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

[Paul Moore]

On Thu, Sep 13, 2018 at 2:26 AM Tetsuo Handa
<penguin-kernel@xxxxxxxxxxxxxxxxxxx> 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.

I'm not to worried about the failure messages, returning -ENOMEM
should be sufficient in this case.

-- 
paul moore
www.paul-moore.com