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Subject: Re: [PATCH] lib/strscpy: remove word-at-a-time optimization.
To:     Linus Torvalds <torvalds@linux-foundation.org>,
        Dmitry Vyukov <dvyukov@google.com>
Cc:     Rasmus Villemoes <rasmus.villemoes@prevas.dk>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Eryu Guan <eguan@redhat.com>,
        Alexander Potapenko <glider@google.com>,
        Chris Metcalf <metcalf@alum.mit.edu>,
        David Laight <David.Laight@aculab.com>,
        stable <stable@vger.kernel.org>,
        kasan-dev <kasan-dev@googlegroups.com>
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Date:   Thu, 25 Jan 2018 22:13:17 +0300
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On 01/25/2018 08:55 PM, Linus Torvalds wrote:
> On Thu, Jan 25, 2018 at 12:32 AM, Dmitry Vyukov <dvyukov@google.com> wrote:
>> On Wed, Jan 24, 2018 at 6:52 PM, Linus Torvalds
>> <torvalds@linux-foundation.org> wrote:
>>>
>>> So I'd *much* rather have some way to tell KASAN that word-at-a-time
>>> is going on. Because that approach definitely makes a difference in
>>> other places.
>>
>> The other option was to use READ_ONCE_NOCHECK().
> 
> How about just using the same accessor that we do for the dcache case.
> That gives a reasonable example of the whole word-at-a-time model, and
> should be good.
> 

If we also instrument load_unaligned_zeropad() with kasan_check_read(addr, 1),
than it should be fine. We don't want completely unchecked read of a source string.

But I also would like to revert df4c0e36f1b1 ("fs: dcache: manually unpoison dname after allocation to shut up kasan's reports")
So I was going to send something like the hunk bellow (split in several patches).

Or we could just use instrumented load_unalingned_zeropad() everywhere, but it seems wrong
to use it to load *cs only to shut up KASAN.


---
 fs/dcache.c              |  2 +-
 include/linux/compiler.h | 11 +++++++++++
 lib/string.c             |  2 +-
 3 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index 5c7df1df81ff..6aa7be55a96d 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -195,7 +195,7 @@ 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_PARTIAL_CHECK(*(unsigned long *)cs);
 		b = load_unaligned_zeropad(ct);
 		if (tcount < sizeof(unsigned long))
 			break;
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 52e611ab9a6c..85b63c2e196e 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -240,6 +240,7 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
  * required ordering.
  */
 #include <asm/barrier.h>
+#include <linux/kasan-checks.h>
 
 #define __READ_ONCE(x, check)						\
 ({									\
@@ -259,6 +260,16 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
  */
 #define READ_ONCE_NOCHECK(x) __READ_ONCE(x, 0)
 
+#ifdef CONFIG_KASAN
+#define READ_PARTIAL_CHECK(x)		\
+({					\
+	kasan_check_read(&(x), 1);	\
+	READ_ONCE_NOCHECK(x);		\
+})
+#else
+#define READ_PARTIAL_CHECK(x) (x)
+#endif
+
 #define WRITE_ONCE(x, val) \
 ({							\
 	union { typeof(x) __val; char __c[1]; } __u =	\
diff --git a/lib/string.c b/lib/string.c
index 64a9e33f1daa..2396856e4c56 100644
--- a/lib/string.c
+++ b/lib/string.c
@@ -203,7 +203,7 @@ ssize_t strscpy(char *dest, const char *src, size_t count)
 	while (max >= sizeof(unsigned long)) {
 		unsigned long c, data;
 
-		c = *(unsigned long *)(src+res);
+		c = READ_PARTIAL_CHECK(*(unsigned long *)(src+res));
 		if (has_zero(c, &data, &constants)) {
 			data = prep_zero_mask(c, data, &constants);
 			data = create_zero_mask(data);
-- 
2.13.6