| Message ID | 20200730194736.173994-5-vgoyal@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | virtiofsd: Add a unprivileged passthrough mode | expand |
On Thu, Jul 30, 2020 at 03:47:35PM -0400, Vivek Goyal wrote: > In sandbox=NONE mode, lo->source points to the directory which is being > exported. We have not done any chroot()/pivot_root(). So open lo->source. > > Signed-off-by: Vivek Goyal <vgoyal@redhat.com> > --- > tools/virtiofsd/passthrough_ll.c | 5 ++++- > 1 file changed, 4 insertions(+), 1 deletion(-) > > diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c > index 76ef891105..a6fa816b6c 100644 > --- a/tools/virtiofsd/passthrough_ll.c > +++ b/tools/virtiofsd/passthrough_ll.c > @@ -3209,7 +3209,10 @@ static void setup_root(struct lo_data *lo, struct lo_inode *root) > int fd, res; > struct stat stat; > > - fd = open("/", O_PATH); > + if (lo->sandbox == SANDBOX_NONE) > + fd = open(lo->source, O_PATH); > + else > + fd = open("/", O_PATH); Up until now virtiofsd has been able to assume that path traversal has the shared directory as "/". Now this is no longer true and it is necessary to audit all syscalls that take path arguments. They must ensure that: 1. Path components are safe (no ".." or "/" allowed) 2. Symlinks are not followed. Did you audit all syscalls made by passthrough_ll.c? virtiofsd still needs to restrict the client to the shared directory for two reasons: 1. The guest may not be trusted. An unprivileged sandbox=none mount can be used with a malicious guest. 2. If accidental escapes are possible then the guest could accidentally corrupt or delete files outside the shared directory. Stefan
On Mon, Aug 03, 2020 at 10:54:59AM +0100, Stefan Hajnoczi wrote: > On Thu, Jul 30, 2020 at 03:47:35PM -0400, Vivek Goyal wrote: > > In sandbox=NONE mode, lo->source points to the directory which is being > > exported. We have not done any chroot()/pivot_root(). So open lo->source. > > > > Signed-off-by: Vivek Goyal <vgoyal@redhat.com> > > --- > > tools/virtiofsd/passthrough_ll.c | 5 ++++- > > 1 file changed, 4 insertions(+), 1 deletion(-) > > > > diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c > > index 76ef891105..a6fa816b6c 100644 > > --- a/tools/virtiofsd/passthrough_ll.c > > +++ b/tools/virtiofsd/passthrough_ll.c > > @@ -3209,7 +3209,10 @@ static void setup_root(struct lo_data *lo, struct lo_inode *root) > > int fd, res; > > struct stat stat; > > > > - fd = open("/", O_PATH); > > + if (lo->sandbox == SANDBOX_NONE) > > + fd = open(lo->source, O_PATH); > > + else > > + fd = open("/", O_PATH); > > Up until now virtiofsd has been able to assume that path traversal has > the shared directory as "/". > > Now this is no longer true and it is necessary to audit all syscalls > that take path arguments. They must ensure that: > 1. Path components are safe (no ".." or "/" allowed) > 2. Symlinks are not followed. This code does not change the path client is passing in and we are already doing the checks on passed in paths/names. So existing checks should work even for this case, isn't it? lo_lookup() { if (strchr(name, '/')) { fuse_reply_err(req, EINVAL); return; } } lo_do_lookup() { /* Do not allow escaping root directory */ if (dir == &lo->root && strcmp(name, "..") == 0) { name = "."; } } > > Did you audit all syscalls made by passthrough_ll.c? > > virtiofsd still needs to restrict the client to the shared directory for > two reasons: > 1. The guest may not be trusted. An unprivileged sandbox=none mount can > be used with a malicious guest. > 2. If accidental escapes are possible then the guest could accidentally > corrupt or delete files outside the shared directory. Even if escape is possible, its no different than a malicious user application running. Given sandbox=none can be used in case of unpriviliged mode, that means user app can only affect files owned by that user. Given we are not doing chroot()/pivot_root(), key question will be what additional path we are enabling using which one can escape out of this shared directory. Having said that, In case of unpriviliged mode, it feels that chroot() is more of a nice to have kind of functionality. Even if guest manages to break out of shared directory, impact is equivalent to user running a malicious app directly. If doing chroot/pivot_root is must, then we need additional capabilities. And that probably means we need to launch virtiofsd in a user namespace with required caps. And that will fall back into the territory of running virtiofsd in a user namespace. That indeed is an important use case which needs to be solved. Thanks Vivek
