[Qemu-devel] [PATCH v2 00/28] Series short description

[Greg Kurz]

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.  An alternative is to walk through all
path elements manually with openat(O_NOFOLLOW).

v2:
        - /proc based implementation for xattr code (fixes metadata perf drop
          observed with v1)
        - some code refactoring

Stefan.

I had to rework some patches you had already reviewed, please consider
giving your Reviewed-by again if the changes are ok.

Thanks.

--
Greg

---

Greg Kurz (28):
      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: 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      | 1023 ++++++++++++++++++++++++++---------------------
 hw/9pfs/9p-local.h      |   20 +
 hw/9pfs/9p-posix-acl.c  |   44 --
 hw/9pfs/9p-util.c       |   65 +++
 hw/9pfs/9p-util.h       |   52 ++
 hw/9pfs/9p-xattr-user.c |   24 -
 hw/9pfs/9p-xattr.c      |  166 +++++++-
 hw/9pfs/9p-xattr.h      |   87 +---
 hw/9pfs/Makefile.objs   |    2 
 9 files changed, 887 insertions(+), 596 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