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powerpc/mm: Don't be too strict with _etext alignment on PPC32

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Similar to PPC64, accept to map RO data as ROX as a trade off between
between security and memory usage.

Having RO data executable is not a high risk as RO data can't be
modified to forge an exploit.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/8c4a0d89d944eed984dd941e509614031a5ace2b.1589866984.git.christophe.leroy@csgroup.eu
This commit is contained in:
Christophe Leroy 2020-05-19 05:49:19 +00:00 committed by Michael Ellerman
parent 0c8c2c9c20
commit a0591b60ee
2 changed files with 1 additions and 28 deletions
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26
arch/powerpc/Kconfig
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@ -778,32 +778,6 @@ config THREAD_SHIFT
Used to define the stack size. The default is almost always what you Used to define the stack size. The default is almost always what you
want. Only change this if you know what you are doing. want. Only change this if you know what you are doing.
config ETEXT_SHIFT_BOOL
bool "Set custom etext alignment" if STRICT_KERNEL_RWX && \
(PPC_BOOK3S_32 || PPC_8xx)
depends on ADVANCED_OPTIONS
help
This option allows you to set the kernel end of text alignment. When
RAM is mapped by blocks, the alignment needs to fit the size and
number of possible blocks. The default should be OK for most configs.
Say N here unless you know what you are doing.
config ETEXT_SHIFT
int "_etext shift" if ETEXT_SHIFT_BOOL
range 17 28 if STRICT_KERNEL_RWX && PPC_BOOK3S_32
range 19 23 if STRICT_KERNEL_RWX && PPC_8xx
default 17 if STRICT_KERNEL_RWX && PPC_BOOK3S_32
default 19 if STRICT_KERNEL_RWX && PPC_8xx
default PPC_PAGE_SHIFT
help
On Book3S 32 (603+), IBATs are used to map kernel text.
Smaller is the alignment, greater is the number of necessary IBATs.
On 8xx, large pages (512kb or 8M) are used to map kernel linear
memory. Aligning to 8M reduces TLB misses as only 8M pages are used
in that case.
config DATA_SHIFT_BOOL config DATA_SHIFT_BOOL
bool "Set custom data alignment" if STRICT_KERNEL_RWX && \ bool "Set custom data alignment" if STRICT_KERNEL_RWX && \
(PPC_BOOK3S_32 || PPC_8xx) (PPC_BOOK3S_32 || PPC_8xx)

3
arch/powerpc/kernel/vmlinux.lds.S
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@ -15,7 +15,6 @@
#include <asm/thread_info.h> #include <asm/thread_info.h>
#define STRICT_ALIGN_SIZE (1 << CONFIG_DATA_SHIFT) #define STRICT_ALIGN_SIZE (1 << CONFIG_DATA_SHIFT)
#define ETEXT_ALIGN_SIZE (1 << CONFIG_ETEXT_SHIFT)
ENTRY(_stext) ENTRY(_stext)
@ -116,7 +115,7 @@ SECTIONS
} :text } :text
. = ALIGN(ETEXT_ALIGN_SIZE); . = ALIGN(PAGE_SIZE);
_etext = .; _etext = .;
PROVIDE32 (etext = .); PROVIDE32 (etext = .);