userfaultfd: require CAP_SYS_PTRACE for UFFD_FEATURE_EVENT_FORK

A while ago Andy noticed
(http://lkml.kernel.org/r/CALCETrWY+5ynDct7eU_nDUqx=okQvjm=Y5wJvA4ahBja=CQXGw@mail.gmail.com)
that UFFD_FEATURE_EVENT_FORK used by an unprivileged user may have
security implications.

As the first step of the solution the following patch limits the availably
of UFFD_FEATURE_EVENT_FORK only for those having CAP_SYS_PTRACE.

The usage of CAP_SYS_PTRACE ensures compatibility with CRIU.

Yet, if there are other users of non-cooperative userfaultfd that run
without CAP_SYS_PTRACE, they would be broken :(

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.

Link: http://lkml.kernel.org/r/1572967777-8812-2-git-send-email-rppt@linux.ibm.com
Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Reviewed-by: Andrea Arcangeli <aarcange@redhat.com>
Cc: Daniel Colascione <dancol@google.com>
Cc: Jann Horn <jannh@google.com>
Cc: Lokesh Gidra <lokeshgidra@google.com>
Cc: Nick Kralevich <nnk@google.com>
Cc: Nosh Minwalla <nosh@google.com>
Cc: Pavel Emelyanov <ovzxemul@gmail.com>
Cc: Tim Murray <timmurray@google.com>
Cc: Aleksa Sarai <cyphar@cyphar.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index d90c4c5aa3cc34b2979b74780a4033f68f1633d3..90acd2812ea73be0213d8a1d49baf43b5bb53f29 100644 (file)
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -1835,13 +1835,12 @@ 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;
@@ -1854,6 +1853,11 @@ 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)))
+               ret = -EFAULT;
+       goto out;
 }
 
 static long userfaultfd_ioctl(struct file *file, unsigned cmd,