[PATCH v2 05/15] x86/alternatives: Use optimized NOPs for padding

[Borislav Petkov]

From: Borislav Petkov <bp@xxxxxxx>

Alternatives allow now for an empty old instruction. In this case we go
and pad the space with NOPs at assembly time. However, there are the
optimal, longer NOPs which should be used. Do that at patching time by
adding alt_instr.padlen-sized NOPs at the old instruction address.

Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
---
 arch/x86/kernel/alternative.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 715af37bf008..af397cc98d05 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -323,6 +323,14 @@ done:
 		n_dspl, (unsigned long)orig_insn + n_dspl + repl_len);
 }
 
+static void __init_or_module optimize_nops(struct alt_instr *a, u8 *instr)
+{
+	add_nops(instr + (a->instrlen - a->padlen), a->padlen);
+
+	DUMP_BYTES(instr, a->instrlen, "%p: [%d:%d) optimized NOPs: ",
+		   instr, a->instrlen - a->padlen, a->padlen);
+}
+
 /*
  * Replace instructions with better alternatives for this CPU type. This runs
  * before SMP is initialized to avoid SMP problems with self modifying code.
@@ -354,8 +362,12 @@ void __init_or_module apply_alternatives(struct alt_instr *start,
 		replacement = (u8 *)&a->repl_offset + a->repl_offset;
 		BUG_ON(a->instrlen > sizeof(insnbuf));
 		BUG_ON(a->cpuid >= (NCAPINTS + NBUGINTS) * 32);
-		if (!boot_cpu_has(a->cpuid))
+		if (!boot_cpu_has(a->cpuid)) {
+			if (a->padlen > 1)
+				optimize_nops(a, instr);
+
 			continue;
+		}
 
 		DPRINTK("feat: %d*32+%d, old: (%p, len: %d), repl: (%p, len: %d)",
 			a->cpuid >> 5,
-- 
2.2.0.33.gc18b867

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/