x86/alternatives: Use the correct length when optimizing NOPs

Commit in Fixes moved the optimize_nops() call inside apply_relocation()
and made it a second optimization pass after the relocations have been
done.

Since optimize_nops() works only on NOPs, that is fine and it'll simply
jump over instructions which are not NOPs.

However, it made that call with repl_len as the buffer length to
optimize.

However, it can happen that there are alternatives calls like this one:

  alternative("mfence; lfence", "", ALT_NOT(X86_FEATURE_APIC_MSRS_FENCE));

where the replacement length is 0. And using repl_len is wrong because
apply_alternatives() expands the buffer size to the length of the source
insn that is being patched, by padding it with one-byte NOPs:

	for (; insn_buff_sz < a->instrlen; insn_buff_sz++)
		insn_buff[insn_buff_sz] = 0x90;

Long story short: pass the length of the original instruction(s) as the
length of the temporary buffer which to optimize.

Result:

  SMP alternatives: feat: 11*32+27, old: (lapic_next_deadline+0x9/0x50 (ffffffff81061829) len: 6), repl: (ffffffff89b1cc60, len: 0) flags: 0x1
  SMP alternatives: ffffffff81061829:   old_insn: 0f ae f0 0f ae e8
  SMP alternatives: ffffffff81061829: final_insn: 90 90 90 90 90 90

=>

  SMP alternatives: feat: 11*32+27, old: (lapic_next_deadline+0x9/0x50 (ffffffff81061839) len: 6), repl: (ffffffff89b1cc60, len: 0) flags: 0x1
  SMP alternatives: ffffffff81061839: [0:6) optimized NOPs: 66 0f 1f 44 00 00
  SMP alternatives: ffffffff81061839:   old_insn: 0f ae f0 0f ae e8
  SMP alternatives: ffffffff81061839: final_insn: 66 0f 1f 44 00 00

Fixes: da8f9cf7 ("x86/alternatives: Get rid of __optimize_nops()")
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20240515104804.32004-1-bp@kernel.org

arch/x86/kernel/alternative.c

@@ -372,7 +372,7 @@ static void __apply_relocation(u8 *buf, const u8 * const instr, size_t instrlen,
 void apply_relocation(u8 *buf, const u8 * const instr, size_t instrlen, u8 *repl, size_t repl_len)
 {
 	__apply_relocation(buf, instr, instrlen, repl, repl_len);
-	optimize_nops(instr, buf, repl_len);
+	optimize_nops(instr, buf, instrlen);
 }
 
 /* Low-level backend functions usable from alternative code replacements. */