linux-kernel - [RFC PATCH 1/2] x86: Don't let pgprot_modify() change the page encryption bit

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Message-Id: <20190905103541.4161-2-thomas_os@shipmail.org>
Date:   Thu,  5 Sep 2019 12:35:40 +0200
From:   Thomas Hellström (VMware) 
        <thomas_os@...pmail.org>
To:     linux-kernel@...r.kernel.org, x86@...nel.org, pv-drivers@...are.com
Cc:     Thomas Hellstrom <thellstrom@...are.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Christoph Hellwig <hch@...radead.org>,
        Christian König <christian.koenig@....com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Tom Lendacky <thomas.lendacky@....com>
Subject: [RFC PATCH 1/2] x86: Don't let pgprot_modify() change the page encryption bit

From: Thomas Hellstrom <thellstrom@...are.com>

When SEV or SME is enabled and active, vm_get_page_prot() typically
returns with the encryption bit set. This means that users of
pgprot_modify(, vm_get_page_prot()) (mprotect_fixup, do_mmap) typically
unintentionally sets encrypted page protection even on mmap'd coherent
memory where the mmap callback has cleared the bit. Fix this by not
allowing pgprot_modify() to change the encryption bit, similar to
how it's done for PAT bits.

Cc: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: Andy Lutomirski <luto@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Borislav Petkov <bp@...en8.de>
Cc: "H. Peter Anvin" <hpa@...or.com>
Cc: Christoph Hellwig <hch@...radead.org>
Cc: Christian König <christian.koenig@....com>
Cc: Marek Szyprowski <m.szyprowski@...sung.com>
Cc: Tom Lendacky <thomas.lendacky@....com>
Signed-off-by: Thomas Hellstrom <thellstrom@...are.com>
---
 arch/x86/include/asm/pgtable.h | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index 0bc530c4eb13..8e507169fd90 100644
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h
@@ -624,12 +624,16 @@ static inline pmd_t pmd_modify(pmd_t pmd, pgprot_t newprot)
 	return __pmd(val);
 }
 
-/* mprotect needs to preserve PAT bits when updating vm_page_prot */
+/*
+ * mprotect needs to preserve PAT and encryption bits when updating
+ * vm_page_prot
+ */
 #define pgprot_modify pgprot_modify
 static inline pgprot_t pgprot_modify(pgprot_t oldprot, pgprot_t newprot)
 {
-	pgprotval_t preservebits = pgprot_val(oldprot) & _PAGE_CHG_MASK;
-	pgprotval_t addbits = pgprot_val(newprot);
+	pgprotval_t preservebits = pgprot_val(oldprot) &
+		(_PAGE_CHG_MASK | sme_me_mask);
+	pgprotval_t addbits = pgprot_val(newprot) & ~sme_me_mask;
 	return __pgprot(preservebits | addbits);
 }
 
-- 
2.20.1