Subject: [RFC/PATCH RESEND -next 20/21] fs: dcache: manually unpoison dname after allocation to shut up kasan's reports

From: [email-masked] (Andrey Ryabinin)
Date: Tue, 15 Jul 2014 13:34:33 +0400

On 07/15/14 10:12, Joonsoo Kim wrote:
> On Wed, Jul 09, 2014 at 03:30:14PM +0400, Andrey Ryabinin wrote:
>> 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.
> 
>>
>> Reported-by: Dmitry Vyukov <[email-masked]>
>> Signed-off-by: Andrey Ryabinin <[email-masked]>
>> ---
>>  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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