Re: [PATCH] userfaultfd: provide properly masked address for huge-pages

[Mike Rapoport <rppt@linux.ibm.com>]

On Mon, Jul 11, 2022 at 09:59:06AM -0700, Nadav Amit wrote:
> From: Nadav Amit <namit@vmware.com>
> 
> Commit 824ddc601adc ("userfaultfd: provide unmasked address on
> page-fault") was introduced to fix an old bug, in which the offset in
> the address of a page-fault was masked. Concerns were raised - although
> were never backed by actual code - that some userspace code might break
> because the bug has been around for quite a while. To address these
> concerns a new flag was introduced, and only when this flag is set by
> the user, userfaultfd provides the exact address of the page-fault.
> 
> The commit however had a bug, and if the flag is unset, the offset was
> always masked based on a base-page granularity. Yet, for huge-pages, the
> behavior prior to the commit was that the address is masked to the
> huge-page granulrity.
> 
> While there are no reports on real breakage, fix this issue. If the flag
> is unset, use the address with the masking that was done before.
> 
> Fixes: 824ddc601adc ("userfaultfd: provide unmasked address on page-fault")
> Reported-by: James Houghton <jthoughton@google.com>
> Cc: Peter Xu <peterx@redhat.com>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Mike Rapoport <rppt@linux.ibm.com>
> Cc: Jan Kara <jack@suse.cz>
> Cc: Andrea Arcangeli <aarcange@redhat.com>
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Nadav Amit <namit@vmware.com>

Reviewed-by: Mike Rapoport <rppt@linux.ibm.com>

> ---
>  fs/userfaultfd.c | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index e943370107d0..de86f5b2859f 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -192,17 +192,19 @@ static inline void msg_init(struct uffd_msg *msg)
>  }
>  
>  static inline struct uffd_msg userfault_msg(unsigned long address,
> +					    unsigned long real_address,
>  					    unsigned int flags,
>  					    unsigned long reason,
>  					    unsigned int features)
>  {
>  	struct uffd_msg msg;
> +
>  	msg_init(&msg);
>  	msg.event = UFFD_EVENT_PAGEFAULT;
>  
> -	if (!(features & UFFD_FEATURE_EXACT_ADDRESS))
> -		address &= PAGE_MASK;
> -	msg.arg.pagefault.address = address;
> +	msg.arg.pagefault.address = (features & UFFD_FEATURE_EXACT_ADDRESS) ?
> +				    real_address : address;
> +
>  	/*
>  	 * These flags indicate why the userfault occurred:
>  	 * - UFFD_PAGEFAULT_FLAG_WP indicates a write protect fault.
> @@ -488,8 +490,8 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
>  
>  	init_waitqueue_func_entry(&uwq.wq, userfaultfd_wake_function);
>  	uwq.wq.private = current;
> -	uwq.msg = userfault_msg(vmf->real_address, vmf->flags, reason,
> -			ctx->features);
> +	uwq.msg = userfault_msg(vmf->address, vmf->real_address, vmf->flags,
> +				reason, ctx->features);
>  	uwq.ctx = ctx;
>  	uwq.waken = false;
>  
> -- 
> 2.25.1
> 

-- 
Sincerely yours,
Mike.