Re: [PATCH] selinux: Add __GFP_NOWARN to allocation at str_read()

[Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>]

On 2018/09/13 12:02, Paul Moore wrote:
> On Fri, Sep 7, 2018 at 12:43 PM Tetsuo Handa
> <penguin-kernel@i-love.sakura.ne.jp> wrote:
>> syzbot is hitting warning at str_read() [1] because len parameter can
>> become larger than KMALLOC_MAX_SIZE. We don't need to emit warning for
>> this case.
>>
>> [1] https://syzkaller.appspot.com/bug?id=7f2f5aad79ea8663c296a2eedb81978401a908f0
>>
>> Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
>> Reported-by: syzbot <syzbot+ac488b9811036cea7ea0@syzkaller.appspotmail.com>
>> ---
>>  security/selinux/ss/policydb.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/security/selinux/ss/policydb.c b/security/selinux/ss/policydb.c
>> index e9394e7..f4eadd3 100644
>> --- a/security/selinux/ss/policydb.c
>> +++ b/security/selinux/ss/policydb.c
>> @@ -1101,7 +1101,7 @@ static int str_read(char **strp, gfp_t flags, void *fp, u32 len)
>>         if ((len == 0) || (len == (u32)-1))
>>                 return -EINVAL;
>>
>> -       str = kmalloc(len + 1, flags);
>> +       str = kmalloc(len + 1, flags | __GFP_NOWARN);
>>         if (!str)
>>                 return -ENOMEM;
> 
> Thanks for the patch.
> 
> My eyes are starting to glaze over a bit chasing down all of the
> different kmalloc() code paths trying to ensure that this always does
> the right thing based on size of the allocation and the different slab
> allocators ... are we sure that this will always return NULL when (len
> + 1) is greater than KMALLOC_MAX_SIZE for the different slab allocator
> configurations?
> 

Yes, for (len + 1) cannot become 0 (which causes kmalloc() to return
ZERO_SIZE_PTR) due to (len == (u32)-1) check above.

The only concern would be whether you want allocation failure messages.
I assumed you don't need it because we are returning -ENOMEM to the caller.