linux-kernel - [PATCH 3/3] x86/alternative: Patch a single alternative location only once

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Message-ID: <20250929112947.27267-4-jgross@suse.com>
Date: Mon, 29 Sep 2025 13:29:47 +0200
From: Juergen Gross <jgross@...e.com>
To: linux-kernel@...r.kernel.org,
	x86@...nel.org
Cc: Juergen Gross <jgross@...e.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: [PATCH 3/3] x86/alternative: Patch a single alternative location only once

Instead of patching a single location potentially multiple times in
case of nested ALTERNATIVE()s, do the patching only after having
evaluated all alt_instr instances for that location.

This has multiple advantages:

- In case of replacing an indirect with a direct call using the
  ALT_FLAG_DIRECT_CALL flag, there is no longer the need to have that
  instance before any other instances at the same location (the
  original instruction is needed for finding the target of the direct
  call).

- In case of nested ALTERNATIVE()s there is no intermediate replacement
  visible. This avoids any problems in case e.g. an interrupt is
  happening between the single instances and the patched location is
  used during handling the interrupt.

Signed-off-by: Juergen Gross <jgross@...e.com>
---
V2:
- new patch
---
 arch/x86/kernel/alternative.c | 27 +++++++++++++++++++--------
 1 file changed, 19 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 735cc017f2d3..ccf07131cd47 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -648,6 +648,8 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start,
 	u8 insn_buff[MAX_PATCH_LEN];
 	u8 *instr;
 	struct alt_instr *a, *b;
+	unsigned int instances = 0;
+	bool patched = false;
 
 	DPRINTK(ALT, "alt table %px, -> %px", start, end);
 
@@ -677,9 +679,13 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start,
 		 * padding for all alt_instr entries for this site (nested
 		 * alternatives result in consecutive entries).
 		 */
-		for (b = a+1; b < end && instr_va(b) == instr_va(a); b++) {
-			u8 len = max(a->instrlen, b->instrlen);
-			a->instrlen = b->instrlen = len;
+		if (!instances) {
+			for (b = a+1; b < end && instr_va(b) == instr_va(a); b++) {
+				u8 len = max(a->instrlen, b->instrlen);
+				a->instrlen = b->instrlen = len;
+			}
+			instances = b - a;
+			patched = false;
 		}
 
 		instr = instr_va(a);
@@ -692,14 +698,19 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start,
 		 * - feature not present but ALT_FLAG_NOT is set to mean,
 		 *   patch if feature is *NOT* present.
 		 */
-		if (!boot_cpu_has(a->cpuid) == !(a->flags & ALT_FLAG_NOT)) {
-			memcpy(insn_buff, instr, a->instrlen);
-			optimize_nops(instr, insn_buff, a->instrlen);
-		} else {
+		if (!boot_cpu_has(a->cpuid) != !(a->flags & ALT_FLAG_NOT)) {
 			apply_one_alternative(instr, insn_buff, a);
+			patched = true;
 		}
 
-		text_poke_early(instr, insn_buff, a->instrlen);
+		instances--;
+		if (!instances) {
+			if (!patched) {
+				memcpy(insn_buff, instr, a->instrlen);
+				optimize_nops(instr, insn_buff, a->instrlen);
+			}
+			text_poke_early(instr, insn_buff, a->instrlen);
+		}
 	}
 
 	kasan_enable_current();
-- 
2.51.0