linux-kernel - [PATCH AUTOSEL 5.4 3/6] x86: __memcpy_flushcache: fix wrong alignment if size

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Message-Id: <20220426190243.2351733-3-sashal@kernel.org>
Date:   Tue, 26 Apr 2022 15:02:39 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Mikulas Patocka <mpatocka@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Sasha Levin <sashal@...nel.org>, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        x86@...nel.org, peterz@...radead.org, jpoimboe@...hat.com
Subject: [PATCH AUTOSEL 5.4 3/6] x86: __memcpy_flushcache: fix wrong alignment if size > 2^32

From: Mikulas Patocka <mpatocka@...hat.com>

[ Upstream commit a6823e4e360fe975bd3da4ab156df7c74c8b07f3 ]

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>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 arch/x86/lib/usercopy_64.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/lib/usercopy_64.c b/arch/x86/lib/usercopy_64.c
index 1847e993ac63..f3f7f4cb15a6 100644
--- a/arch/x86/lib/usercopy_64.c
+++ b/arch/x86/lib/usercopy_64.c
@@ -142,7 +142,7 @@ void __memcpy_flushcache(void *_dst, const void *_src, size_t size)
 
 	/* 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);
-- 
2.35.1