On Mon, Aug 03, 2020 at 09:57:15AM -0400, Vivek Goyal wrote: > On Mon, Aug 03, 2020 at 10:54:59AM +0100, Stefan Hajnoczi wrote: > > On Thu, Jul 30, 2020 at 03:47:35PM -0400, Vivek Goyal wrote: > > > In sandbox=NONE mode, lo->source points to the directory which is being > > > exported. We have not done any chroot()/pivot_root(). So open lo->source. > > > > > > Signed-off-by: Vivek Goyal <vgoyal@redhat.com> > > > --- > > > tools/virtiofsd/passthrough_ll.c | 5 ++++- > > > 1 file changed, 4 insertions(+), 1 deletion(-) > > > > > > diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c > > > index 76ef891105..a6fa816b6c 100644 > > > --- a/tools/virtiofsd/passthrough_ll.c > > > +++ b/tools/virtiofsd/passthrough_ll.c > > > @@ -3209,7 +3209,10 @@ static void setup_root(struct lo_data *lo, struct lo_inode *root) > > > int fd, res; > > > struct stat stat; > > > > > > - fd = open("/", O_PATH); > > > + if (lo->sandbox == SANDBOX_NONE) > > > + fd = open(lo->source, O_PATH); > > > + else > > > + fd = open("/", O_PATH); > > > > Up until now virtiofsd has been able to assume that path traversal has > > the shared directory as "/". > > > > Now this is no longer true and it is necessary to audit all syscalls > > that take path arguments. They must ensure that: > > 1. Path components are safe (no ".." or "/" allowed) > > 2. Symlinks are not followed. > > This code does not change the path client is passing in and we are > already doing the checks on passed in paths/names. So existing checks > should work even for this case, isn't it? > > lo_lookup() { > if (strchr(name, '/')) { > fuse_reply_err(req, EINVAL); > return; > } > } > > lo_do_lookup() { > /* Do not allow escaping root directory */ > if (dir == &lo->root && strcmp(name, "..") == 0) { > name = "."; > } > } Yes, hopefully paths are already checked and syscalls do not follow symlinks. However, we wouldn't have noticed if either of those were missing since the pivot_root(2) ensured that path traversal stays within the shared directory. Now that a layer of protection has been removed it's necessary to check again whether everything is really alright. > > > > > Did you audit all syscalls made by passthrough_ll.c? > > > > virtiofsd still needs to restrict the client to the shared directory for > > two reasons: > > 1. The guest may not be trusted. An unprivileged sandbox=none mount can > > be used with a malicious guest. > > 2. If accidental escapes are possible then the guest could accidentally > > corrupt or delete files outside the shared directory. > > Even if escape is possible, its no different than a malicious user > application running. Given sandbox=none can be used in case of > unpriviliged mode, that means user app can only affect files owned by > that user. Users may run untrusted guests. Those guests should not gain access to the user's files outside the shared directory. > If doing chroot/pivot_root is must, then we need additional capabilities. I think it's not a must, but just an extra layer of security. What I don't know 100% is whether virtiofsd accidentally relies on that extra layer of security today since it never ran without it :). Stefan
diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c index 76ef891105..a6fa816b6c 100644 --- a/tools/virtiofsd/passthrough_ll.c +++ b/tools/virtiofsd/passthrough_ll.c @@ -3209,7 +3209,10 @@ static void setup_root(struct lo_data *lo, struct lo_inode *root) int fd, res; struct stat stat; - fd = open("/", O_PATH); + if (lo->sandbox == SANDBOX_NONE) + fd = open(lo->source, O_PATH); + else + fd = open("/", O_PATH); if (fd == -1) { fuse_log(FUSE_LOG_ERR, "open(%s, O_PATH): %m\n", lo->source); exit(1);
In sandbox=NONE mode, lo->source points to the directory which is being exported. We have not done any chroot()/pivot_root(). So open lo->source. Signed-off-by: Vivek Goyal <vgoyal@redhat.com> --- tools/virtiofsd/passthrough_ll.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-)