On Mon, Jul 11, 2022 at 09:59:06AM -0700, Nadav Amit wrote:
> From: Nadav Amit <namit@xxxxxxxxxx>
>
> 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@xxxxxxxxxx>
> Cc: Peter Xu <peterx@xxxxxxxxxx>
> Cc: David Hildenbrand <david@xxxxxxxxxx>
> Cc: Mike Rapoport <rppt@xxxxxxxxxxxxx>
> Cc: Jan Kara <jack@xxxxxxx>
> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>
> Signed-off-by: Nadav Amit <namit@xxxxxxxxxx>
Reviewed-by: Mike Rapoport <rppt@xxxxxxxxxxxxx>
> ---
> 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.
