Re: [PATCH v2] userfaultfd: provide unmasked address on page-fault

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

On Fri, Feb 18, 2022 at 04:10:03AM +0000, Nadav Amit wrote:
> From: Nadav Amit <namit@vmware.com>
> 
> Userfaultfd is supposed to provide the full address (i.e., unmasked) of
> the faulting access back to userspace. However, that is not the case for
> quite some time.
> 
> Even running "userfaultfd_demo" from the userfaultfd man page provides
> the wrong output (and contradicts the man page). Notice that
> "UFFD_EVENT_PAGEFAULT event" shows the masked address (7fc5e30b3000)
> and not the first read address (0x7fc5e30b300f).
> 
> 	Address returned by mmap() = 0x7fc5e30b3000
> 
> 	fault_handler_thread():
> 	    poll() returns: nready = 1; POLLIN = 1; POLLERR = 0
> 	    UFFD_EVENT_PAGEFAULT event: flags = 0; address = 7fc5e30b3000
> 		(uffdio_copy.copy returned 4096)
> 	Read address 0x7fc5e30b300f in main(): A
> 	Read address 0x7fc5e30b340f in main(): A
> 	Read address 0x7fc5e30b380f in main(): A
> 	Read address 0x7fc5e30b3c0f in main(): A
> 
> The exact address is useful for various reasons and specifically for
> prefetching decisions. If it is known that the memory is populated by
> certain objects whose size is not page-aligned, then based on the
> faulting address, the uffd-monitor can decide whether to prefetch and
> prefault the adjacent page.
> 
> This bug has been for quite some time in the kernel: since commit
> 1a29d85eb0f1 ("mm: use vmf->address instead of of vmf->virtual_address")
> vmf->virtual_address"), which dates back to 2016. A concern has been
> raised that existing userspace application might rely on the old/wrong
> behavior in which the address is masked. Therefore, it was suggested to
> provide the masked address unless the user explicitly asks for the exact
> address.
> 
> Add a new userfaultfd feature UFFD_FEATURE_EXACT_ADDRESS to direct
> userfaultfd to provide the exact address. Add a new "real_address" field
> to vmf to hold the unmasked address. Provide the address to userspace
> accordingly.
> 
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Andrea Arcangeli <aarcange@redhat.com>
> Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
> Cc: Peter Xu <peterx@redhat.com>
> Cc: Jan Kara <jack@suse.cz>
> Signed-off-by: Nadav Amit <namit@vmware.com>

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

> 
> ---
> 
> v1->v2:
> * Add uffd feature to selectively enable [David, Andrea]
> ---
>  fs/userfaultfd.c                 | 5 ++++-
>  include/linux/mm.h               | 3 ++-
>  include/uapi/linux/userfaultfd.h | 8 +++++++-
>  mm/memory.c                      | 1 +
>  4 files changed, 14 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index e26b10132d47..826927026fe7 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -198,6 +198,9 @@ static inline struct uffd_msg userfault_msg(unsigned long address,
>  	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;
>  	/*
>  	 * These flags indicate why the userfault occurred:
> @@ -482,7 +485,7 @@ 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->address, vmf->flags, reason,
> +	uwq.msg = userfault_msg(vmf->real_address, vmf->flags, reason,
>  			ctx->features);
>  	uwq.ctx = ctx;
>  	uwq.waken = false;
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 213cc569b192..27df0ca0a36a 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -478,7 +478,8 @@ struct vm_fault {
>  		struct vm_area_struct *vma;	/* Target VMA */
>  		gfp_t gfp_mask;			/* gfp mask to be used for allocations */
>  		pgoff_t pgoff;			/* Logical page offset based on vma */
> -		unsigned long address;		/* Faulting virtual address */
> +		unsigned long address;		/* Faulting virtual address - masked */
> +		unsigned long real_address;	/* Faulting virtual address - unmaked */
>  	};
>  	enum fault_flag flags;		/* FAULT_FLAG_xxx flags
>  					 * XXX: should really be 'const' */
> diff --git a/include/uapi/linux/userfaultfd.h b/include/uapi/linux/userfaultfd.h
> index 05b31d60acf6..ef739054cb1c 100644
> --- a/include/uapi/linux/userfaultfd.h
> +++ b/include/uapi/linux/userfaultfd.h
> @@ -32,7 +32,8 @@
>  			   UFFD_FEATURE_SIGBUS |		\
>  			   UFFD_FEATURE_THREAD_ID |		\
>  			   UFFD_FEATURE_MINOR_HUGETLBFS |	\
> -			   UFFD_FEATURE_MINOR_SHMEM)
> +			   UFFD_FEATURE_MINOR_SHMEM |		\
> +			   UFFD_FEATURE_EXACT_ADDRESS)
>  #define UFFD_API_IOCTLS				\
>  	((__u64)1 << _UFFDIO_REGISTER |		\
>  	 (__u64)1 << _UFFDIO_UNREGISTER |	\
> @@ -189,6 +190,10 @@ struct uffdio_api {
>  	 *
>  	 * UFFD_FEATURE_MINOR_SHMEM indicates the same support as
>  	 * UFFD_FEATURE_MINOR_HUGETLBFS, but for shmem-backed pages instead.
> +	 *
> +	 * UFFD_FEATURE_EXACT_ADDRESS indicates that the exact address of page
> +	 * faults would be provided and the offset within the page would not be
> +	 * masked.
>  	 */
>  #define UFFD_FEATURE_PAGEFAULT_FLAG_WP		(1<<0)
>  #define UFFD_FEATURE_EVENT_FORK			(1<<1)
> @@ -201,6 +206,7 @@ struct uffdio_api {
>  #define UFFD_FEATURE_THREAD_ID			(1<<8)
>  #define UFFD_FEATURE_MINOR_HUGETLBFS		(1<<9)
>  #define UFFD_FEATURE_MINOR_SHMEM		(1<<10)
> +#define UFFD_FEATURE_EXACT_ADDRESS		(1<<11)
>  	__u64 features;
> 
>  	__u64 ioctls;
> diff --git a/mm/memory.c b/mm/memory.c
> index c125c4969913..aae53fde13d9 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4622,6 +4622,7 @@ static vm_fault_t __handle_mm_fault(struct vm_area_struct *vma,
>  	struct vm_fault vmf = {
>  		.vma = vma,
>  		.address = address & PAGE_MASK,
> +		.real_address = address,
>  		.flags = flags,
>  		.pgoff = linear_page_index(vma, address),
>  		.gfp_mask = __get_fault_gfp_mask(vma),
> -- 
> 2.25.1
> 

-- 
Sincerely yours,
Mike.