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From:   Jann Horn <jannh@google.com>
Date:   Mon, 1 Apr 2019 17:04:00 +0200
Message-ID: <CAG48ez05JxAAfen6DanFWigBDPLCOEKYZ4ZL2RQ1bRcRVTf13A@mail.gmail.com>
Subject: Re: [PATCH 2/2] x86: fix __user annotations
To:     David Laight <David.Laight@aculab.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        "x86@kernel.org" <x86@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Qiaowei Ren <qiaowei.ren@intel.com>
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On Mon, Apr 1, 2019 at 4:36 PM David Laight <David.Laight@aculab.com> wrote:
>
> From: Jann Horn
> > Sent: 28 March 2019 21:23
> > Fix __user annotations in various places across the x86 tree:
> >
> ...
> >  - generic_load_microcode() deals with a pointer that can be either a
> >    kernel pointer or a user pointer; change the code to pass it around as
> >    a __user pointer, and add explicit casts to convert between __user and
> >    __kernel
> ...
> > -static int get_ucode_fw(void *to, const void *from, size_t n)
> > +static int get_ucode_fw(void *to, const void __user *from, size_t n)
> >  {
> > -     memcpy(to, from, n);
> > +     /* cast paired with request_microcode_fw() */
> > +     memcpy(to, (const void __force *)from, n);
> >       return 0;
> >  }
> >
> > @@ -993,7 +996,8 @@ static enum ucode_state request_microcode_fw(int cpu, struct device *device,
> >               return UCODE_NFOUND;
> >       }
> >
> > -     ret = generic_load_microcode(cpu, (void *)firmware->data,
> > +     /* cast paired with get_ucode_fw() */
> > +     ret = generic_load_microcode(cpu, (void __force __user *)firmware->data,
> >                                    firmware->size, &get_ucode_fw);
> >
> >       release_firmware(firmware);
> > @@ -1001,7 +1005,7 @@ static enum ucode_state request_microcode_fw(int cpu, struct device *device,
> >       return ret;
> >  }
> >
> > -static int get_ucode_user(void *to, const void *from, size_t n)
> > +static int get_ucode_user(void *to, const void __user *from, size_t n)
> >  {
> >       return copy_from_user(to, from, n);
> >  }
> > @@ -1012,7 +1016,7 @@ request_microcode_user(int cpu, const void __user *buf, size_t size)
> >       if (is_blacklisted(cpu))
> >               return UCODE_NFOUND;
> >
> > -     return generic_load_microcode(cpu, (void *)buf, size, &get_ucode_user);
> > +     return generic_load_microcode(cpu, buf, size, &get_ucode_user);
>
> That is all an 'accident waiting to happen' ...

What's your suggestion? The code used to store user pointers in
kernel-typed pointers. Now it only stores kernel pointers in
user-typed pointers, which is much less hazardous.