[PATCH 5.6] arm64: Always force a branch protection mode when the compiler has one

[PATCH 5.6] arm64: Always force a branch protection mode when the compiler has one

[Mark Brown]

Compilers with branch protection support can be configured to enable it by
default, it is likely that distributions will do this as part of deploying
branch protection system wide. As well as the slight overhead from having
some extra NOPs for unused branch protection features this can cause more
serious problems when the kernel is providing pointer authentication to
userspace but not built for pointer authentication itself. In that case our
switching of keys for userspace can affect the kernel unexpectedly, causing
pointer authentication instructions in the kernel to corrupt addresses.

To ensure that we get consistent and reliable behaviour always explicitly
initialise the branch protection mode, ensuring that the kernel is built
the same way regardless of the compiler defaults.

[This is a reworked version of b8fdef311a0bd9223f1075 ("arm64: Always
force a branch protection mode when the compiler has one") for backport.
Kernels prior to 74afda4016a7 ("arm64: compile the kernel with ptrauth
return address signing") don't have any Makefile machinery for forcing
on pointer auth but still have issues if the compiler defaults it on so
need this reworked version. -- broonie]

Fixes: 7503197562567 (arm64: add basic pointer authentication support)
Reported-by: Szabolcs Nagy <szabolcs.nagy@arm.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org
[catalin.marinas@arm.com: remove Kconfig option in favour of Makefile check]
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
---
 arch/arm64/Makefile | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
index dca1a97751ab..4e6ce2d9196e 100644
--- a/arch/arm64/Makefile
+++ b/arch/arm64/Makefile
@@ -65,6 +65,10 @@ stack_protector_prepare: prepare0
 					include/generated/asm-offsets.h))
 endif
 
+# Ensure that if the compiler supports branch protection we default it
+# off.
+KBUILD_CFLAGS += $(call cc-option,-mbranch-protection=none)
+
 ifeq ($(CONFIG_CPU_BIG_ENDIAN), y)
 KBUILD_CPPFLAGS	+= -mbig-endian
 CHECKFLAGS	+= -D__AARCH64EB__
-- 
2.20.1

[PATCH 5.6] arm64: Always force a branch protection mode when the compiler has one

[Mark Brown]

Compilers with branch protection support can be configured to enable it by
default, it is likely that distributions will do this as part of deploying
branch protection system wide. As well as the slight overhead from having
some extra NOPs for unused branch protection features this can cause more
serious problems when the kernel is providing pointer authentication to
userspace but not built for pointer authentication itself. In that case our
switching of keys for userspace can affect the kernel unexpectedly, causing
pointer authentication instructions in the kernel to corrupt addresses.

To ensure that we get consistent and reliable behaviour always explicitly
initialise the branch protection mode, ensuring that the kernel is built
the same way regardless of the compiler defaults.

[This is a reworked version of b8fdef311a0bd9223f1075 ("arm64: Always
force a branch protection mode when the compiler has one") for backport.
Kernels prior to 74afda4016a7 ("arm64: compile the kernel with ptrauth
return address signing") don't have any Makefile machinery for forcing
on pointer auth but still have issues if the compiler defaults it on so
need this reworked version. -- broonie]

Fixes: 7503197562567 (arm64: add basic pointer authentication support)
Reported-by: Szabolcs Nagy <szabolcs.nagy@arm.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org
[catalin.marinas@arm.com: remove Kconfig option in favour of Makefile check]
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
---
 arch/arm64/Makefile | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
index dca1a97751ab..4e6ce2d9196e 100644
--- a/arch/arm64/Makefile
+++ b/arch/arm64/Makefile
@@ -65,6 +65,10 @@ stack_protector_prepare: prepare0
 					include/generated/asm-offsets.h))
 endif
 
+# Ensure that if the compiler supports branch protection we default it
+# off.
+KBUILD_CFLAGS += $(call cc-option,-mbranch-protection=none)
+
 ifeq ($(CONFIG_CPU_BIG_ENDIAN), y)
 KBUILD_CPPFLAGS	+= -mbig-endian
 CHECKFLAGS	+= -D__AARCH64EB__
-- 
2.20.1


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Re: [PATCH 5.6] arm64: Always force a branch protection mode when the compiler has one

[Greg KH]

On Thu, Apr 16, 2020 at 12:24:30PM +0100, Mark Brown wrote:
> Compilers with branch protection support can be configured to enable it by
> default, it is likely that distributions will do this as part of deploying
> branch protection system wide. As well as the slight overhead from having
> some extra NOPs for unused branch protection features this can cause more
> serious problems when the kernel is providing pointer authentication to
> userspace but not built for pointer authentication itself. In that case our
> switching of keys for userspace can affect the kernel unexpectedly, causing
> pointer authentication instructions in the kernel to corrupt addresses.
> 
> To ensure that we get consistent and reliable behaviour always explicitly
> initialise the branch protection mode, ensuring that the kernel is built
> the same way regardless of the compiler defaults.
> 
> [This is a reworked version of b8fdef311a0bd9223f1075 ("arm64: Always
> force a branch protection mode when the compiler has one") for backport.
> Kernels prior to 74afda4016a7 ("arm64: compile the kernel with ptrauth
> return address signing") don't have any Makefile machinery for forcing
> on pointer auth but still have issues if the compiler defaults it on so
> need this reworked version. -- broonie]
> 
> Fixes: 7503197562567 (arm64: add basic pointer authentication support)
> Reported-by: Szabolcs Nagy <szabolcs.nagy@arm.com>
> Signed-off-by: Mark Brown <broonie@kernel.org>
> Cc: stable@vger.kernel.org
> [catalin.marinas@arm.com: remove Kconfig option in favour of Makefile check]
> Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
> ---
>  arch/arm64/Makefile | 4 ++++
>  1 file changed, 4 insertions(+)

Now queued up, thanks!

greg k-h

Re: [PATCH 5.6] arm64: Always force a branch protection mode when the compiler has one

[Greg KH]

On Thu, Apr 16, 2020 at 12:24:30PM +0100, Mark Brown wrote:
> Compilers with branch protection support can be configured to enable it by
> default, it is likely that distributions will do this as part of deploying
> branch protection system wide. As well as the slight overhead from having
> some extra NOPs for unused branch protection features this can cause more
> serious problems when the kernel is providing pointer authentication to
> userspace but not built for pointer authentication itself. In that case our
> switching of keys for userspace can affect the kernel unexpectedly, causing
> pointer authentication instructions in the kernel to corrupt addresses.
> 
> To ensure that we get consistent and reliable behaviour always explicitly
> initialise the branch protection mode, ensuring that the kernel is built
> the same way regardless of the compiler defaults.
> 
> [This is a reworked version of b8fdef311a0bd9223f1075 ("arm64: Always
> force a branch protection mode when the compiler has one") for backport.
> Kernels prior to 74afda4016a7 ("arm64: compile the kernel with ptrauth
> return address signing") don't have any Makefile machinery for forcing
> on pointer auth but still have issues if the compiler defaults it on so
> need this reworked version. -- broonie]
> 
> Fixes: 7503197562567 (arm64: add basic pointer authentication support)
> Reported-by: Szabolcs Nagy <szabolcs.nagy@arm.com>
> Signed-off-by: Mark Brown <broonie@kernel.org>
> Cc: stable@vger.kernel.org
> [catalin.marinas@arm.com: remove Kconfig option in favour of Makefile check]
> Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
> ---
>  arch/arm64/Makefile | 4 ++++
>  1 file changed, 4 insertions(+)

Now queued up, thanks!

greg k-h

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