x86/alternatives: Use the correct length when optimizing NOPs

JIRA: https://issues.redhat.com/browse/RHEL-68940

commit 9dba9c6
Author: Borislav Petkov (AMD) <bp@alien8.de>
Date:   Wed, 15 May 2024 12:48:04 +0200

    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: da8f9cf ("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

Signed-off-by: Waiman Long <longman@redhat.com>

Author: Waiman-Long

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. */