kprobes: Avoid false KASAN reports during stack copy

Kprobes save and restore raw stack chunks with memcpy().
With KASAN these chunks can contain poisoned stack redzones,
as the result memcpy() interceptor produces false
stack out-of-bounds reports.

Use __memcpy() instead of memcpy() for stack copying.
__memcpy() is not instrumented by KASAN and does not lead
to the false reports.

Currently there is a spew of KASAN reports during boot
if CONFIG_KPROBES_SANITY_TEST is enabled:

[   ] Kprobe smoke test: started
[   ] ==================================================================
[   ] BUG: KASAN: stack-out-of-bounds in setjmp_pre_handler+0x17c/0x280 at addr ffff88085259fba8
[   ] Read of size 64 by task swapper/0/1
[   ] page:ffffea00214967c0 count:0 mapcount:0 mapping:          (null) index:0x0
[   ] flags: 0x2fffff80000000()
[   ] page dumped because: kasan: bad access detected
[...]

Reported-by: CAI Qian <[email protected]>
Tested-by: CAI Qian <[email protected]>
Signed-off-by: Dmitry Vyukov <[email protected]>
Acked-by: Masami Hiramatsu <[email protected]>
Cc: Alexander Potapenko <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Ananth N Mavinakayanahalli <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Andrey Ryabinin <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Anil S Keshavamurthy <[email protected]>
Cc: Arnaldo Carvalho de Melo <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Brian Gerst <[email protected]>
Cc: David S. Miller <[email protected]>
Cc: Denys Vlasenko <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Josh Poimboeuf <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: [email protected]
[ Improved various details. ]
Signed-off-by: Ingo Molnar <[email protected]>

arch/x86/kernel/kprobes/core.c

@@ -1057,9 +1057,10 @@ int setjmp_pre_handler(struct kprobe *p, struct pt_regs *regs)
 	 * tailcall optimization. So, to be absolutely safe
 	 * we also save and restore enough stack bytes to cover
 	 * the argument area.
+	 * Use __memcpy() to avoid KASAN stack out-of-bounds reports as we copy
+	 * raw stack chunk with redzones:
 	 */
-	memcpy(kcb->jprobes_stack, (kprobe_opcode_t *)addr,
-	       MIN_STACK_SIZE(addr));
+	__memcpy(kcb->jprobes_stack, (kprobe_opcode_t *)addr, MIN_STACK_SIZE(addr));
 	regs->flags &= ~X86_EFLAGS_IF;
 	trace_hardirqs_off();
 	regs->ip = (unsigned long)(jp->entry);
@@ -1118,7 +1119,7 @@ int longjmp_break_handler(struct kprobe *p, struct pt_regs *regs)
 		/* It's OK to start function graph tracing again */
 		unpause_graph_tracing();
 		*regs = kcb->jprobe_saved_regs;
-		memcpy(saved_sp, kcb->jprobes_stack, MIN_STACK_SIZE(saved_sp));
+		__memcpy(saved_sp, kcb->jprobes_stack, MIN_STACK_SIZE(saved_sp));
 		preempt_enable_no_resched();
 		return 1;
 	}