On 2019-11-05, Mike Rapoport <rppt@xxxxxxxxxxxxx> wrote:
> Current implementation of UFFD_FEATURE_EVENT_FORK modifies the file
> descriptor table from the read() implementation of uffd, which may have
> security implications for unprivileged use of the userfaultfd.
>
> Limit availability of UFFD_FEATURE_EVENT_FORK only for callers that have
> CAP_SYS_PTRACE.
>
> Signed-off-by: Mike Rapoport <rppt@xxxxxxxxxxxxx>
> ---
> fs/userfaultfd.c | 18 +++++++++++-------
> 1 file changed, 11 insertions(+), 7 deletions(-)
>
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index f9fd18670e22..d99d166fd892 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -1834,13 +1834,12 @@ static int userfaultfd_api(struct userfaultfd_ctx *ctx,
> if (copy_from_user(&uffdio_api, buf, sizeof(uffdio_api)))
> goto out;
> features = uffdio_api.features;
> - if (uffdio_api.api != UFFD_API || (features & ~UFFD_API_FEATURES)) {
> - memset(&uffdio_api, 0, sizeof(uffdio_api));
> - if (copy_to_user(buf, &uffdio_api, sizeof(uffdio_api)))
> - goto out;
> - ret = -EINVAL;
> - goto out;
> - }
> + 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;
> /* report all available features and ioctls to userland */
> uffdio_api.features = UFFD_API_FEATURES;
> uffdio_api.ioctls = UFFD_API_IOCTLS;
> @@ -1853,6 +1852,11 @@ static int userfaultfd_api(struct userfaultfd_ctx *ctx,
> ret = 0;
> out:
> return ret;
> +err_out:
> + memset(&uffdio_api, 0, sizeof(uffdio_api));
> + if (copy_to_user(buf, &uffdio_api, sizeof(uffdio_api)))
Wouldn't it be simpler to do clear_user()?
> + ret = -EFAULT;
> + goto out;
> }
>
> static long userfaultfd_ioctl(struct file *file, unsigned cmd,
--
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>
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