* [PATCH 1/2] arm64: Always load shadow stack pointer directly from the task struct
2022-12-05 20:03 [PATCH 0/2] arm64: harden shadow call stack pointer handling Ard Biesheuvel
@ 2022-12-05 20:03 ` Ard Biesheuvel
2022-12-05 20:03 ` [PATCH 2/2] arm64: Stash shadow stack pointer in the task struct on interrupt Ard Biesheuvel
1 sibling, 0 replies; 3+ messages in thread
From: Ard Biesheuvel @ 2022-12-05 20:03 UTC (permalink / raw)
To: linux-arm-kernel
Cc: will, catalin.marinas, mark.rutland, Ard Biesheuvel,
Sami Tolvanen, Kees Cook
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>
---
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 8297bccf0784577e..5cd4d09bc69d7f6d 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
- 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
str scs_sp, [\tsk, #TSK_TI_SCS_SP]
.endm
#else
- .macro scs_load tsk
+ .macro scs_load_current
.endm
.macro scs_save tsk
diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index e28137d64b7688e2..20e25083eced13f5 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -272,7 +272,7 @@ alternative_if ARM64_HAS_ADDRESS_AUTH
alternative_else_nop_endif
1:
- scs_load tsk
+ scs_load_current
.else
add x21, sp, #PT_REGS_SIZE
get_current_task tsk
@@ -845,7 +845,7 @@ SYM_FUNC_START(cpu_switch_to)
msr sp_el0, x1
ptrauth_keys_install_kernel x1, x8, x9, x10
scs_save x0
- scs_load x1
+ 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 2196aad7b55bcef0..cdbbc95eb49d025a 100644
--- a/arch/arm64/kernel/head.S
+++ b/arch/arm64/kernel/head.S
@@ -404,7 +404,7 @@ SYM_FUNC_END(create_kernel_mapping)
stp xzr, xzr, [sp, #S_STACKFRAME]
add x29, sp, #S_STACKFRAME
- scs_load \tsk
+ scs_load_current
adr_l \tmp1, __per_cpu_offset
ldr w\tmp2, [\tsk, #TSK_TI_CPU]
--
2.35.1
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* [PATCH 2/2] arm64: Stash shadow stack pointer in the task struct on interrupt
2022-12-05 20:03 [PATCH 0/2] arm64: harden shadow call stack pointer handling Ard Biesheuvel
2022-12-05 20:03 ` [PATCH 1/2] arm64: Always load shadow stack pointer directly from the task struct Ard Biesheuvel
@ 2022-12-05 20:03 ` Ard Biesheuvel
1 sibling, 0 replies; 3+ messages in thread
From: Ard Biesheuvel @ 2022-12-05 20:03 UTC (permalink / raw)
To: linux-arm-kernel
Cc: will, catalin.marinas, mark.rutland, Ard Biesheuvel,
Sami Tolvanen, Kees Cook
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)
While at it, fold a mov/add/mov sequence into a single add.
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
arch/arm64/kernel/entry.S | 12 +++++-------
1 file changed, 5 insertions(+), 7 deletions(-)
diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 20e25083eced13f5..3671d9521d4f559e 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -873,19 +873,19 @@ NOKPROBE(ret_from_fork)
*/
SYM_FUNC_START(call_on_irq_stack)
#ifdef CONFIG_SHADOW_CALL_STACK
- stp scs_sp, xzr, [sp, #-16]!
+ get_current_task x16
+ scs_save x16
ldr_this_cpu scs_sp, irq_shadow_call_stack_ptr, x17
#endif
+
/* Create a frame record to save our LR and SP (implicit in FP) */
stp x29, x30, [sp, #-16]!
mov x29, sp
ldr_this_cpu x16, irq_stack_ptr, x17
- mov x15, #IRQ_STACK_SIZE
- add x16, x16, x15
/* Move to the new stack and call the function there */
- mov sp, x16
+ add sp, x16, #IRQ_STACK_SIZE
blr x1
/*
@@ -894,9 +894,7 @@ SYM_FUNC_START(call_on_irq_stack)
*/
mov sp, x29
ldp x29, x30, [sp], #16
-#ifdef CONFIG_SHADOW_CALL_STACK
- ldp scs_sp, xzr, [sp], #16
-#endif
+ scs_load_current
ret
SYM_FUNC_END(call_on_irq_stack)
NOKPROBE(call_on_irq_stack)
--
2.35.1
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http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
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