FAILED: patch "[PATCH] arm64: Always force a branch protection mode when the" failed to apply to 5.6-stable tree

FAILED: patch "[PATCH] arm64: Always force a branch protection mode when the" failed to apply to 5.6-stable tree

[gregkh]

The patch below does not apply to the 5.6-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From b8fdef311a0bd9223f10754f94fdcf1a594a3457 Mon Sep 17 00:00:00 2001
From: Mark Brown <broonie@kernel.org>
Date: Tue, 31 Mar 2020 20:44:59 +0100
Subject: [PATCH] arm64: Always force a branch protection mode when the
 compiler has one

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.

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>

diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
index f15f92ba53e6..85e4149cc5d5 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, this will be overridden if we are using branch protection.
+branch-prot-flags-y += $(call cc-option,-mbranch-protection=none)
+
 ifeq ($(CONFIG_ARM64_PTR_AUTH),y)
 branch-prot-flags-$(CONFIG_CC_HAS_SIGN_RETURN_ADDRESS) := -msign-return-address=all
 branch-prot-flags-$(CONFIG_CC_HAS_BRANCH_PROT_PAC_RET) := -mbranch-protection=pac-ret+leaf
@@ -73,9 +77,10 @@ branch-prot-flags-$(CONFIG_CC_HAS_BRANCH_PROT_PAC_RET) := -mbranch-protection=pa
 # we pass it only to the assembler. This option is utilized only in case of non
 # integrated assemblers.
 branch-prot-flags-$(CONFIG_AS_HAS_PAC) += -Wa,-march=armv8.3-a
-KBUILD_CFLAGS += $(branch-prot-flags-y)
 endif
 
+KBUILD_CFLAGS += $(branch-prot-flags-y)
+
 ifeq ($(CONFIG_CPU_BIG_ENDIAN), y)
 KBUILD_CPPFLAGS	+= -mbig-endian
 CHECKFLAGS	+= -D__AARCH64EB__

Re: FAILED: patch "[PATCH] arm64: Always force a branch protection mode when the" failed to apply to 5.6-stable tree

[Sasha Levin]

On Wed, Apr 15, 2020 at 01:23:24PM +0200, gregkh@linuxfoundation.org wrote:
>
>The patch below does not apply to the 5.6-stable tree.
>If someone wants it applied there, or to any other stable or longterm
>tree, then please email the backport, including the original git commit
>id to <stable@vger.kernel.org>.
>
>thanks,
>
>greg k-h
>
>------------------ original commit in Linus's tree ------------------
>
>From b8fdef311a0bd9223f10754f94fdcf1a594a3457 Mon Sep 17 00:00:00 2001
>From: Mark Brown <broonie@kernel.org>
>Date: Tue, 31 Mar 2020 20:44:59 +0100
>Subject: [PATCH] arm64: Always force a branch protection mode when the
> compiler has one
>
>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.
>
>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>

I don't think that this is needed anywhere without 74afda4016a7 ("arm64:
compile the kernel with ptrauth return address signing")?

-- 
Thanks,
Sasha

Re: FAILED: patch "[PATCH] arm64: Always force a branch protection mode when the" failed to apply to 5.6-stable tree

[Catalin Marinas]

On Wed, Apr 15, 2020 at 09:51:21PM -0400, Sasha Levin wrote:
> On Wed, Apr 15, 2020 at 01:23:24PM +0200, gregkh@linuxfoundation.org wrote:
> > The patch below does not apply to the 5.6-stable tree.
> > If someone wants it applied there, or to any other stable or longterm
> > tree, then please email the backport, including the original git commit
> > id to <stable@vger.kernel.org>.
> > 
> > thanks,
> > 
> > greg k-h
> > 
> > ------------------ original commit in Linus's tree ------------------
> > 
> > From b8fdef311a0bd9223f10754f94fdcf1a594a3457 Mon Sep 17 00:00:00 2001
> > From: Mark Brown <broonie@kernel.org>
> > Date: Tue, 31 Mar 2020 20:44:59 +0100
> > Subject: [PATCH] arm64: Always force a branch protection mode when the
> > compiler has one
> > 
> > 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.
> > 
> > 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>
> 
> I don't think that this is needed anywhere without 74afda4016a7 ("arm64:
> compile the kernel with ptrauth return address signing")?

Good point. Mark, is the Fixes line above correct or it should have been
the one Sasha mentions?

-- 
Catalin

Re: FAILED: patch "[PATCH] arm64: Always force a branch protection mode when the" failed to apply to 5.6-stable tree

[Mark Brown]

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On Thu, Apr 16, 2020 at 09:32:05AM +0100, Catalin Marinas wrote:
> On Wed, Apr 15, 2020 at 09:51:21PM -0400, Sasha Levin wrote:

> > I don't think that this is needed anywhere without 74afda4016a7 ("arm64:
> > compile the kernel with ptrauth return address signing")?

> Good point. Mark, is the Fixes line above correct or it should have been
> the one Sasha mentions?

Yes, Sasha's is right.

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Re: FAILED: patch "[PATCH] arm64: Always force a branch protection mode when the" failed to apply to 5.6-stable tree

[Mark Brown]

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On Thu, Apr 16, 2020 at 10:35:17AM +0100, Mark Brown wrote:
> On Thu, Apr 16, 2020 at 09:32:05AM +0100, Catalin Marinas wrote:
> > On Wed, Apr 15, 2020 at 09:51:21PM -0400, Sasha Levin wrote:

> > > I don't think that this is needed anywhere without 74afda4016a7 ("arm64:
> > > compile the kernel with ptrauth return address signing")?

> > Good point. Mark, is the Fixes line above correct or it should have been
> > the one Sasha mentions?

> Yes, Sasha's is right.

Actually, no - we do need a version of the fix for older versions prior
to the kernel having pointer auth support since the goal was to ensure
that the kernel doesn't actually get built with pointer auth when we are
doing it for userspace as that leaves us with half baked pointer auth in
the code.  The original fixes was right and we want a redone backport.

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