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Date:   Thu, 13 Sep 2018 08:42:42 +0200
From:   Martin Schwidefsky <schwidefsky@de.ibm.com>
To:     Arnd Bergmann <arnd@arndb.de>
Cc:     Al Viro <viro@zeniv.linux.org.uk>, "Theodore Ts'o" <tytso@mit.edu>,
        gregkh <gregkh@linuxfoundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
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        linux-s390 <linux-s390@vger.kernel.org>,
        Heiko Carstens <heiko.carstens@de.ibm.com>
Subject: Re: [PATCH 06/11] compat_ioctl: remove /dev/random commands
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On Wed, 12 Sep 2018 16:02:40 +0200
Arnd Bergmann <arnd@arndb.de> wrote:

> On Wed, Sep 12, 2018 at 7:29 AM Martin Schwidefsky
> <schwidefsky@de.ibm.com> wrote:
> > On Tue, 11 Sep 2018 22:26:54 +0200 Arnd Bergmann <arnd@arndb.de> wrote:  
> > > On Sun, Sep 9, 2018 at 6:12 AM Al Viro <viro@zeniv.linux.org.uk> wrote:  
> 
> > > Out of those, there are only a few that may get used on s390,
> > > in particular at most infiniband/uverbs, nvme, nvdimm,
> > > btrfs, ceph, fuse, fanotify and userfaultfd.
> > > [Note: there are three s390 drivers in the list, which use
> > > a different method: they check in_compat_syscall() from
> > > a shared handler to decide whether to do compat_ptr().  
> >
> > Using in_compat_syscall() seems to be a good solution, no?  
> 
> It works fine for you, but wouldn't work on architecture-independent
> code, since 32-bit architectures generally don't provide
> a compat_ptr() implementation. This could of course
> be changed easily, but after this change it, your drivers
> work just as well with a couple few lines, and more consistent
> with other drivers:
> 
> --- a/drivers/s390/char/sclp_ctl.c
> +++ b/drivers/s390/char/sclp_ctl.c
> @@ -93,12 +93,8 @@ static int sclp_ctl_ioctl_sccb(void __user *user_area)
>  static long sclp_ctl_ioctl(struct file *filp, unsigned int cmd,
>                            unsigned long arg)
>  {
> -       void __user *argp;
> +       void __user *argp = (void __user *)arg;
> 
> -       if (is_compat_task())
> -               argp = compat_ptr(arg);
> -       else
> -               argp = (void __user *) arg;
>         switch (cmd) {
>         case SCLP_CTL_SCCB:
>                 return sclp_ctl_ioctl_sccb(argp);
> @@ -114,7 +110,7 @@ static const struct file_operations sclp_ctl_fops = {
>         .owner = THIS_MODULE,
>         .open = nonseekable_open,
>         .unlocked_ioctl = sclp_ctl_ioctl,
> -       .compat_ioctl = sclp_ctl_ioctl,
> +       .compat_ioctl = generic_compat_ioctl_ptrarg,
>         .llseek = no_llseek,
>  };
> 
> This should probably be separate from the change to using compat_ptr()
> in all other drivers, and I could easily drop this change if you prefer,
> it is meant only as a cosmetic change.

So generic_compat_ioctl_ptrarg will to the compat_ptr thing on the
"unsigned int cmd" argument? Should work just fine.


> > > According to my memory from when I last worked on this,
> > > the compat_ptr() is mainly a safeguard for legacy binaries
> > > that got created with ancient C compilers (or compilers for
> > > something other than C)  and might leave the high bit set
> > > in a pointer, but modern C compilers (gcc-3+) won't ever
> > > do that.  
> >
> > And compat_ptr clears the upper 32-bit of the register. If
> > the register is loaded to e.g. "lr" or "l" there will be
> > junk in the 4 upper bytes.  
> 
> I don't think we hit that problem anywhere: in the ioctl
> argument we pass an 'unsigned long' that has already
> been zero-extended by the compat_sys_ioctl() wrapper,
> while any other usage would get extended by the compiler
> when casting from compat_uptr_t to a 64-bit type.
> This would be different if you had a function call with the
> wrong prototype, i.e. calling a function declared as taking
> an compat_uptr_t, but defining it as taking a void __user*.
> (I suppose that is undefined behavior).

That is true. For the ioctls we have a compat "unsigned int"
or "unsigned long" and the system call wrapper must have cleared
the upper half already. There are a few places where we copy
a data structure from user space, then read a 32-bit pointer
from the structure. These get the compat_ptr treatment as well.
All of those structure definitions should use compat_uptr_t
though, the compiler has to do the zero extension at the time
the 32-bit value is cast to a pointer.

> Unless I'm missing something, compat_ptr() should
> always just clear bit 31. What I'd like to confirm is whether
> you have encountered any code that actually passes
> a pointer with that bit set by a user application in the
> past 15 years. As Al said, it's probably best to just always
> apply the compat_ptr() conversion in each case that s390
> needs it even for drivers that don't run on s390, but I'd also
> like to understand how much it matters in practice.
> (A separate question would be how long we expect to need
> 32 bit compat mode on arch/s390 at all, but for the moment
> I assume this is not up for debate at all).

I don't know if that is worth the risk, yes it should work if
compat_ptr just removes bit 31 and leaves the other bits alone.
But if you have to clear one bit, you can as well remove all
the other bits as well.

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.