[PATCH v8 06/12] arm64: preserve x18 when CPU is suspended

Sami Tolvanen
2020-02-19T00:10:23Z

Don't lose the current task's shadow stack when the CPU is suspended.

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Will Deacon <will@kernel.org>
---
arch/arm64/include/asm/suspend.h | 2 +-
arch/arm64/mm/proc.S | 14 ++++++++++++++
2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/suspend.h b/arch/arm64/include/asm/suspend.h
index 8939c87c4dce..0cde2f473971 100644
--- a/arch/arm64/include/asm/suspend.h
+++ b/arch/arm64/include/asm/suspend.h
@@ -2,7 +2,7 @@
#ifndef __ASM_SUSPEND_H
#define __ASM_SUSPEND_H

-#define NR_CTX_REGS 12
+#define NR_CTX_REGS 13
#define NR_CALLEE_SAVED_REGS 12

/*
diff --git a/arch/arm64/mm/proc.S b/arch/arm64/mm/proc.S
index aafed6902411..7d37e3c70ff5 100644
--- a/arch/arm64/mm/proc.S
+++ b/arch/arm64/mm/proc.S
@@ -56,6 +56,8 @@
* cpu_do_suspend - save CPU registers context
*
* x0: virtual address of context pointer
+ *
+ * This must be kept in sync with struct cpu_suspend_ctx in <asm/suspend.h>.
*/
SYM_FUNC_START(cpu_do_suspend)
mrs x2, tpidr_el0
@@ -80,6 +82,11 @@ alternative_endif
stp x8, x9, [x0, #48]
stp x10, x11, [x0, #64]
stp x12, x13, [x0, #80]
+ /*
+ * Save x18 as it may be used as a platform register, e.g. by shadow
+ * call stack.
+ */
+ str x18, [x0, #96]
ret
SYM_FUNC_END(cpu_do_suspend)

@@ -96,6 +103,13 @@ SYM_FUNC_START(cpu_do_resume)
ldp x9, x10, [x0, #48]
ldp x11, x12, [x0, #64]
ldp x13, x14, [x0, #80]
+ /*
+ * Restore x18, as it may be used as a platform register, and clear
+ * the buffer to minimize the risk of exposure when used for shadow
+ * call stack.
+ */
+ ldr x18, [x0, #96]
+ str xzr, [x0, #96]
msr tpidr_el0, x2
msr tpidrro_el0, x3
msr contextidr_el1, x4
--
2.25.0.265.gbab2e86ba0-goog