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

[Scotty Bauer]

This is a multi-part message in MIME format.

On 01/05/2016 02:13 PM, Mike Snitzer wrote:
> On Tue, Jan 05 2016 at  3:16pm -0500,
> Mike Snitzer <snitzer@xxxxxxxxxx> wrote:
> 
>> On Tue, Dec 08 2015 at  1:26pm -0500,
>> Scotty Bauer <sbauer@xxxxxxxxxxxx> wrote:
>>
>>> 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
> 
> Alasdair helped me understand that we do need your original fix.
> I've staged it for 4.5 (and stable@) here:
> https://git.kernel.org/cgit/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-4.5&id=ead3db62bf10fe143bec99e7b7ff370d7a6d23ef
> 
> Thanks again,
> Mike
> --

This broke linux-next because I'm dumb and didn't test it. I thought it was a trivial enough of a patch that I wouldn't screw it up, but I did.

I incorrectly assumed that user->version was essentially a pointer in userland, not a flat chunk of memory. Ie it was a pointer to some malloc'd region, not an inlined version[3].

I thought it was this:
struct dm_ioctl {

uint32_t *version;
...
}

It is really this:

struct dm_ioctl {

uint32_t version[3];

}

I was trying to get the values out of *version, which would have been a pointer, but instead what the code ended up doing was actually getting 8 bytes of the version (think 4,3,1) out and trying to access that version as a memory address, oops.

It turns out that the original code is correct and doesn't actually touch user memory without a copy_from_user().  Gcc is smart enough to see that version[3] is inlined, and it can emit code which simply takes the userland pointer (struct dm_ioctl __user user), and calculates on offset based on the pointer, thus no actual user dereference occurs. Had the struct looked like the first example I believe the patch would work.

I'm wondering now if we should switch the code a bit to make it less ambiguous, so someone like me doesn't come along again thinking the code dereferences userland memory and waste everyones time.

I've attached a patch based off linux-next-20150616 which reverts my broken code but adds an & to the front of user->version so it looks like the code is doing the right thing.

If I should be basing my patch off something other than linux-next let me know and I'll rewrite it, or we can just revert the old patch and ignore this one.

Thanks and very sorry for the confusion and breakage.