[PATCH v8 27/40] x86/mm: Warn if create Write=0,Dirty=1 with raw prot

From: Rick Edgecombe <rick.p.edgecombe@intel.com>
To: x86@kernel.org, "H . Peter Anvin" <hpa@zytor.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>,
	linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
	linux-mm@kvack.org, linux-arch@vger.kernel.org,
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	Andy Lutomirski <luto@kernel.org>,
	Balbir Singh <bsingharora@gmail.com>,
	Borislav Petkov <bp@alien8.de>,
	Cyrill Gorcunov <gorcunov@gmail.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Eugene Syromiatnikov <esyr@redhat.com>,
	Florian Weimer <fweimer@redhat.com>,
	"H . J . Lu" <hjl.tools@gmail.com>, Jann Horn <jannh@google.com>,
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Cc: rick.p.edgecombe@intel.com
Subject: [PATCH v8 27/40] x86/mm: Warn if create Write=0,Dirty=1 with raw prot
Date: Sat, 18 Mar 2023 17:15:22 -0700	[thread overview]
Message-ID: <20230319001535.23210-28-rick.p.edgecombe@intel.com> (raw)
In-Reply-To: <20230319001535.23210-1-rick.p.edgecombe@intel.com>

When user shadow stack is in use, Write=0,Dirty=1 is treated by the CPU as
shadow stack memory. So for shadow stack memory this bit combination is
valid, but when Dirty=1,Write=1 (conventionally writable) memory is being
write protected, the kernel has been taught to transition the Dirty=1
bit to SavedDirty=1, to avoid inadvertently creating shadow stack
memory. It does this inside pte_wrprotect() because it knows the PTE is
not intended to be a writable shadow stack entry, it is supposed to be
write protected.

However, when a PTE is created by a raw prot using mk_pte(), mk_pte()
can't know whether to adjust Dirty=1 to SavedDirty=1. It can't
distinguish between the caller intending to create a shadow stack PTE or
needing the SavedDirty shift.

The kernel has been updated to not do this, and so Write=0,Dirty=1
memory should only be created by the pte_mkfoo() helpers. Add a warning
to make sure no new mk_pte() start doing this, like, for example,
set_memory_rox() did.

Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>
Tested-by: Pengfei Xu <pengfei.xu@intel.com>
Tested-by: John Allen <john.allen@amd.com>
Tested-by: Kees Cook <keescook@chromium.org>
---
v8:
 - Update commit log verbiage (Boris)

v6:
 - New patch (Note, this has already been a useful warning, it caught the
   newly added set_memory_rox() doing this)
---
 arch/x86/include/asm/pgtable.h | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index e5b3dce0d9fe..7142f99d3fbb 100644
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h
@@ -1032,7 +1032,15 @@ static inline unsigned long pmd_page_vaddr(pmd_t pmd)
  * (Currently stuck as a macro because of indirect forward reference
  * to linux/mm.h:page_to_nid())
  */
-#define mk_pte(page, pgprot)   pfn_pte(page_to_pfn(page), (pgprot))
+#define mk_pte(page, pgprot)						 \
+({									 \
+	pgprot_t __pgprot = pgprot;					 \
+									 \
+	WARN_ON_ONCE(cpu_feature_enabled(X86_FEATURE_USER_SHSTK) &&	 \
+		    (pgprot_val(__pgprot) & (_PAGE_DIRTY | _PAGE_RW)) == \
+		    _PAGE_DIRTY);					 \
+	pfn_pte(page_to_pfn(page), __pgprot);				 \
+})
 
 static inline int pmd_bad(pmd_t pmd)
 {
-- 
2.17.1


