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From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Thu, 13 Sep 2018 10:01:47 +0200
Message-ID: <CACT4Y+Y0ShhnxOXdFHYz40zq58Je1eZ47oLLYaC4gH1zwQNN7Q@mail.gmail.com>
Subject: Re: [PATCH] selinux: Add __GFP_NOWARN to allocation at str_read()
To:     Paul Moore <paul@paul-moore.com>
Cc:     Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
        SELinux <selinux@tycho.nsa.gov>,
        syzbot+ac488b9811036cea7ea0@syzkaller.appspotmail.com,
        Eric Paris <eparis@parisplace.org>,
        LKML <linux-kernel@vger.kernel.org>, peter.enderborg@sony.com,
        Stephen Smalley <sds@tycho.nsa.gov>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>
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On Thu, Sep 13, 2018 at 5:02 AM, Paul Moore <paul@paul-moore.com> 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, it's the blessed way to do it. We have lots of similar cases:
https://elixir.bootlin.com/linux/v4.19-rc3/ident/__GFP_NOWARN