[Qemu-devel] [PATCH 00/29] 9pfs: local: fix vulnerability to symlink attacks

[Greg Kurz <groug@kaod.org>]

This series tries to fix CVE-2016-9602 reported by Jann Horn of Google
Project Zero:

https://bugzilla.redhat.com/show_bug.cgi?id=1413929

This vulnerability affects all accesses to the underlying filesystem in
the "local" backend code.

If QEMU is started with:

-fsdev local,security_model=<passthrough|none>,path=/foo/bar

then the guest can cause QEMU to create symlinks in /foo/bar.

This causes accesses to any path /foo/bar/some/path to be unsafe, since
untrusted code within the guest (or in another guest sharing the same
virtfs folder) could change some/path to point to a random path of the
host filesystem.

The core problem is that the "local" backend relies on path-based syscalls
to access the underlying filesystem. All path-based syscalls are vulnerable
to this issue, even open(O_NOFOLLOW) or syscalls that explicitly don't
dereference symlinks, since the kernel only checks the rightmost element of
the path. Depending on the privilege level of the QEMU process, a guest can
end up opening, renaming, changing ACLs, unlinking... files on the host
filesystem.

The right way to address this is to use "at" variants of all syscalls in
the "local" backend code. This requires to open directories without
traversing any symlink in the intermediate path elements. There was a
tentative to introduce an O_BENEATH flag for openat() that would address
this:

https://patchwork.kernel.org/patch/7007181/

Unfortunately this never got merged. I shall contact the author and try
to revive this kernel patchset.

In the meantime, an alternative is to walk through all path elements
manually with openat(O_NOFOLLOW). This is likely to degrade performances,
but I don't see any better way to get the vulnerability fixed in 2.9.
I'll try to come up with some numbers later.

Stefan and Daniel, I've Cc'ed you because we talked about the issue
on irc already. Feel free to comment/review if you have some spare
cycles, it will be appreciated (but of course, I'll understand if
you don't :)

---

Greg Kurz (29):
      9pfs: local: move xattr security ops to 9p-xattr.c
      9pfs: remove side-effects in local_init()
      9pfs: remove side-effects in local_open() and local_opendir()
      9pfs: introduce openat_nofollow() helper
      9pfs: local: keep a file descriptor on the shared folder
      9pfs: local: open/opendir: don't follow symlinks
      9pfs: local: introduce symlink-attack safe xattr helpers
      9pfs: local: lgetxattr: don't follow symlinks
      9pfs: local: llistxattr: don't follow symlinks
      9pfs: local: lsetxattr: don't follow symlinks
      9pfs: local: lremovexattr: don't follow symlinks
      9pfs: local: unlinkat: don't follow symlinks
      9pfs: local: remove: don't follow symlinks
      9pfs: local: utimensat: don't follow symlinks
      9pfs: local: statfs: don't follow symlinks
      9pfs: local: truncate: don't follow symlinks
      9pfs: local: readlink: don't follow symlinks
      9pfs: local: lstat: don't follow symlinks
      9pfs: local: renameat: don't follow symlinks
      9pfs: local: rename: use renameat
      9pfs: local: improve error handling in link op
      9pfs: local: link: don't follow symlinks
      9pfs: local: chmod: don't follow symlinks
      9pfs: local: chown: don't follow symlinks
      9pfs: local: symlink: don't follow symlinks
      9pfs: local: mknod: don't follow symlinks
      9pfs: local: mkdir: don't follow symlinks
      9pfs: local: open2: don't follow symlinks
      9pfs: local: drop unused code


 hw/9pfs/9p-local.c      | 1020 ++++++++++++++++++++++++++---------------------
 hw/9pfs/9p-local.h      |   20 +
 hw/9pfs/9p-posix-acl.c  |   44 --
 hw/9pfs/9p-util.c       |   69 +++
 hw/9pfs/9p-util.h       |   25 +
 hw/9pfs/9p-xattr-user.c |   24 -
 hw/9pfs/9p-xattr.c      |  231 ++++++++++-
 hw/9pfs/9p-xattr.h      |   93 +---
 hw/9pfs/Makefile.objs   |    2 
 9 files changed, 934 insertions(+), 594 deletions(-)
 create mode 100644 hw/9pfs/9p-local.h
 create mode 100644 hw/9pfs/9p-util.c
 create mode 100644 hw/9pfs/9p-util.h

--
Greg