linux-kernel - [PATCH] x86: __memcpy_flushcache: fix wrong alignment if size

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Message-ID: <alpine.LRH.2.02.2204190948080.18399@file01.intranet.prod.int.rdu2.redhat.com>
Date:   Tue, 19 Apr 2022 09:56:23 -0400 (EDT)
From:   Mikulas Patocka <mpatocka@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        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>
cc:     x86@...nel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] x86: __memcpy_flushcache: fix wrong alignment if size >
 2^32

The first "if" condition in __memcpy_flushcache is supposed to align the
"dest" variable to 8 bytes and copy data up to this alignment. However,
this condition may misbehave if "size" is greater than 4GiB.

The statement min_t(unsigned, size, ALIGN(dest, 8) - dest); casts both
arguments to unsigned int and selects the smaller one. However, the cast
truncates high bits in "size" and it results in misbehavior.

For example:
	suppose that size == 0x100000001, dest == 0x200000002
	min_t(unsigned, size, ALIGN(dest, 8) - dest) == min_t(0x1, 0xe) == 0x1;
	...
	dest += 0x1;
so we copy just one byte "and" dest remains unaligned.

This patch fixes the bug by replacing unsigned with size_t.

Signed-off-by: Mikulas Patocka <mpatocka@...hat.com>

---
 arch/x86/lib/usercopy_64.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Index: linux-2.6/arch/x86/lib/usercopy_64.c
===================================================================
--- linux-2.6.orig/arch/x86/lib/usercopy_64.c
+++ linux-2.6/arch/x86/lib/usercopy_64.c
@@ -119,7 +119,7 @@ void __memcpy_flushcache(void *_dst, con
 
 	/* cache copy and flush to align dest */
 	if (!IS_ALIGNED(dest, 8)) {
-		unsigned len = min_t(unsigned, size, ALIGN(dest, 8) - dest);
+		size_t len = min_t(size_t, size, ALIGN(dest, 8) - dest);
 
 		memcpy((void *) dest, (void *) source, len);
 		clean_cache_range((void *) dest, len);