Re: [PATCH] security/keyring: avoid pagefaults in keyring_read_iterator

[Chris von Recklinghausen]

On 10/21/2019 10:21 AM, David Howells wrote:
> Chris von Recklinghausen <crecklin@xxxxxxxxxx> wrote:
>
>> The put_user call from keyring_read_iterator caused a page fault which
>> attempts to lock mm->mmap_sem and type->lock_class (key->sem) in the reverse
>> order that keyring_read_iterator did, thus causing the circular locking
>> dependency.
>>
>> Remedy this by using access_ok and __put_user instead of put_user so we'll
>> return an error instead of faulting in the page.
> I wonder if it's better to create a kernel buffer outside of the lock in
> keyctl_read_key().  Hmmm...  The reason I didn't want to do that is that
> keyrings have don't have limits on the size.  Maybe that's not actually a
> problem, since 1MiB would be able to hold a list of a quarter of a million
> keys.
>
> David
>

Hi David,

Thanks for the feedback.

I can try to prototype that, but regardless of where the kernel buffer
is allocated, the important part is causing the initial pagefault in the
read path outside the lock so __put_user won't fail due to a valid user
address but page backing the user address isn't in-core.

I'll start work on v2.

Thanks,

Chris