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arm64: gcs: Honour mprotect(PROT_NONE) on shadow stack mappings

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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: 6497b66ba6 ("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>
This commit is contained in:
Catalin Marinas 2026-02-23 17:45:31 +00:00 • committed by Will Deacon
parent 8a85b31312
commit 47a8aad135
1 changed files with 5 additions and 1 deletions
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6
arch/arm64/mm/mmap.c
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@ -91,7 +91,11 @@ pgprot_t vm_get_page_prot(vm_flags_t vm_flags)
/* 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)]);