Date:   Thu, 22 Jun 2023 14:43:29 -0400
From:   Demi Marie Obenour <demi@...isiblethingslab.com>
To:     Mike Snitzer <snitzer@...nel.org>
Cc:     Alasdair Kergon <agk@...hat.com>, dm-devel@...hat.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 4/6] device-mapper: Avoid double-fetch of version

On Thu, Jun 22, 2023 at 12:20:40PM -0400, Mike Snitzer wrote:
> On Sat, Jun 03 2023 at 10:52P -0400,
> Demi Marie Obenour <demi@...isiblethingslab.com> wrote:
> 
> > The version is fetched once in check_version(), which then does some
> > validation and then overwrites the version in userspace with the API
> > version supported by the kernel.  copy_params() then fetches the version
> > from userspace *again*, and this time no validation is done.  The result
> > is that the kernel's version number is completely controllable by
> > userspace, provided that userspace can win a race condition.
> > 
> > Fix this flaw by not copying the version back to the kernel the second
> > time.  This is not exploitable as the version is not further used in the
> > kernel.  However, it could become a problem if future patches start
> > relying on the version field.
> > 
> > Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> > Cc: stable@...r.kernel.org
> > Signed-off-by: Demi Marie Obenour <demi@...isiblethingslab.com>
> > ---
> >  drivers/md/dm-ioctl.c | 30 ++++++++++++++++++------------
> >  1 file changed, 18 insertions(+), 12 deletions(-)
> > 
> > diff --git a/drivers/md/dm-ioctl.c b/drivers/md/dm-ioctl.c
> > index da6ca26b51d0953df380582bb3a51c2ec22c27cb..7510afe237d979a5ee71afe87a20d49f631de1aa 100644
> > --- a/drivers/md/dm-ioctl.c
> > +++ b/drivers/md/dm-ioctl.c
> > @@ -1873,30 +1873,33 @@ static ioctl_fn lookup_ioctl(unsigned int cmd, int *ioctl_flags)
> >   * As well as checking the version compatibility this always
> >   * copies the kernel interface version out.
> >   */
> > -static int check_version(unsigned int cmd, struct dm_ioctl __user *user)
> > +static int check_version(unsigned int cmd, struct dm_ioctl __user *user,
> > +			 struct dm_ioctl *kernel_params)
> >  {
> > -	uint32_t version[3];
> >  	int r = 0;
> >  
> > -	if (copy_from_user(version, user->version, sizeof(version)))
> > +	if (copy_from_user(kernel_params->version, user->version, sizeof(kernel_params->version)))
> >  		return -EFAULT;
> >  
> > -	if ((version[0] != DM_VERSION_MAJOR) ||
> > -	    (version[1] > DM_VERSION_MINOR)) {
> > +	if ((kernel_params->version[0] != DM_VERSION_MAJOR) ||
> > +	    (kernel_params->version[1] > DM_VERSION_MINOR)) {
> >  		DMERR("ioctl interface mismatch: kernel(%u.%u.%u), user(%u.%u.%u), cmd(%d)",
> >  		      DM_VERSION_MAJOR, DM_VERSION_MINOR,
> >  		      DM_VERSION_PATCHLEVEL,
> > -		      version[0], version[1], version[2], cmd);
> > +		      kernel_params->version[0],
> > +		      kernel_params->version[1],
> > +		      kernel_params->version[2],
> > +		      cmd);
> >  		r = -EINVAL;
> >  	}
> >  
> >  	/*
> >  	 * Fill in the kernel version.
> >  	 */
> > -	version[0] = DM_VERSION_MAJOR;
> > -	version[1] = DM_VERSION_MINOR;
> > -	version[2] = DM_VERSION_PATCHLEVEL;
> > -	if (copy_to_user(user->version, version, sizeof(version)))
> > +	kernel_params->version[0] = DM_VERSION_MAJOR;
> > +	kernel_params->version[1] = DM_VERSION_MINOR;
> > +	kernel_params->version[2] = DM_VERSION_PATCHLEVEL;
> > +	if (copy_to_user(user->version, kernel_params->version, sizeof(kernel_params->version)))
> >  		return -EFAULT;
> >  
> >  	return r;
> > @@ -1922,7 +1925,10 @@ static int copy_params(struct dm_ioctl __user *user, struct dm_ioctl *param_kern
> >  	const size_t minimum_data_size = offsetof(struct dm_ioctl, data);
> >  	unsigned int noio_flag;
> >  
> > -	if (copy_from_user(param_kernel, user, minimum_data_size))
> > +	/* Version has been copied from userspace already, avoid TOCTOU */
> > +	if (copy_from_user((char *)param_kernel + sizeof(param_kernel->version),
> > +			   (char __user *)user + sizeof(param_kernel->version),
> > +			   minimum_data_size - sizeof(param_kernel->version)))
> >  		return -EFAULT;
> >  
> >  	if (param_kernel->data_size < minimum_data_size) {
> > @@ -2034,7 +2040,7 @@ static int ctl_ioctl(struct file *file, uint command, struct dm_ioctl __user *us
> >  	 * Check the interface version passed in.  This also
> >  	 * writes out the kernel's interface version.
> >  	 */
> > -	r = check_version(cmd, user);
> > +	r = check_version(cmd, user, &param_kernel);
> >  	if (r)
> >  		return r;
> >  
> 
> I picked this patch up for 6.5, please see:
> https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-6.5&id=c71878e9982075eab2e9f6dc5a09ba7b60ac1e24

That’s great, thanks!

> But FYI, I folded these changes in:
> 
> diff --git a/drivers/md/dm-ioctl.c b/drivers/md/dm-ioctl.c
> index 526464904fc1..b58a15e212a3 100644
> --- a/drivers/md/dm-ioctl.c
> +++ b/drivers/md/dm-ioctl.c
> @@ -1838,6 +1838,9 @@ static int check_version(unsigned int cmd, struct dm_ioctl __user *user,
>  {
>  	int r = 0;
>  
> +	/* Make certain version is first member of dm_ioctl struct */
> +	BUILD_BUG_ON(offsetof(struct dm_ioctl, version) != 0);
> +
>  	if (copy_from_user(kernel_params->version, user->version, sizeof(kernel_params->version)))
>  		return -EFAULT;
>  
> @@ -1885,7 +1888,7 @@ static int copy_params(struct dm_ioctl __user *user, struct dm_ioctl *param_kern
>  	const size_t minimum_data_size = offsetof(struct dm_ioctl, data);
>  	unsigned int noio_flag;
>  
> -	/* Version has been copied from userspace already, avoid TOCTOU */
> +	/* check_version() already copied version from userspace, avoid TOCTOU */
>  	if (copy_from_user((char *)param_kernel + sizeof(param_kernel->version),
>  			   (char __user *)user + sizeof(param_kernel->version),
>  			   minimum_data_size - sizeof(param_kernel->version)))

Those changes indeed make the code better, thanks for including them!
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab

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