linux-kernel - [PATCH 2/3] x86/cpu: fix unbootable VMs by inlining memcmp() in hypervisor_cpuid

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Message-ID: <20240802154253.482658-2-adobriyan@gmail.com>
Date: Fri,  2 Aug 2024 18:42:52 +0300
From: Alexey Dobriyan <adobriyan@...il.com>
To: jgross@...e.com,
	boris.ostrovsky@...cle.com,
	tglx@...utronix.de,
	mingo@...hat.com,
	bp@...en8.de,
	dave.hansen@...ux.intel.com
Cc: adobriyan@...il.com,
	linux-kernel@...r.kernel.org,
	x86@...nel.org,
	hpa@...or.com
Subject: [PATCH 2/3] x86/cpu: fix unbootable VMs by inlining memcmp() in hypervisor_cpuid_base()

If this memcmp() is not inlined then PVH early boot code can call
into KASAN-instrumented memcmp() which results in unbootable VMs:

	pvh_start_xen
	xen_prepare_pvh
	xen_cpuid_base
	hypervisor_cpuid_base
	memcmp

Signed-off-by: Alexey Dobriyan <adobriyan@...il.com>
---
 arch/x86/include/asm/cpuid.h | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/cpuid.h b/arch/x86/include/asm/cpuid.h
index 6b122a31da06..80cc6386d7b1 100644
--- a/arch/x86/include/asm/cpuid.h
+++ b/arch/x86/include/asm/cpuid.h
@@ -196,7 +196,12 @@ static inline uint32_t hypervisor_cpuid_base(const char *sig, uint32_t leaves)
 	for_each_possible_hypervisor_cpuid_base(base) {
 		cpuid(base, &eax, &signature[0], &signature[1], &signature[2]);
 
-		if (!memcmp(sig, signature, 12) &&
+		/*
+		 * This must not compile to "call memcmp" because it's called
+		 * from PVH early boot code before instrumentation is set up
+		 * and memcmp() itself may be instrumented.
+		 */
+		if (!__builtin_memcmp(sig, signature, 12) &&
 		    (leaves == 0 || ((eax - base) >= leaves)))
 			return base;
 	}
-- 
2.45.0