On 07/15/14 10:12, Joonsoo Kim wrote:

On Wed, Jul 09, 2014 at 03:30:14PM +0400, Andrey Ryabinin wrote:

quoted

We need to manually unpoison rounded up allocation size for dname
to avoid kasan's reports in __d_lookup_rcu.
__d_lookup_rcu may validly read a little beyound allocated size.

If it read a little beyond allocated size, IMHO, it is better to
allocate correct size.

kmalloc(name->len + 1, GFP_KERNEL); -->
kmalloc(roundup(name->len + 1, sizeof(unsigned long ), GFP_KERNEL);

Isn't it?

It's not needed here because kmalloc always roundup allocation size.

This out of bound access happens in dentry_string_cmp() if CONFIG_DCACHE_WORD_ACCESS=y.
dentry_string_cmp() relays on fact that kmalloc always round up allocation size,
in other words it's by design.

That was discussed some time ago here - https://lkml.org/lkml/2013/10/3/493.
Since filesystem's maintainer don't want to add needless round up here, I'm not going to do it.

I think this patch needs only more detailed description why we not simply allocate more.
Also I think it would be better to rename unpoisoin_shadow to something like kasan_mark_allocated().

Thanks.

quoted

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Andrey Ryabinin <redacted>
---
 fs/dcache.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/dcache.c b/fs/dcache.c
index b7e8b20..dff64f2 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c

@@ -38,6 +38,7 @@
 #include <linux/prefetch.h>
 #include <linux/ratelimit.h>
 #include <linux/list_lru.h>
+#include <linux/kasan.h>
 #include "internal.h"
 #include "mount.h"
 

@@ -1412,6 +1413,8 @@ struct dentry *__d_alloc(struct super_block *sb, const struct qstr *name)
 			kmem_cache_free(dentry_cache, dentry); 
 			return NULL;
 		}
+		unpoison_shadow(dname,
+				roundup(name->len + 1, sizeof(unsigned long)));
 	} else  {
 		dname = dentry->d_iname;
 	}	

-- 
1.8.5.5

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