Commit dda7bb76 authored by Juergen Gross's avatar Juergen Gross Committed by Borislav Petkov
Browse files

x86/alternative: Support not-feature 



Add support for alternative patching for the case a feature is not
present on the current CPU. For users of ALTERNATIVE() and friends, an
inverted feature is specified by applying the ALT_NOT() macro to it,
e.g.:

  ALTERNATIVE(old, new, ALT_NOT(feature));

Committer note:

The decision to encode the NOT-bit in the feature bit itself is because
a future change which would make objtool generate such alternative
calls, would keep the code in objtool itself fairly simple.

Also, this allows for the alternative macros to support the NOT feature
without having to change them.

Finally, the u16 cpuid member encoding the X86_FEATURE_ flags is not an
ABI so if more bits are needed, cpuid itself can be enlarged or a flags
field can be added to struct alt_instr after having considered the size
growth in either cases.

Signed-off-by: default avatarJuergen Gross <jgross@suse.com>
Signed-off-by: default avatarBorislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20210311142319.4723-6-jgross@suse.com
parent a0e2bf7c

arch/x86/include/asm/alternative.h

+3 −0
Original line number Diff line number Diff line
@@ -6,6 +6,9 @@ 
#include <linux/stringify.h>

#include <asm/asm.h>



#define ALTINSTR_FLAG_INV	(1 << 15)

#define ALT_NOT(feat)		((feat) | ALTINSTR_FLAG_INV)



#ifndef __ASSEMBLY__



#include <linux/stddef.h>

arch/x86/kernel/alternative.c

+15 −5
Original line number Diff line number Diff line
@@ -388,21 +388,31 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start, 
	 */

	for (a = start; a < end; a++) {

		int insn_buff_sz = 0;

		/* Mask away "NOT" flag bit for feature to test. */

		u16 feature = a->cpuid & ~ALTINSTR_FLAG_INV;



		instr = (u8 *)&a->instr_offset + a->instr_offset;

		replacement = (u8 *)&a->repl_offset + a->repl_offset;

		BUG_ON(a->instrlen > sizeof(insn_buff));

		BUG_ON(a->cpuid >= (NCAPINTS + NBUGINTS) * 32);

		if (!boot_cpu_has(a->cpuid)) {

		BUG_ON(feature >= (NCAPINTS + NBUGINTS) * 32);



		/*

		 * Patch if either:

		 * - feature is present

		 * - feature not present but ALTINSTR_FLAG_INV is set to mean,

		 *   patch if feature is *NOT* present.

		 */

		if (!boot_cpu_has(feature) == !(a->cpuid & ALTINSTR_FLAG_INV)) {

			if (a->padlen > 1)

				optimize_nops(a, instr);



			continue;

		}



		DPRINTK("feat: %d*32+%d, old: (%pS (%px) len: %d), repl: (%px, len: %d), pad: %d",

			a->cpuid >> 5,

			a->cpuid & 0x1f,

		DPRINTK("feat: %s%d*32+%d, old: (%pS (%px) len: %d), repl: (%px, len: %d), pad: %d",

			(a->cpuid & ALTINSTR_FLAG_INV) ? "!" : "",

			feature >> 5,

			feature & 0x1f,

			instr, instr, a->instrlen,

			replacement, a->replacementlen, a->padlen);