[PATCH v2] x86/mm: Refuse W^X violations

From: Peter Zijlstra
Date: Mon Aug 29 2022 - 06:18:16 EST



x86 has STRICT_*_RWX, but not even a warning when someone violates it.

Add this warning and fully refuse the transition.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
arch/x86/mm/pat/set_memory.c | 32 ++++++++++++++++++++++++++++++++
1 file changed, 32 insertions(+)

--- a/arch/x86/mm/pat/set_memory.c
+++ b/arch/x86/mm/pat/set_memory.c
@@ -580,6 +580,33 @@ static inline pgprot_t static_protection
}

/*
+ * Validate and enforce strict W^X semantics.
+ */
+static inline pgprot_t verify_rwx(pgprot_t old, pgprot_t new, unsigned long start,
+ unsigned long pfn, unsigned long npg)
+{
+ unsigned long end;
+
+ if (!cpu_feature_enabled(X86_FEATURE_NX))
+ return new;
+
+ if (!((pgprot_val(old) ^ pgprot_val(new)) & (_PAGE_RW | _PAGE_NX)))
+ return new;
+
+ if ((pgprot_val(new) & (_PAGE_RW | _PAGE_NX)) != _PAGE_RW)
+ return new;
+
+ end = start + npg * PAGE_SIZE - 1;
+ WARN_ONCE(1, "CPA refuse W^X violation: %016llx -> %016llx range: 0x%016lx - 0x%016lx PFN %lx\n",
+ (unsigned long long)pgprot_val(old),
+ (unsigned long long)pgprot_val(new),
+ start, end, pfn);
+
+ /* refuse the transition into WX */
+ return old;
+}
+
+/*
* Lookup the page table entry for a virtual address in a specific pgd.
* Return a pointer to the entry and the level of the mapping.
*/
@@ -885,6 +912,8 @@ static int __should_split_large_page(pte
new_prot = static_protections(req_prot, lpaddr, old_pfn, numpages,
psize, CPA_DETECT);

+ new_prot = verify_rwx(old_prot, new_prot, lpaddr, old_pfn, numpages);
+
/*
* If there is a conflict, split the large page.
*
@@ -1525,6 +1554,7 @@ static int __change_page_attr(struct cpa

if (level == PG_LEVEL_4K) {
pte_t new_pte;
+ pgprot_t old_prot = pte_pgprot(old_pte);
pgprot_t new_prot = pte_pgprot(old_pte);
unsigned long pfn = pte_pfn(old_pte);

@@ -1536,6 +1566,8 @@ static int __change_page_attr(struct cpa
new_prot = static_protections(new_prot, address, pfn, 1, 0,
CPA_PROTECT);

+ new_prot = verify_rwx(old_prot, new_prot, address, pfn, 1);
+
new_prot = pgprot_clear_protnone_bits(new_prot);

/*