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

[Paul Moore]

On Thu, Sep 13, 2018 at 8:55 AM peter enderborg
<peter.enderborg@xxxxxxxx> wrote:
> On 09/13/2018 01:11 PM, Michal Hocko wrote:
> > On Thu 13-09-18 09:12:04, peter enderborg wrote:
> >> On 09/13/2018 08:26 AM, Tetsuo Handa wrote:
> >>> On 2018/09/13 12:02, Paul Moore wrote:
> >>>> On Fri, Sep 7, 2018 at 12:43 PM Tetsuo Handa
> >>>> <penguin-kernel@xxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxx>
> >>>>> Reported-by: syzbot <syzbot+ac488b9811036cea7ea0@xxxxxxxxxxxxxxxxxxxxxxxxx>
> >>>>> ---
> >>>>>  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.
> >>>
> >> Would it not be better with
> >>
> >>     char *str;
> >>
> >>     if ((len == 0) || (len == (u32)-1) || (len >= KMALLOC_MAX_SIZE))
> >>         return -EINVAL;
> >>
> >>     str = kmalloc(len + 1, flags);
> >>     if (!str)
> >>         return -ENOMEM;
> > I strongly suspect that you want kvmalloc rather than kmalloc here. The
> > larger the request the more likely is the allocation to fail.
> >
> > I am not familiar with the code but I assume this is a root only
> > interface so we don't have to worry about nasty users scenario.
> >
> I don't think we get any big data there at all. Usually less than 32 bytes. However this data can be in fast path so a vmalloc is not an option.
>
> And some of the calls are GFP_ATOMC.

Based on all the comments it looks like Tetsuo's original patch is
probably the best fix right now.  I'm going to merge this into
selinux/next.

Tetsuo, thanks for the patch, and thanks to everyone else for the
comments/review.

-- 
paul moore
www.paul-moore.com