[PATCH 4.14 54/91] userfaultfd: require CAP_SYS_PTRACE for UFFD_FEATURE_EVENT_FORK

Greg Kroah-Hartman
2020-01-02T22:49:34Z

From: Mike Rapoport <rppt@linux.ibm.com>

[ Upstream commit 3c1c24d91ffd536de0a64688a9df7f49e58fadbc ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
fs/userfaultfd.c | 18 +++++++++++-------
1 file changed, 11 insertions(+), 7 deletions(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index a609d480606d..e2b2196fd942 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -1807,13 +1807,12 @@ 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;
@@ -1826,6 +1825,11 @@ 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,
--
2.20.1