Re: [PATCH 01/29] Revert "userfaultfd: don't fail on unrecognized features"

[David Hildenbrand <david@redhat.com>]

On 30.03.23 17:56, Peter Xu wrote:
> This is a proposal to revert commit 914eedcb9ba0ff53c33808.
> 
> I found this when writting a simple UFFDIO_API test to be the first unit
> test in this set.  Two things breaks with the commit:
> 
>    - UFFDIO_API check was lost and missing.  According to man page, the
>    kernel should reject ioctl(UFFDIO_API) if uffdio_api.api != 0xaa.  This
>    check is needed if the api version will be extended in the future, or
>    user app won't be able to identify which is a new kernel.

Agreed.

> 
>    - Feature flags checks were removed, which means UFFDIO_API with a
>    feature that does not exist will also succeed.  According to the man
>    page, we should (and it makes sense) to reject ioctl(UFFDIO_API) if
>    unknown features passed in.
> 

Agreed.

I understand the motivation of the original commit, but it should not 
have changed existing checks/functionality. Introducing a different way 
to enable such functionality on explicit request would be better. But 
maybe simple feature probing (is X support? is Y supported? is Z 
supported) might be easier without requiring ABI changes.

I assume we better add

Fixes: 914eedcb9ba0 ("userfaultfd: don't fail on unrecognized features")


Acked-by: David Hildenbrand <david@redhat.com>

> Link: https://lore.kernel.org/r/20220722201513.1624158-1-axelrasmussen@google.com
> Cc: Axel Rasmussen <axelrasmussen@google.com>
> Cc: linux-stable <stable@vger.kernel.org>
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>   fs/userfaultfd.c | 6 ++++--
>   1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index 8395605790f6..3b2a41c330e6 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -1977,8 +1977,10 @@ static int userfaultfd_api(struct userfaultfd_ctx *ctx,
>   	ret = -EFAULT;
>   	if (copy_from_user(&uffdio_api, buf, sizeof(uffdio_api)))
>   		goto out;
> -	/* Ignore unsupported features (userspace built against newer kernel) */
> -	features = uffdio_api.features & UFFD_API_FEATURES;
> +	features = uffdio_api.features;
> +	ret = -EINVAL;
> +	if (uffdio_api.api != UFFD_API || (features & ~UFFD_API_FEATURES))
> +		goto err_out;
>   	ret = -EPERM;
>   	if ((features & UFFD_FEATURE_EVENT_FORK) && !capable(CAP_SYS_PTRACE))
>   		goto err_out;

-- 
Thanks,

David / dhildenb