[Mike Rapoport]

On Wed, Feb 25, 2026 at 04:13:57PM +0000, Catalin Marinas wrote:
> This is v2 of the series extracting the common shadow stack mmap into a
> separate helper for arm64, riscv and x86. Thanks for the review.
> 
> Andrew, if you are happy with this, I think taking the patches through
> the mm tree makes most sense.
> 
> Minor changes since v1:
> 
> - Removed #ifdef from the helper function declaration
> - Used two tabs for the alignment of multi-line function arguments
> - Added "There is no functional change" to the arch patches log
> - Collected ack/review/test tags
> 
> v1:
> 
> https://lore.kernel.org/r/20260224175800.2500729-1-catalin.marinas@arm.com
> 
> Catalin Marinas (5):
>   mm: Introduce vm_mmap_shadow_stack() as a helper for VM_SHADOW_STACK
>     mappings
>   arm64: gcs: Use the new common vm_mmap_shadow_stack() helper
>   riscv: shstk: Use the new common vm_mmap_shadow_stack() helper
>   x86: shstk: Use the new common vm_mmap_shadow_stack() helper
>   mm: Do not map the shadow stack as THP

Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
 
>  arch/arm64/mm/gcs.c         | 14 +-------------
>  arch/riscv/kernel/usercfi.c | 12 +-----------
>  arch/x86/kernel/shstk.c     | 12 ++----------
>  include/linux/mm.h          |  2 ++
>  mm/util.c                   | 29 +++++++++++++++++++++++++++++
>  5 files changed, 35 insertions(+), 34 deletions(-)
> 
> 

-- 
Sincerely yours,
Mike.