Re: [PATCH v5 03/16] x86/alternatives: Disable LASS when patching kernel alternatives

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Oct 29, 2024 at 12:36:11PM +0100, Peter Zijlstra wrote:

> Anyway, looking at this, I see we grew rep_{movs,stos}_alternative, as
> used in copy_user_generic() and __clear_user(). Which are all somewhat
> similar.

That is, we could consider something like the completely untested and
probably broken, will light your granny on fire and maul pets like
below..

---
diff --git a/arch/x86/include/asm/string.h b/arch/x86/include/asm/string.h
index 9cb5aae7fba9..e25a988360a1 100644
--- a/arch/x86/include/asm/string.h
+++ b/arch/x86/include/asm/string.h
@@ -2,31 +2,50 @@
 #ifndef _ASM_X86_STRING_H
 #define _ASM_X86_STRING_H
 
+#include <asm/asm.h>
+#include <asm/alternative.h>
+
 #ifdef CONFIG_X86_32
 # include <asm/string_32.h>
 #else
 # include <asm/string_64.h>
 #endif
 
+#ifdef CONFIG_X86_64
+#define ALT_64(orig, alt, feat) ALTERNATIVE(orig, alt, feat)
+#else
+#define ALT_64(orig, alt, feat) orig
+#endif
+
 static __always_inline void *__inline_memcpy(void *to, const void *from, size_t len)
 {
 	void *ret = to;
 
-	asm volatile("rep movsb"
-		     : "+D" (to), "+S" (from), "+c" (len)
-		     : : "memory");
-	return ret;
+	asm volatile("1:\n\t"
+		     ALT_64("rep movsb",
+			    "call rep_movs_alternative", ALT_NOT(X86_FEATURE_FSRM))
+		     "2:\n\t"
+		     _ASM_EXTABLE_UA(1b, 2b)
+		     : "+D" (to), "+S" (from), "+c" (len), ASM_CALL_CONSTRAINT
+		     : : "memory", _ASM_AX);
+
+	return ret + len;
 }
 
 static __always_inline void *__inline_memset(void *s, int v, size_t n)
 {
 	void *ret = s;
 
-	asm volatile("rep stosb"
-		     : "+D" (s), "+c" (n)
+	asm volatile("1:\n\t"
+		     ALT_64("rep stosb",
+			    "call rep_stos_alternative", ALT_NOT(X86_FEATURE_FSRM))
+		     "2:\n\t"
+		     _ASM_EXTABLE_UA(1b, 2b)
+		     : "+D" (s), "+c" (n), ASM_CALL_CONSTRAINT
 		     : "a" ((uint8_t)v)
-		     : "memory");
-	return ret;
+		     : "memory", _ASM_SI);
+
+	return ret + len;
 }
 
 #endif /* _ASM_X86_STRING_H */
diff --git a/arch/x86/include/asm/uaccess_64.h b/arch/x86/include/asm/uaccess_64.h
index b0a887209400..9f2d2c2ca731 100644
--- a/arch/x86/include/asm/uaccess_64.h
+++ b/arch/x86/include/asm/uaccess_64.h
@@ -13,6 +13,7 @@
 #include <asm/page.h>
 #include <asm/percpu.h>
 #include <asm/runtime-const.h>
+#include <asm/string.h>
 
 /*
  * Virtual variable: there's no actual backing store for this,
@@ -118,21 +119,12 @@ rep_movs_alternative(void *to, const void *from, unsigned len);
 static __always_inline __must_check unsigned long
 copy_user_generic(void *to, const void *from, unsigned long len)
 {
+	void *ret;
+
 	stac();
-	/*
-	 * If CPU has FSRM feature, use 'rep movs'.
-	 * Otherwise, use rep_movs_alternative.
-	 */
-	asm volatile(
-		"1:\n\t"
-		ALTERNATIVE("rep movsb",
-			    "call rep_movs_alternative", ALT_NOT(X86_FEATURE_FSRM))
-		"2:\n"
-		_ASM_EXTABLE_UA(1b, 2b)
-		:"+c" (len), "+D" (to), "+S" (from), ASM_CALL_CONSTRAINT
-		: : "memory", "rax");
+	ret = __inline_memcpy(to, from, len);
 	clac();
-	return len;
+	return ret - to;
 }
 
 static __always_inline __must_check unsigned long
@@ -178,25 +170,15 @@ rep_stos_alternative(void __user *addr, unsigned long len);
 
 static __always_inline __must_check unsigned long __clear_user(void __user *addr, unsigned long size)
 {
-	might_fault();
-	stac();
+	void *ret;
 
-	/*
-	 * No memory constraint because it doesn't change any memory gcc
-	 * knows about.
-	 */
-	asm volatile(
-		"1:\n\t"
-		ALTERNATIVE("rep stosb",
-			    "call rep_stos_alternative", ALT_NOT(X86_FEATURE_FSRS))
-		"2:\n"
-	       _ASM_EXTABLE_UA(1b, 2b)
-	       : "+c" (size), "+D" (addr), ASM_CALL_CONSTRAINT
-	       : "a" (0));
+	might_fault();
 
+	stac();
+	ret = __inline_memset(addr, 0, size);
 	clac();
 
-	return size;
+	return ret - addr;
 }
 
 static __always_inline unsigned long clear_user(void __user *to, unsigned long n)
diff --git a/arch/x86/lib/clear_page_64.S b/arch/x86/lib/clear_page_64.S
index 2760a15fbc00..17d4bf6f50e5 100644
--- a/arch/x86/lib/clear_page_64.S
+++ b/arch/x86/lib/clear_page_64.S
@@ -53,16 +53,22 @@ SYM_FUNC_END(clear_page_erms)
 EXPORT_SYMBOL_GPL(clear_page_erms)
 
 /*
- * Default clear user-space.
+ * Default memset
  * Input:
  * rdi destination
+ * rsi scratch
  * rcx count
- * rax is zero
+ * al is value
  *
  * Output:
- * rcx: uncleared bytes or 0 if successful.
+ * rcx: unset bytes or 0 if successful.
  */
 SYM_FUNC_START(rep_stos_alternative)
+
+	movzbl %al, %rsi
+	movabs $0x0101010101010101, %rax
+	mulq %rsi
+
 	cmpq $64,%rcx
 	jae .Lunrolled