linux-kernel - [PATCH 4/5] fs/dcache: Use read_word_at_a_time() in dentry_string

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Date:   Thu,  1 Feb 2018 21:00:51 +0300
From:   Andrey Ryabinin <aryabinin@...tuozzo.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Rasmus Villemoes <rasmus.villemoes@...vas.dk>,
        linux-kernel@...r.kernel.org, Dmitry Vyukov <dvyukov@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Eryu Guan <eguan@...hat.com>,
        Alexander Potapenko <glider@...gle.com>,
        Chris Metcalf <metcalf@...m.mit.edu>,
        David Laight <David.Laight@...lab.com>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>
Subject: [PATCH 4/5] fs/dcache: Use read_word_at_a_time() in dentry_string_cmp()

dentry_string_cmp() performs the word-at-a-time reads from 'cs' and may
read slightly more than it was requested in kmallac(). Normally this
would make KASAN to report out-of-bounds access, but this was workarounded
by commit df4c0e36f1b1 ("fs: dcache: manually unpoison dname after
allocation to shut up kasan's reports").

This workaround is not perfect, since it allows out-of-bounds access
to dentry's name for all the code, not just in dentry_string_cmp().

So it would be better to use read_word_at_a_time() instead and
revert commit df4c0e36f1b1.

Signed-off-by: Andrey Ryabinin <aryabinin@...tuozzo.com>
---
 fs/dcache.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index c99812fd54b3..195999c8d1b4 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -193,7 +193,7 @@ static inline int dentry_string_cmp(const unsigned char *cs, const unsigned char
 	unsigned long a,b,mask;
 
 	for (;;) {
-		a = *(unsigned long *)cs;
+		a = read_word_at_a_time(cs);
 		b = load_unaligned_zeropad(ct);
 		if (tcount < sizeof(unsigned long))
 			break;
-- 
2.13.6