Re: [PATCH v6 0/2] Control over userfaultfd kernel-fault handling

Re: [PATCH v6 0/2] Control over userfaultfd kernel-fault handling

[Lokesh Gidra]

On Thu, Nov 19, 2020 at 7:04 PM Lokesh Gidra <lokeshgidra@google.com> wrote:
>
> This patch series is split from [1]. The other series enables SELinux
> support for userfaultfd file descriptors so that its creation and
> movement can be controlled.
>
> It has been demonstrated on various occasions that suspending kernel
> code execution for an arbitrary amount of time at any access to
> userspace memory (copy_from_user()/copy_to_user()/...) can be exploited
> to change the intended behavior of the kernel. For instance, handling
> page faults in kernel-mode using userfaultfd has been exploited in [2, 3].
> Likewise, FUSE, which is similar to userfaultfd in this respect, has been
> exploited in [4, 5] for similar outcome.
>
> This small patch series adds a new flag to userfaultfd(2) that allows
> callers to give up the ability to handle kernel-mode faults with the
> resulting UFFD file object. It then adds a 'user-mode only' option to
> the unprivileged_userfaultfd sysctl knob to require unprivileged
> callers to use this new flag.
>
> The purpose of this new interface is to decrease the chance of an
> unprivileged userfaultfd user taking advantage of userfaultfd to
> enhance security vulnerabilities by lengthening the race window in
> kernel code.
>
> [1] https://lore.kernel.org/lkml/20200211225547.235083-1-dancol@google.com/
> [2] https://duasynt.com/blog/linux-kernel-heap-spray
> [3] https://duasynt.com/blog/cve-2016-6187-heap-off-by-one-exploit
> [4] https://googleprojectzero.blogspot.com/2016/06/exploiting-recursion-in-linux-kernel_20.html
> [5] https://bugs.chromium.org/p/project-zero/issues/detail?id=808
>
> Changes since v5:
>
>   - Added printk_once when unprivileged_userfaultfd is set to 0 and
>     userfaultfd syscall is called without UFFD_USER_MODE_ONLY in the
>     absence of CAP_SYS_PTRACE capability.
>
> Changes since v4:
>
>   - Added warning when bailing out from handling kernel fault.
>
> Changes since v3:
>
>   - Modified the meaning of value '0' of unprivileged_userfaultfd
>     sysctl knob. Setting this knob to '0' now allows unprivileged users
>     to use userfaultfd, but can handle page faults in user-mode only.
>   - The default value of unprivileged_userfaultfd sysctl knob is changed
>     to '0'.
>
> Changes since v2:
>
>   - Removed 'uffd_flags' and directly used 'UFFD_USER_MODE_ONLY' in
>     userfaultfd().
>
> Changes since v1:
>
>   - Added external references to the threats from allowing unprivileged
>     users to handle page faults from kernel-mode.
>   - Removed the new sysctl knob restricting handling of page
>     faults from kernel-mode, and added an option for the same
>     in the existing 'unprivileged_userfaultfd' knob.
>
> Lokesh Gidra (2):
>   Add UFFD_USER_MODE_ONLY
>   Add user-mode only option to unprivileged_userfaultfd sysctl knob
>
>  Documentation/admin-guide/sysctl/vm.rst | 15 ++++++++++-----
>  fs/userfaultfd.c                        | 20 +++++++++++++++++---
>  include/uapi/linux/userfaultfd.h        |  9 +++++++++
>  3 files changed, 36 insertions(+), 8 deletions(-)
>
> --
> 2.29.0.rc1.297.gfa9743e501-goog
>
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Re: [PATCH v6 1/2] Add UFFD_USER_MODE_ONLY

[Lokesh Gidra]

On Thu, Nov 19, 2020 at 7:04 PM Lokesh Gidra <lokeshgidra@google.com> 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.
>
> Signed-off-by: Daniel Colascione <dancol@google.com>
> Signed-off-by: Lokesh Gidra <lokeshgidra@google.com>
> Reviewed-by: Andrea Arcangeli <aarcange@redhat.com>
> ---
>  fs/userfaultfd.c                 | 10 +++++++++-
>  include/uapi/linux/userfaultfd.h |  9 +++++++++
>  2 files changed, 18 insertions(+), 1 deletion(-)
>
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index 000b457ad087..605599fde015 100644
> --- 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);
>         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 */
> --
> 2.29.0.rc1.297.gfa9743e501-goog
>
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Re: [PATCH v6 1/2] Add UFFD_USER_MODE_ONLY

[Lokesh Gidra]

On Fri, Nov 20, 2020 at 3:33 PM Andrew Morton <akpm@linux-foundation.org> wrote:
>
> On Thu, 19 Nov 2020 19:04:10 -0800 Lokesh Gidra <lokeshgidra@google.com> 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?
>
Yes, I'm working on it. Can the kernel version which will have these
patches be known now so that I can mention it in the manpage?

