Re: [RFC PATCH 01/13] fs/userfaultfd: fix wrong error code on WP & !VM_MAYWRITE

[Mike Kravetz]

On 11/28/20 4:45 PM, Nadav Amit wrote:
> From: Nadav Amit <namit@xxxxxxxxxx>
> 
> It is possible to get an EINVAL error instead of EPERM if the following
> test vm_flags have VM_UFFD_WP but do not have VM_MAYWRITE, as "ret" is
> overwritten since commit cab350afcbc9 ("userfaultfd: hugetlbfs: allow
> registration of ranges containing huge pages").
> 
> Fix it.
> 
> Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> Cc: Jens Axboe <axboe@xxxxxxxxx>
> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> Cc: Peter Xu <peterx@xxxxxxxxxx>
> Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
> Cc: io-uring@xxxxxxxxxxxxxxx
> Cc: linux-fsdevel@xxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: linux-mm@xxxxxxxxx
> Fixes: cab350afcbc9 ("userfaultfd: hugetlbfs: allow registration of ranges containing huge pages")
> Signed-off-by: Nadav Amit <namit@xxxxxxxxxx>
> ---
>  fs/userfaultfd.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index 000b457ad087..c8ed4320370e 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -1364,6 +1364,7 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx,
>  			if (end & (vma_hpagesize - 1))
>  				goto out_unlock;
>  		}
> +		ret = -EPERM;
>  		if ((vm_flags & VM_UFFD_WP) && !(cur->vm_flags & VM_MAYWRITE))
>  			goto out_unlock;
>  

Thanks!  We should return EPERM in that case.

However, the check for VM_UFFD_WP && !VM_MAYWRITE went in after commit
cab350afcbc9.  I think it is more accurate to say that the issue was
introduced with commit 63b2d4174c4a ("Introduce the new uffd-wp APIs
for userspace.").  The convention in userfaultfd_register() is that the
return code is set before testing condition which could cause return.
Therefore, when 63b2d4174c4a added the VM_UFFD_WP && !VM_MAYWRITE check,
it should have also added the 'ret = -EPERM;' statement.

With changes to commit message and Fixes tag,

Reviewed-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
-- 
Mike Kravetz