[PATCH 5.10.y 1/2] arm64: Always load shadow stack pointer directly from the task struct

[PATCH 5.10.y 1/2] arm64: Always load shadow stack pointer directly from the task struct

[Ard Biesheuvel]

All occurrences of the scs_load macro load the value of the shadow call
stack pointer from the task which is current at that point. So instead
of taking a task struct register argument in the scs_load macro to
specify the task struct to load from, let's always reference the current
task directly. This should make it much harder to exploit any
instruction sequences reloading the shadow call stack pointer register
from memory.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20230109174800.3286265-2-ardb@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 arch/arm64/include/asm/scs.h | 7 ++++---
 arch/arm64/kernel/entry.S    | 4 ++--
 arch/arm64/kernel/head.S     | 2 +-
 3 files changed, 7 insertions(+), 6 deletions(-)

diff --git a/arch/arm64/include/asm/scs.h b/arch/arm64/include/asm/scs.h
index eaa2cd92e4c10122..7155055a5bebc17d 100644
--- a/arch/arm64/include/asm/scs.h
+++ b/arch/arm64/include/asm/scs.h
@@ -9,15 +9,16 @@
 #ifdef CONFIG_SHADOW_CALL_STACK
 	scs_sp	.req	x18
 
-	.macro scs_load tsk, tmp
-	ldr	scs_sp, [\tsk, #TSK_TI_SCS_SP]
+	.macro scs_load_current
+	get_current_task scs_sp
+	ldr	scs_sp, [scs_sp, #TSK_TI_SCS_SP]
 	.endm
 
 	.macro scs_save tsk, tmp
 	str	scs_sp, [\tsk, #TSK_TI_SCS_SP]
 	.endm
 #else
-	.macro scs_load tsk, tmp
+	.macro scs_load_current
 	.endm
 
 	.macro scs_save tsk, tmp
diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index d5bc1dbdd2fda84c..28d4cdeee5ae6083 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -221,7 +221,7 @@ alternative_else_nop_endif
 
 	ptrauth_keys_install_kernel tsk, x20, x22, x23
 
-	scs_load tsk, x20
+	scs_load_current
 	.else
 	add	x21, sp, #S_FRAME_SIZE
 	get_current_task tsk
@@ -1025,7 +1025,7 @@ SYM_FUNC_START(cpu_switch_to)
 	msr	sp_el0, x1
 	ptrauth_keys_install_kernel x1, x8, x9, x10
 	scs_save x0, x8
-	scs_load x1, x8
+	scs_load_current
 	ret
 SYM_FUNC_END(cpu_switch_to)
 NOKPROBE(cpu_switch_to)
diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
index e1c25fa3b8e6ca65..351ee64c7deb4c96 100644
--- a/arch/arm64/kernel/head.S
+++ b/arch/arm64/kernel/head.S
@@ -747,7 +747,7 @@ SYM_FUNC_START_LOCAL(__secondary_switched)
 	ldr	x2, [x0, #CPU_BOOT_TASK]
 	cbz	x2, __secondary_too_slow
 	msr	sp_el0, x2
-	scs_load x2, x3
+	scs_load_current
 	mov	x29, #0
 	mov	x30, #0
 
-- 
2.39.2

[PATCH 5.10.y 2/2] arm64: Stash shadow stack pointer in the task struct on interrupt

[Ard Biesheuvel]

commit 59b37fe52f49955791a460752c37145f1afdcad1 upstream.

Instead of reloading the shadow call stack pointer from the ordinary
stack *, which may be vulnerable to the kind of gadget based attacks
shadow call stacks were designed to prevent, let's store a task's shadow
call stack pointer in the task struct when switching to the shadow IRQ
stack.

Given that currently, the task_struct::scs_sp field is only used to
preserve the shadow call stack pointer while a task is scheduled out or
running in user space, reusing this field to preserve and restore it
while running off the IRQ stack must be safe, as those occurrences are
guaranteed to never overlap. (The stack switching logic only switches
stacks when running from the task stack, and so the value being saved
here always corresponds to the task mode shadow stack)

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20230109174800.3286265-3-ardb@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
[ardb: v5.10 backport, which doesn't have call_on_irq_stack() yet *]
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 arch/arm64/kernel/entry.S | 8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 28d4cdeee5ae6083..55e477f73158d6f8 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -431,9 +431,7 @@ SYM_CODE_END(__swpan_exit_el0)
 
 	.macro	irq_stack_entry
 	mov	x19, sp			// preserve the original sp
-#ifdef CONFIG_SHADOW_CALL_STACK
-	mov	x24, scs_sp		// preserve the original shadow stack
-#endif
+	scs_save tsk			// preserve the original shadow stack
 
