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.

Save the original values of 'to' and 'n', and pass those to
kmsan_unpoison_memory(), so that the entire region is unpoisoned.
Signed-off-by: Brian Johannesmeyer <bjohannesmeyer@gmail.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Alexander Potapenko <glider@google.com>
Link: https://lore.kernel.org/r/20240523215029.4160518-1-bjohannesmeyer@gmail.com

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)