linux-kernel - [PATCH 3.16 102/136] ARM: 8721/1: mm: dump: check hardware RO bit for LPAE

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Message-ID: <lsq.1518323471.993322817@decadent.org.uk>
Date:   Sun, 11 Feb 2018 04:31:11 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, "Neil Dick" <neil@....systems>,
        "Russell King" <rmk+kernel@...linux.org.uk>,
        "Philip Derrin" <philip@....systems>,
        "Kees Cook" <keescook@...omium.org>
Subject: [PATCH 3.16 102/136] ARM: 8721/1: mm: dump: check hardware RO bit
 for LPAE

3.16.54-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Philip Derrin <philip@....systems>

commit 3b0c0c922ff4be275a8beb87ce5657d16f355b54 upstream.

When CONFIG_ARM_LPAE is set, the PMD dump relies on the software
read-only bit to determine whether a page is writable. This
concealed a bug which left the kernel text section writable
(AP2=0) while marked read-only in the software bit.

In a kernel with the AP2 bug, the dump looks like this:

    ---[ Kernel Mapping ]---
    0xc0000000-0xc0200000           2M RW NX SHD
    0xc0200000-0xc0600000           4M ro x  SHD
    0xc0600000-0xc0800000           2M ro NX SHD
    0xc0800000-0xc4800000          64M RW NX SHD

The fix is to check that the software and hardware bits are both
set before displaying "ro". The dump then shows the true perms:

    ---[ Kernel Mapping ]---
    0xc0000000-0xc0200000           2M RW NX SHD
    0xc0200000-0xc0600000           4M RW x  SHD
    0xc0600000-0xc0800000           2M RW NX SHD
    0xc0800000-0xc4800000          64M RW NX SHD

Fixes: ded947798469 ("ARM: 8109/1: mm: Modify pte_write and pmd_write logic for LPAE")
Signed-off-by: Philip Derrin <philip@....systems>
Tested-by: Neil Dick <neil@....systems>
Reviewed-by: Kees Cook <keescook@...omium.org>
Signed-off-by: Russell King <rmk+kernel@...linux.org.uk>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 arch/arm/mm/dump.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/arch/arm/mm/dump.c
+++ b/arch/arm/mm/dump.c
@@ -126,8 +126,8 @@ static const struct prot_bits section_bi
 		.val	= PMD_SECT_USER,
 		.set	= "USR",
 	}, {
-		.mask	= L_PMD_SECT_RDONLY,
-		.val	= L_PMD_SECT_RDONLY,
+		.mask	= L_PMD_SECT_RDONLY | PMD_SECT_AP2,
+		.val	= L_PMD_SECT_RDONLY | PMD_SECT_AP2,
 		.set	= "ro",
 		.clear	= "RW",
 #elif __LINUX_ARM_ARCH__ >= 6