[PATCH 6.18 199/333] arm64: gcs: Honour mprotect(PROT_NONE) on shadow stack mappings

Greg Kroah-Hartman
2026-03-17T17:19:16Z

From: Catalin Marinas <catalin.marinas@arm.com>

commit 47a8aad135ac1aed04b7b0c0a8157fd208075827 upstream.

vm_get_page_prot() short-circuits the protection_map[] lookup for a
VM_SHADOW_STACK mapping since it uses a different PIE index from the
typical read/write/exec permissions. However, the side effect is that it
also ignores mprotect(PROT_NONE) by creating an accessible PTE.

Special-case the !(vm_flags & VM_ACCESS_FLAGS) flags to use the
protection_map[VM_NONE] permissions instead. No GCS attributes are
required for an inaccessible PTE.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Fixes: 6497b66ba694 ("arm64/mm: Map pages for guarded control stack")
Cc: stable@vger.kernel.org
Cc: Mark Brown <broonie@kernel.org>
Cc: Will Deacon <will@kernel.org>
Cc: David Hildenbrand <david@kernel.org>
Reviewed-by: David Hildenbrand (Arm) <david@kernel.org>
Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
arch/arm64/mm/mmap.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)

--- a/arch/arm64/mm/mmap.c
+++ b/arch/arm64/mm/mmap.c
@@ -91,7 +91,11 @@ pgprot_t vm_get_page_prot(vm_flags_t vm_

/* Short circuit GCS to avoid bloating the table. */
if (system_supports_gcs() && (vm_flags & VM_SHADOW_STACK)) {
- prot = gcs_page_prot;
+ /* Honour mprotect(PROT_NONE) on shadow stack mappings */
+ if (vm_flags & VM_ACCESS_FLAGS)
+ prot = gcs_page_prot;
+ else
+ prot = pgprot_val(protection_map[VM_NONE]);
} else {
prot = pgprot_val(protection_map[vm_flags &
(VM_READ|VM_WRITE|VM_EXEC|VM_SHARED)]);