linux-kernel - Re: [RFC PATCH] asm/generic: introduce if_nospec and nospec

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Message-ID: <CAPcyv4gKQD68dCCD-Nz78ZQ9ttung5yMzG3f+JRmGUxuKhK4Pw@mail.gmail.com>
Date:   Wed, 3 Jan 2018 21:44:33 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "alan@...ux.intel.com" <alan@...ux.intel.com>,
        "Reshetova, Elena" <elena.reshetova@...el.com>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "gnomes@...rguk.ukuu.org.uk" <gnomes@...rguk.ukuu.org.uk>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "jikos@...nel.org" <jikos@...nel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>
Subject: Re: [RFC PATCH] asm/generic: introduce if_nospec and nospec_barrier

On Wed, Jan 3, 2018 at 8:44 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
> On Thu, Jan 04, 2018 at 03:10:51AM +0000, Williams, Dan J wrote:
>
>> diff --git a/include/linux/fdtable.h b/include/linux/fdtable.h
>> index 1c65817673db..dbc12007da51 100644
>> --- a/include/linux/fdtable.h
>> +++ b/include/linux/fdtable.h
>> @@ -82,8 +82,10 @@ static inline struct file *__fcheck_files(struct files_struct *files, unsigned i
>>  {
>>       struct fdtable *fdt = rcu_dereference_raw(files->fdt);
>>
>> -     if (fd < fdt->max_fds)
>> +     if (fd < fdt->max_fds) {
>> +             osb();
>>               return rcu_dereference_raw(fdt->fd[fd]);
>> +     }
>>       return NULL;
>>  }
>
> ... and the point of that would be?  Possibly revealing the value of files->fdt?
> Why would that be a threat, assuming you manage to extract the information in
> question in the first place?

No, the concern is that an fd value >= fdt->max_fds may cause the cpu
to read arbitrary memory addresses relative to files->fdt and
userspace can observe that it got loaded. With the barrier the
speculation stops and never allows that speculative read to issue.
With the change, the cpu only issues a read for fdt->fd[fd] when fd is
valid.