Re: [GIT PULL] USB/PHY driver patches for 4.17-rc1

[Lars-Peter Clausen]

On 04/05/2018 08:31 AM, Kees Cook wrote:
> On Wed, Apr 4, 2018 at 3:31 AM, Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
>> Lars-Peter Clausen (2):
>>       usb: gadget: ffs: Execute copy_to_user() with USER_DS set
> 
> https://git.kernel.org/linus/4058ebf33cb0be88ca516f968eda24ab7b6b93e4
> 
> Isn't there a better way to do this without the set_fs() usage? We've
> been try to eliminate it in the kernel. I thought there was a safer
> way to use iters now?

The problem is use_mm(). It needs to be accompanied with set_fs(DS_USER) to
work reliably. This has simply been missing for this particular instance of
use_mm().

Now, in my opinion, use_mm() is not the best approach here in the first
place and instead of using copy_to_user() it is probably better to map the
userspace pages to kernel space and then access them directly. But that's a
lot more intrusive and separate from this issue.

- Lars