On Thu, 19 Nov 2020 19:04:10 -0800 Lokesh Gidra <lokeshgidra@xxxxxxxxxx> wrote:
> userfaultfd handles page faults from both user and kernel code.
> Add a new UFFD_USER_MODE_ONLY flag for userfaultfd(2) that makes
> the resulting userfaultfd object refuse to handle faults from kernel
> mode, treating these faults as if SIGBUS were always raised, causing
> the kernel code to fail with EFAULT.
>
> A future patch adds a knob allowing administrators to give some
> processes the ability to create userfaultfd file objects only if they
> pass UFFD_USER_MODE_ONLY, reducing the likelihood that these processes
> will exploit userfaultfd's ability to delay kernel page faults to open
> timing windows for future exploits.
Can we assume that an update to the userfaultfd(2) manpage is in the
works?
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -405,6 +405,13 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
>
> if (ctx->features & UFFD_FEATURE_SIGBUS)
> goto out;
> + if ((vmf->flags & FAULT_FLAG_USER) == 0 &&
> + ctx->flags & UFFD_USER_MODE_ONLY) {
> + printk_once(KERN_WARNING "uffd: Set unprivileged_userfaultfd "
> + "sysctl knob to 1 if kernel faults must be handled "
> + "without obtaining CAP_SYS_PTRACE capability\n");
> + goto out;
> + }
>
> /*
> * If it's already released don't get it. This avoids to loop
> @@ -1965,10 +1972,11 @@ SYSCALL_DEFINE1(userfaultfd, int, flags)
> BUG_ON(!current->mm);
>
> /* Check the UFFD_* constants for consistency. */
> + BUILD_BUG_ON(UFFD_USER_MODE_ONLY & UFFD_SHARED_FCNTL_FLAGS);
Are we sure this is true for all architectures?
> BUILD_BUG_ON(UFFD_CLOEXEC != O_CLOEXEC);
> BUILD_BUG_ON(UFFD_NONBLOCK != O_NONBLOCK);
>
> - if (flags & ~UFFD_SHARED_FCNTL_FLAGS)
> + if (flags & ~(UFFD_SHARED_FCNTL_FLAGS | UFFD_USER_MODE_ONLY))
> return -EINVAL;
>
> ctx = kmem_cache_alloc(userfaultfd_ctx_cachep, GFP_KERNEL);
> diff --git a/include/uapi/linux/userfaultfd.h b/include/uapi/linux/userfaultfd.h
> index e7e98bde221f..5f2d88212f7c 100644
> --- a/include/uapi/linux/userfaultfd.h
> +++ b/include/uapi/linux/userfaultfd.h
> @@ -257,4 +257,13 @@ struct uffdio_writeprotect {
> __u64 mode;
> };
>
> +/*
> + * Flags for the userfaultfd(2) system call itself.
> + */
> +
> +/*
> + * Create a userfaultfd that can handle page faults only in user mode.
> + */
> +#define UFFD_USER_MODE_ONLY 1
> +
> #endif /* _LINUX_USERFAULTFD_H */
It would be nice to define this in include/linux/userfaultfd_k.h,
alongside the other flags. But I guess it has to be here because it's
part of the userspace API.
