linux-kernel - [PATCH 18/20] x86/mm: fix NX bit clearing issue in kernel_map_pages_in

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Date:   Fri,  3 Jan 2020 12:39:51 +0100
From:   Ard Biesheuvel <ardb@...nel.org>
To:     linux-efi@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     Ard Biesheuvel <ardb@...nel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        linux-kernel@...r.kernel.org, Andy Lutomirski <luto@...nel.org>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        Matthew Garrett <mjg59@...gle.com>
Subject: [PATCH 18/20] x86/mm: fix NX bit clearing issue in kernel_map_pages_in_pgd

Commit 15f003d20782 ("x86/mm/pat: Don't implicitly allow _PAGE_RW in
kernel_map_pages_in_pgd()") modified kernel_map_pages_in_pgd() to
manage writable permissions of memory mappings in the EFI page
table in a different way, but in the process, it removed the
ability to clear NX attributes from read-only mappings, by
clobbering the clear mask if _PAGE_RW is not being requested.

Failure to remove the NX attribute from read-only mappings is
unlikely to be a security issue, but it does prevent us from
tightening the permissions in the EFI page tables going forward,
so let's fix it now.

Fixes: 15f003d20782 ("x86/mm/pat: Don't implicitly allow _PAGE_RW in kernel_map_pages_in_pgd()
Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
---
 arch/x86/mm/pageattr.c | 8 +-------
 1 file changed, 1 insertion(+), 7 deletions(-)

diff --git a/arch/x86/mm/pageattr.c b/arch/x86/mm/pageattr.c
index 1b99ad05b117..f42780ba0893 100644
--- a/arch/x86/mm/pageattr.c
+++ b/arch/x86/mm/pageattr.c
@@ -2215,7 +2215,7 @@ int __init kernel_map_pages_in_pgd(pgd_t *pgd, u64 pfn, unsigned long address,
 		.pgd = pgd,
 		.numpages = numpages,
 		.mask_set = __pgprot(0),
-		.mask_clr = __pgprot(0),
+		.mask_clr = __pgprot(~page_flags & (_PAGE_NX|_PAGE_RW)),
 		.flags = 0,
 	};
 
@@ -2224,12 +2224,6 @@ int __init kernel_map_pages_in_pgd(pgd_t *pgd, u64 pfn, unsigned long address,
 	if (!(__supported_pte_mask & _PAGE_NX))
 		goto out;
 
-	if (!(page_flags & _PAGE_NX))
-		cpa.mask_clr = __pgprot(_PAGE_NX);
-
-	if (!(page_flags & _PAGE_RW))
-		cpa.mask_clr = __pgprot(_PAGE_RW);
-
 	if (!(page_flags & _PAGE_ENC))
 		cpa.mask_clr = pgprot_encrypted(cpa.mask_clr);
 
-- 
2.20.1