Re: [PATCH 4/9] x86/alternative: Implement .retpoline_sites support

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Oct 13, 2021 at 01:39:27PM -0700, Josh Poimboeuf wrote:
> On Wed, Oct 13, 2021 at 02:22:21PM +0200, Peter Zijlstra wrote:
> > +static int patch_retpoline(void *addr, struct insn *insn, u8 *bytes)
> > +{
> > +	void (*target)(void);
> > +	int reg, i = 0;
> > +
> > +	if (cpu_feature_enabled(X86_FEATURE_RETPOLINE))
> > +		return -1;
> 
> Better to do this check further up the call stack in apply_retpolines()
> before looping through all the call sites?

In fact, I've pushed it further down, in order to always validate the
absense of rsp.

> > +
> > +	target = addr + insn->length + insn->immediate.value;
> > +	reg = (target - &__x86_indirect_thunk_rax) /
> > +	      (&__x86_indirect_thunk_rcx - &__x86_indirect_thunk_rax);
> > +
> > +	if (WARN_ON_ONCE(reg & ~0xf))
> > +		return -1;
> 
> It would be more robust and less magical to just have a basic lookup
> table array which converts a thunk address to a reg.  Then you can just
> avoid all the safety checks because it's no longer insane ;-)

Andrew suggested the reverse lookup to validate. That should give the
same robustness but lacks the linear lookup.

---
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -392,6 +392,12 @@ static int emit_indirect(int op, int reg
  */
 static int patch_retpoline(void *addr, struct insn *insn, u8 *bytes)
 {
+	static const void *reg_to_thunk[] = {
+#undef GEN
+#define GEN(reg) &__x86_indirect_thunk_ ## reg,
+#include <asm/GEN-for-each-reg.h>
+	};
+
 	void (*target)(void);
 	int reg, i = 0;
 
@@ -402,6 +408,8 @@ static int patch_retpoline(void *addr, s
 	if (WARN_ON_ONCE(reg & ~0xf))
 		return -1;
 
+	BUG_ON(target != reg_to_thunk[reg]);
+
 	/*
 	 * If anyone ever does: CALL/JMP *%rsp, we're in deep trouble.
 	 */