 	/*
 	 * Compare sp with the base of the task stack.
@@ -467,9 +465,7 @@ SYM_CODE_END(__swpan_exit_el0)
 	 */
 	.macro	irq_stack_exit
 	mov	sp, x19
-#ifdef CONFIG_SHADOW_CALL_STACK
-	mov	scs_sp, x24
-#endif
+	scs_load_current
 	.endm
 
 /* GPRs used by entry code */
-- 
2.39.2

Re: [PATCH 5.10.y 1/2] arm64: Always load shadow stack pointer directly from the task struct

[Greg KH]

On Sat, May 06, 2023 at 02:34:33PM +0200, Ard Biesheuvel wrote:
> All occurrences of the scs_load macro load the value of the shadow call
> stack pointer from the task which is current at that point. So instead
> of taking a task struct register argument in the scs_load macro to
> specify the task struct to load from, let's always reference the current
> task directly. This should make it much harder to exploit any
> instruction sequences reloading the shadow call stack pointer register
> from memory.
> 
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> Acked-by: Mark Rutland <mark.rutland@arm.com>
> Reviewed-by: Kees Cook <keescook@chromium.org>
> Link: https://lore.kernel.org/r/20230109174800.3286265-2-ardb@kernel.org
> Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
>  arch/arm64/include/asm/scs.h | 7 ++++---
>  arch/arm64/kernel/entry.S    | 4 ++--
>  arch/arm64/kernel/head.S     | 2 +-
>  3 files changed, 7 insertions(+), 6 deletions(-)

What is the git commit id of this in Linus's tree?

thanks,

greg k-h

Re: [PATCH 5.10.y 1/2] arm64: Always load shadow stack pointer directly from the task struct

[Ard Biesheuvel]

On Sun, 7 May 2023 at 07:11, Greg KH <gregkh@linuxfoundation.org> wrote:
>
> On Sat, May 06, 2023 at 02:34:33PM +0200, Ard Biesheuvel wrote:
> > All occurrences of the scs_load macro load the value of the shadow call
> > stack pointer from the task which is current at that point. So instead
> > of taking a task struct register argument in the scs_load macro to
> > specify the task struct to load from, let's always reference the current
> > task directly. This should make it much harder to exploit any
> > instruction sequences reloading the shadow call stack pointer register
> > from memory.
> >
> > Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> > Acked-by: Mark Rutland <mark.rutland@arm.com>
> > Reviewed-by: Kees Cook <keescook@chromium.org>
> > Link: https://lore.kernel.org/r/20230109174800.3286265-2-ardb@kernel.org
> > Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
> > Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> > ---
> >  arch/arm64/include/asm/scs.h | 7 ++++---
> >  arch/arm64/kernel/entry.S    | 4 ++--
> >  arch/arm64/kernel/head.S     | 2 +-
> >  3 files changed, 7 insertions(+), 6 deletions(-)
>
> What is the git commit id of this in Linus's tree?
>

commit 2198d07c509f1db4a1185d1f65aaada794c6ea59 upstream.

Thanks,

Re: [PATCH 5.10.y 1/2] arm64: Always load shadow stack pointer directly from the task struct

[Greg KH]

On Sun, May 07, 2023 at 10:11:32AM +0200, Ard Biesheuvel wrote:
> On Sun, 7 May 2023 at 07:11, Greg KH <gregkh@linuxfoundation.org> wrote:
> >
> > On Sat, May 06, 2023 at 02:34:33PM +0200, Ard Biesheuvel wrote:
> > > All occurrences of the scs_load macro load the value of the shadow call
> > > stack pointer from the task which is current at that point. So instead
> > > of taking a task struct register argument in the scs_load macro to
> > > specify the task struct to load from, let's always reference the current
> > > task directly. This should make it much harder to exploit any
> > > instruction sequences reloading the shadow call stack pointer register
> > > from memory.
> > >
> > > Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> > > Acked-by: Mark Rutland <mark.rutland@arm.com>
> > > Reviewed-by: Kees Cook <keescook@chromium.org>
> > > Link: https://lore.kernel.org/r/20230109174800.3286265-2-ardb@kernel.org
> > > Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
> > > Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> > > ---
> > >  arch/arm64/include/asm/scs.h | 7 ++++---
> > >  arch/arm64/kernel/entry.S    | 4 ++--
> > >  arch/arm64/kernel/head.S     | 2 +-
> > >  3 files changed, 7 insertions(+), 6 deletions(-)
> >
> > What is the git commit id of this in Linus's tree?
> >
> 
> commit 2198d07c509f1db4a1185d1f65aaada794c6ea59 upstream.

Thanks, both now queued up!

greg k-h