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

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>

diff --git a/arch/arm64/mm/mmap.c b/arch/arm64/mm/mmap.c
index 75f343009b4b1fa1e0ae461f18ac096ce4987a4a..92b2f5097a96c160f2425f1e11cb61ee326beace 100644 (file)
--- 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_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)]);