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

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Date:   Thu, 5 Apr 2018 09:19:28 +0200
From:   Lars-Peter Clausen <lars@...afoo.de>
To:     Kees Cook <keescook@...omium.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Felipe Balbi <felipe.balbi@...ux.intel.com>,
        Al Viro <viro@...iv.linux.org.uk>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>, linux-usb@...r.kernel.org,
        Christoph Hellwig <hch@...radead.org>
Subject: Re: [GIT PULL] USB/PHY driver patches for 4.17-rc1

On 04/05/2018 08:31 AM, Kees Cook wrote:
> On Wed, Apr 4, 2018 at 3:31 AM, Greg KH <gregkh@...uxfoundation.org> 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