> > --- 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?

Yes, none of the architectures are using the least-significant bit for
O_CLOEXEC or O_NONBLOCK.
>
> >       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.

Re: [PATCH v6 1/2] Add UFFD_USER_MODE_ONLY

[Andrew Morton]

On Mon, 23 Nov 2020 11:17:43 -0800 Lokesh Gidra <lokeshgidra@google.com> wrote:

> > > 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?
> >
> Yes, I'm working on it. Can the kernel version which will have these
> patches be known now so that I can mention it in the manpage?

5.11, if all proceeds smoothly.

Re: [PATCH v6 2/2] Add user-mode only option to unprivileged_userfaultfd sysctl knob

[Lokesh Gidra]

On Thu, Nov 19, 2020 at 7:04 PM Lokesh Gidra <lokeshgidra@google.com> wrote:
>
> With this change, when the knob is set to 0, it allows unprivileged
> users to call userfaultfd, like when it is set to 1, but with the
> restriction that page faults from only user-mode can be handled.
> In this mode, an unprivileged user (without SYS_CAP_PTRACE capability)
> must pass UFFD_USER_MODE_ONLY to userfaultd or the API will fail with
> EPERM.
>
> This enables administrators to reduce the likelihood that an attacker
> with access to userfaultfd can delay faulting kernel code to widen
> timing windows for other exploits.
>
> The default value of this knob is changed to 0. This is required for
> correct functioning of pipe mutex. However, this will fail postcopy
> live migration, which will be unnoticeable to the VM guests. To avoid
> this, set 'vm.userfault = 1' in /sys/sysctl.conf.
>
> The main reason this change is desirable as in the short term is that
> the Android userland will behave as with the sysctl set to zero. So
> without this commit, any Linux binary using userfaultfd to manage its
> memory would behave differently if run within the Android userland.
> For more details, refer to Andrea's reply [1].
>
> [1] https://lore.kernel.org/lkml/20200904033438.GI9411@redhat.com/
>
> Signed-off-by: Lokesh Gidra <lokeshgidra@google.com>
> Reviewed-by: Andrea Arcangeli <aarcange@redhat.com>
> ---
>  Documentation/admin-guide/sysctl/vm.rst | 15 ++++++++++-----
>  fs/userfaultfd.c                        | 10 ++++++++--
>  2 files changed, 18 insertions(+), 7 deletions(-)
>
> diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-guide/sysctl/vm.rst
> index f455fa00c00f..d06a98b2a4e7 100644
> --- a/Documentation/admin-guide/sysctl/vm.rst
> +++ b/Documentation/admin-guide/sysctl/vm.rst
> @@ -873,12 +873,17 @@ file-backed pages is less than the high watermark in a zone.
>  unprivileged_userfaultfd
>  ========================
>
> -This flag controls whether unprivileged users can use the userfaultfd
> -system calls.  Set this to 1 to allow unprivileged users to use the
> -userfaultfd system calls, or set this to 0 to restrict userfaultfd to only
> -privileged users (with SYS_CAP_PTRACE capability).
> +This flag controls the mode in which unprivileged users can use the
> +userfaultfd system calls. Set this to 0 to restrict unprivileged users
> +to handle page faults in user mode only. In this case, users without
> +SYS_CAP_PTRACE must pass UFFD_USER_MODE_ONLY in order for userfaultfd to
> +succeed. Prohibiting use of userfaultfd for handling faults from kernel
> +mode may make certain vulnerabilities more difficult to exploit.
>
> -The default value is 1.
> +Set this to 1 to allow unprivileged users to use the userfaultfd system
> +calls without any restrictions.
> +
> +The default value is 0.
>
>
>  user_reserve_kbytes
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index 605599fde015..894cc28142e7 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -28,7 +28,7 @@
>  #include <linux/security.h>
>  #include <linux/hugetlb.h>
>
> -int sysctl_unprivileged_userfaultfd __read_mostly = 1;
> +int sysctl_unprivileged_userfaultfd __read_mostly;
>
>  static struct kmem_cache *userfaultfd_ctx_cachep __read_mostly;
>
> @@ -1966,8 +1966,14 @@ SYSCALL_DEFINE1(userfaultfd, int, flags)
>         struct userfaultfd_ctx *ctx;
>         int fd;
>
> -       if (!sysctl_unprivileged_userfaultfd && !capable(CAP_SYS_PTRACE))
> +       if (!sysctl_unprivileged_userfaultfd &&
> +           (flags & UFFD_USER_MODE_ONLY) == 0 &&
> +           !capable(CAP_SYS_PTRACE)) {
> +               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");
>                 return -EPERM;
> +       }
>
>         BUG_ON(!current->mm);
>
> --
> 2.29.0.rc1.297.gfa9743e501-goog
>
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