On Thu, Jan 25, 2018 at 8:13 PM, Andrey Ryabinin aryabinin@virtuozzo.com wrote:
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);
Looks good to me a general way to support word-at-a-time pattern.
This will also get rid of this in fs/dcache.c:
if (IS_ENABLED(CONFIG_DCACHE_WORD_ACCESS)) kasan_unpoison_shadow(dname, round_up(name->len + 1, sizeof(unsigned long)));