linux-kernel - [tip: x86/shstk] x86/mm: Warn if create Write=0,Dirty=1 with raw prot

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Message-ID: <167933036405.5837.15340188207531246966.tip-bot2@tip-bot2>
Date:   Mon, 20 Mar 2023 16:39:24 -0000
From:   "tip-bot2 for Rick Edgecombe" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "Borislav Petkov (AMD)" <bp@...en8.de>,
        "Mike Rapoport (IBM)" <rppt@...nel.org>,
        Pengfei Xu <pengfei.xu@...el.com>,
        John Allen <john.allen@....com>,
        Kees Cook <keescook@...omium.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: x86/shstk] x86/mm: Warn if create Write=0,Dirty=1 with raw prot

The following commit has been merged into the x86/shstk branch of tip:

Commit-ID:     6ad563c4b42f64038456f838f3bd712acc95cbcb
Gitweb:        https://git.kernel.org/tip/6ad563c4b42f64038456f838f3bd712acc95cbcb
Author:        Rick Edgecombe <rick.p.edgecombe@...el.com>
AuthorDate:    Sat, 18 Mar 2023 17:15:22 -07:00
Committer:     Dave Hansen <dave.hansen@...ux.intel.com>
CommitterDate: Mon, 20 Mar 2023 09:01:11 -07:00

x86/mm: Warn if create Write=0,Dirty=1 with raw prot

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@...el.com>
Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>
Reviewed-by: Borislav Petkov (AMD) <bp@...en8.de>
Acked-by: Mike Rapoport (IBM) <rppt@...nel.org>
Tested-by: Pengfei Xu <pengfei.xu@...el.com>
Tested-by: John Allen <john.allen@....com>
Tested-by: Kees Cook <keescook@...omium.org>
Link: https://lore.kernel.org/all/20230319001535.23210-28-rick.p.edgecombe%40intel.com
---
 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 e5b3dce..7142f99 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)
 {