arm64: Preserve register x18 when CPU is suspended

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>
Signed-off-by: Will Deacon <will@kernel.org>

diff --git a/arch/arm64/include/asm/suspend.h b/arch/arm64/include/asm/suspend.h
index 8939c87..0cde2f4 100644 (file)
--- 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 197a9ba..ed15be0 100644 (file)
--- a/arch/arm64/mm/proc.S
+++ b/arch/arm64/mm/proc.S
@@ -58,6 +58,8 @@
  * 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
@@ -82,6 +84,11 @@ 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)
 
@@ -98,6 +105,13 @@ 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