Re: [Qemu-devel] [PATCH v2 27/28] 9pfs: local: open2: don't follow symlinks

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Sun, Feb 26, 2017 at 11:45:09PM +0100, Greg Kurz wrote:
> The local_open2() callback is vulnerable to symlink attacks because it
> calls:
> 
> (1) open() which follows symbolic links for all path elements but the
>     rightmost one
> (2) local_set_xattr()->setxattr() which follows symbolic links for all
>     path elements
> (3) local_set_mapped_file_attr() which calls in turn local_fopen() and
>     mkdir(), both functions following symbolic links for all path
>     elements but the rightmost one
> (4) local_post_create_passthrough() which calls in turn lchown() and
>     chmod(), both functions also following symbolic links
> 
> This patch converts local_open2() to rely on opendir_nofollow() and
> mkdirat() to fix (1), as well as local_set_xattrat(),
> local_set_mapped_file_attrat() and local_set_cred_passthrough() to
> fix (2), (3) and (4) respectively. Since local_open2() already opens
> a descriptor to the target file, local_set_cred_passthrough() is
> modified to reuse it instead of opening a new one.
> 
> The mapped and mapped-file security modes are supposed to be identical,
> except for the place where credentials and file modes are stored. While
> here, we also make that explicit by sharing the call to openat().
> 
> This partly fixes CVE-2016-9602.
> 
> Signed-off-by: Greg Kurz <groug@kaod.org>
> ---
> v2: - use openat_file()
>     - pass dirfd and name to local_set_cred_passthrough()
> ---
>  hw/9pfs/9p-local.c |   56 ++++++++++++++++++----------------------------------
>  1 file changed, 19 insertions(+), 37 deletions(-)

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

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