x86/mm: Ease W^X enforcement back to just a warning

Currently, the "change_page_attr" (CPA) code refuses to create
W+X mappings on 64-bit kernels.  There have been reports both
from 32-bit[1] and from BPF[2] users where this change kept the
system from booting.

These reports are showing up even after about a month of soak
time in -next.

To avoid breaking anything, never enforce W^X.  Always warn
and return the requested permissions even if a problem is
detected.

1. https://lore.kernel.org/all/CAMj1kXHcF_iK_g0OZSkSv56Wmr=eQGQwNstcNjLEfS=mm7a06w@mail.gmail.com/
2. https://lore.kernel.org/bpf/c84cc27c1a5031a003039748c3c099732a718aec.camel@kernel.org/T/#u

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: x86@kernel.org
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Peter Zijlstra <peterz@infradead.org>

diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
index efe882c..97342c4 100644 (file)
--- a/arch/x86/mm/pat/set_memory.c
+++ b/arch/x86/mm/pat/set_memory.c
@@ -580,7 +580,7 @@ static inline pgprot_t static_protections(pgprot_t prot, unsigned long start,
 }
 
 /*
- * Validate and enforce strict W^X semantics.
+ * Validate 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)
@@ -595,7 +595,7 @@ static inline pgprot_t verify_rwx(pgprot_t old, pgprot_t new, unsigned long star
        if (IS_ENABLED(CONFIG_X86_32))
                return new;
 
-       /* Only enforce when NX is supported: */
+       /* Only verify when NX is supported: */
        if (!(__supported_pte_mask & _PAGE_NX))
                return new;
 
@@ -606,13 +606,17 @@ static inline pgprot_t verify_rwx(pgprot_t old, pgprot_t new, unsigned long star
                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",
+       WARN_ONCE(1, "CPA detected 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;
+       /*
+        * For now, allow all permission change attempts by returning the
+        * attempted permissions.  This can 'return old' to actively
+        * refuse the permission change at a later time.
+        */
+       return new;
 }
 
 /*