linux-kernel - [PATCH] x86: kmsan: Fix hook for unaligned accesses

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Message-Id: <20240523215029.4160518-1-bjohannesmeyer@gmail.com>
Date: Thu, 23 May 2024 23:50:29 +0200
From: Brian Johannesmeyer <bjohannesmeyer@...il.com>
To: Brian Johannesmeyer <bjohannesmeyer@...il.com>,
	Alexander Potapenko <glider@...gle.com>,
	Marco Elver <elver@...gle.com>,
	Dmitry Vyukov <dvyukov@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	kasan-dev@...glegroups.com,
	linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	x86@...nel.org,
	"H . Peter Anvin" <hpa@...or.com>
Subject: [PATCH] x86: kmsan: Fix hook for unaligned accesses

When called with a 'from' that is not 4-byte-aligned,
string_memcpy_fromio() calls the movs() macro to copy the first few bytes,
so that 'from' becomes 4-byte-aligned before calling rep_movs(). This
movs() macro modifies 'to', and the subsequent line modifies 'n'.

As a result, on unaligned accesses, kmsan_unpoison_memory() uses the
updated (aligned) values of 'to' and 'n'. Hence, it does not unpoison the
entire region.

This patch saves the original values of 'to' and 'n', and passes those to
kmsan_unpoison_memory(), so that the entire region is unpoisoned.

Signed-off-by: Brian Johannesmeyer <bjohannesmeyer@...il.com>
---
 arch/x86/lib/iomem.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/x86/lib/iomem.c b/arch/x86/lib/iomem.c
index e0411a3774d4..5eecb45d05d5 100644
--- a/arch/x86/lib/iomem.c
+++ b/arch/x86/lib/iomem.c
@@ -25,6 +25,9 @@ static __always_inline void rep_movs(void *to, const void *from, size_t n)
 
 static void string_memcpy_fromio(void *to, const volatile void __iomem *from, size_t n)
 {
+	const void *orig_to = to;
+	const size_t orig_n = n;
+
 	if (unlikely(!n))
 		return;
 
@@ -39,7 +42,7 @@ static void string_memcpy_fromio(void *to, const volatile void __iomem *from, si
 	}
 	rep_movs(to, (const void *)from, n);
 	/* KMSAN must treat values read from devices as initialized. */
-	kmsan_unpoison_memory(to, n);
+	kmsan_unpoison_memory(orig_to, orig_n);
 }
 
 static void string_memcpy_toio(volatile void __iomem *to, const void *from, size_t n)
-- 
2.34.1