[PATCH] powerpc: make show_stack's stack walking KASAN-safe

[Daniel Axtens]

Make our stack-walking code KASAN-safe by using READ_ONCE_NOCHECK -
generic code, arm64, s390 and x86 all do this for similar sorts of
reasons: when unwinding a stack, we might touch memory that KASAN has
marked as being out-of-bounds. In ppc64 KASAN development, I hit this
sometimes when checking for an exception frame - because we're checking
an arbitrary offset into the stack frame.

See commit 20955746320e ("s390/kasan: avoid false positives during stack
unwind"), commit bcaf669b4bdb ("arm64: disable kasan when accessing
frame->fp in unwind_frame"), commit 91e08ab0c851 ("x86/dumpstack:
Prevent KASAN false positive warnings") and commit 6e22c8366416
("tracing, kasan: Silence Kasan warning in check_stack of stack_tracer").

Signed-off-by: Daniel Axtens <d...@axtens.net>
---
 arch/powerpc/kernel/process.c | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
index 89e34aa273e2..430cf06f9406 100644
--- a/arch/powerpc/kernel/process.c
+++ b/arch/powerpc/kernel/process.c
@@ -2151,8 +2151,8 @@ void show_stack(struct task_struct *tsk, unsigned long 
*stack,
                        break;
 
                stack = (unsigned long *) sp;
-               newsp = stack[0];
-               ip = stack[STACK_FRAME_LR_SAVE];
+               newsp = READ_ONCE_NOCHECK(stack[0]);
+               ip = READ_ONCE_NOCHECK(stack[STACK_FRAME_LR_SAVE]);
                if (!firstframe || ip != lr) {
                        printk("%s["REG"] ["REG"] %pS",
                                loglvl, sp, ip, (void *)ip);
@@ -2170,17 +2170,19 @@ void show_stack(struct task_struct *tsk, unsigned long 
*stack,
                 * See if this is an exception frame.
                 * We look for the "regshere" marker in the current frame.
                 */
-               if (validate_sp(sp, tsk, STACK_FRAME_WITH_PT_REGS)
-                   && stack[STACK_FRAME_MARKER] == STACK_FRAME_REGS_MARKER) {
+               if (validate_sp(sp, tsk, STACK_FRAME_WITH_PT_REGS) &&
+                   (READ_ONCE_NOCHECK(stack[STACK_FRAME_MARKER]) ==
+                    STACK_FRAME_REGS_MARKER)) {
                        struct pt_regs *regs = (struct pt_regs *)
                                (sp + STACK_FRAME_OVERHEAD);
 
-                       lr = regs->link;
+                       lr = READ_ONCE_NOCHECK(regs->link);
                        printk("%s--- interrupt: %lx at %pS\n",
-                              loglvl, regs->trap, (void *)regs->nip);
+                              loglvl, READ_ONCE_NOCHECK(regs->trap),
+                              (void *)READ_ONCE_NOCHECK(regs->nip));
                        __show_regs(regs);
                        printk("%s--- interrupt: %lx\n",
-                              loglvl, regs->trap);
+                              loglvl, READ_ONCE_NOCHECK(regs->trap));
 
                        firstframe = 1;
                }
-- 
2.27.0