x86/alternatives: Add instruction padding

Up until now we have always paid attention to make sure the length of
the new instruction replacing the old one is at least less or equal to
the length of the old instruction. If the new instruction is longer, at
the time it replaces the old instruction it will overwrite the beginning
of the next instruction in the kernel image and cause your pants to
catch fire.

So instead of having to pay attention, teach the alternatives framework
to pad shorter old instructions with NOPs at buildtime - but only in the
case when

  len(old instruction(s)) < len(new instruction(s))

and add nothing in the >= case. (In that case we do add_nops() when
patching).

This way the alternatives user shouldn't have to care about instruction
sizes and simply use the macros.

Add asm ALTERNATIVE* flavor macros too, while at it.

Also, we need to save the pad length in a separate struct alt_instr
member for NOP optimization and the way to do that reliably is to carry
the pad length instead of trying to detect whether we're looking at
single-byte NOPs or at pathological instruction offsets like e9 90 90 90
90, for example, which is a valid instruction.

Thanks to Michael Matz for the great help with toolchain questions.

Signed-off-by: Borislav Petkov <bp@suse.de>

1 files changed, 3 insertions, 3 deletions

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 1e86e85bcf58..c99b0f13a90e 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -270,7 +270,6 @@ void __init_or_module apply_alternatives(struct alt_instr *start,
 	for (a = start; a < end; a++) {
 		instr = (u8 *)&a->instr_offset + a->instr_offset;
 		replacement = (u8 *)&a->repl_offset + a->repl_offset;
-		BUG_ON(a->replacementlen > a->instrlen);
 		BUG_ON(a->instrlen > sizeof(insnbuf));
 		BUG_ON(a->cpuid >= (NCAPINTS + NBUGINTS) * 32);
 		if (!boot_cpu_has(a->cpuid))
@@ -290,8 +289,9 @@ void __init_or_module apply_alternatives(struct alt_instr *start,
 			DPRINTK("Fix CALL offset: 0x%x", *(s32 *)(insnbuf + 1));
 		}
 
-		add_nops(insnbuf + a->replacementlen,
-			 a->instrlen - a->replacementlen);
+		if (a->instrlen > a->replacementlen)
+			add_nops(insnbuf + a->replacementlen,
+				 a->instrlen - a->replacementlen);
 
 		text_poke_early(instr, insnbuf, a->instrlen);
 	}