Re: dm ioctl: Access user-land memory through safe functions.

[Mike Snitzer <snitzer@redhat.com>]

On Tue, Dec 08 2015 at  1:26pm -0500,
Scotty Bauer <sbauer@eng.utah.edu> wrote:

> 
> On 12/01/2015 11:11 AM, Scotty wrote:
> > 
> > 0001-dm-ioctl-Access-user-land-memory-through-safe-functi.patch
> > 
> > 
> > From b26adf880eba03ac6f2b1dd87426bb96fd2a0282 Mon Sep 17 00:00:00 2001
> > From: Scotty Bauer <sbauer@eng.utah.edu>
> > Date: Tue, 1 Dec 2015 10:52:46 -0700
> > Subject: [PATCH] dm ioctl: Access user-land memory through safe functions.
> > 
> > This patch fixes a user-land dereference. Now we use
> > the safe copy_from_user to access the memory.
> > 
> > Signed-off-by: Scotty Bauer <sbauer@eng.utah.edu>
> > ---
> >  drivers/md/dm-ioctl.c | 8 ++++++--
> >  1 file changed, 6 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/md/dm-ioctl.c b/drivers/md/dm-ioctl.c
> > index 80a4395..39a9d1a 100644
> > --- a/drivers/md/dm-ioctl.c
> > +++ b/drivers/md/dm-ioctl.c
> > @@ -1642,9 +1642,13 @@ static ioctl_fn lookup_ioctl(unsigned int cmd, int *ioctl_flags)
> >  static int check_version(unsigned int cmd, struct dm_ioctl __user *user)
> >  {
> >  	uint32_t version[3];
> > +	uint32_t __user *version_ptr;
> >  	int r = 0;
> >  
> > -	if (copy_from_user(version, user->version, sizeof(version)))
> > +	if (copy_from_user(&version_ptr, &user->version, sizeof(version_ptr)))
> > +		return -EFAULT;
> > +
> > +	if (copy_from_user(version, version_ptr, sizeof(version)))
> >  		return -EFAULT;
> >  
> >  	if ((DM_VERSION_MAJOR != version[0]) ||
> > @@ -1663,7 +1667,7 @@ static int check_version(unsigned int cmd, struct dm_ioctl __user *user)
> >  	version[0] = DM_VERSION_MAJOR;
> >  	version[1] = DM_VERSION_MINOR;
> >  	version[2] = DM_VERSION_PATCHLEVEL;
> > -	if (copy_to_user(user->version, version, sizeof(version)))
> > +	if (copy_to_user(version_ptr, version, sizeof(version)))
> >  		return -EFAULT;
> >  
> >  	return r;
> > -- 
> 
> 
> Friendly ping, is anyone interested in this?

The passed @user argument is flagged via __user so it can be
deferenced directly.  It does look like directly deferencing
user->version is wrong.

But even if such indirect access is needed (because __user flag is only
applicable to @user arg, not the contained version member) we could more
easily just do something like this no?:

  uint32_t __user *versionp = (uint32_t __user *)user->version;
  ...
  if (copy_from_user(version, versionp, sizeof(version)))
     return -EFAULT;

I've staged the following, thanks:
https://git.kernel.org/cgit/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-4.5&id=bffc9e237a0c3176712bcd93fc6a184a61e0df26