x86/alternative: Support not-feature

commit dda7bb76484978316bb412a353789ebc5901de36 upstream.

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: Juergen Gross <jgross@suse.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20210311142319.4723-6-jgross@suse.com
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/x86/include/asm/alternative.h b/arch/x86/include/asm/alternative.h
index aa5f246fdc0747a17daed874f40ba93661884c33..c6690d1e1de6f8aa5d7ab605fb202551539e14e7 100644 (file)
--- a/arch/x86/include/asm/alternative.h
+++ b/arch/x86/include/asm/alternative.h
@@ -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>

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 2400ad62f330b9f518b3bcfa3462f5fd2e659e78..ff2987d5aeed8345ff297e085c3d755ec1af44d7 100644 (file)
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -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);