Re: [CFT][PATCH 00/10] Making new mounts of proc and sysfs as safe as bind mounts (take 2)

"Serge E. Hallyn" [off-list ref] writes:

On Thu, May 28, 2015 at 10:03:28AM -0500, Eric W. Biederman wrote:

quoted

Serge Hallyn [off-list ref] writes:

quoted

Quoting Andy Lutomirski (luto-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org):

quoted

On Fri, May 22, 2015 at 10:39 AM, Eric W. Biederman
[off-list ref] wrote:

quoted

I had hoped to get some Tested-By's on that patch series.

Sorry, I've been totally swamped.

I suspect that Sandstorm is okay, but I haven't had a chance to test
it for real.  Sandstorm makes only limited use of proc and sysfs in
containers, but I'll see if I can test it for real this weekend.

Testing this with unprivileged containers, I get

lxc-start: conf.c: lxc_mount_auto_mounts: 808 Operation not permitted
- error mounting sysfs on
/usr/lib/x86_64-linux-gnu/lxc/sys/devices/virtual/net flags 0

Grr..  I was afraid this would break something. :(

Looking at my system I see that sysfs is currently mounted
"nosuid,nodev,noexec"

Looking at the lxc-start code I don't see it as including any of those
mount options.  In practice for sysfs I think those options are
meaningless (as there should be no devices and nothing executable in
sysfs) but I can understand the past concerns with chmod on virtual
filesystems that would incline people to use them, so I think the
failure is reporting a legitimate security issue in the lxc userspace
code where the the unprivileged code is currently attempting to give
greater access to sysfs than was given by the original mount of sysfs.

As nosuid,nodev,noexec should not impair the operation of sysfs
operation it looks like you can always specify those options and just
make this concern go away.

Something like the untested patch below I expect.

diff --git a/src/lxc/conf.c b/src/lxc/conf.c
index 9870455b3cae..d9ccd03afe68 100644
--- a/src/lxc/conf.c
+++ b/src/lxc/conf.c

@@ -770,8 +770,8 @@ static int lxc_mount_auto_mounts(struct lxc_conf *conf, int flags, struct lxc_ha
 		{ LXC_AUTO_PROC_MASK, LXC_AUTO_PROC_MIXED, "%r/proc/sysrq-trigger",                             "%r/proc/sysrq-trigger",        NULL,       MS_BIND,                        NULL },
 		{ LXC_AUTO_PROC_MASK, LXC_AUTO_PROC_MIXED, NULL,                                                "%r/proc/sysrq-trigger",        NULL,       MS_REMOUNT|MS_BIND|MS_RDONLY,   NULL },
 		{ LXC_AUTO_PROC_MASK, LXC_AUTO_PROC_RW,    "proc",                                              "%r/proc",                      "proc",     MS_NODEV|MS_NOEXEC|MS_NOSUID,   NULL },
-		{ LXC_AUTO_SYS_MASK,  LXC_AUTO_SYS_RW,     "sysfs",                                             "%r/sys",                       "sysfs",    0,                              NULL },
-		{ LXC_AUTO_SYS_MASK,  LXC_AUTO_SYS_RO,     "sysfs",                                             "%r/sys",                       "sysfs",    MS_RDONLY,                      NULL },
+		{ LXC_AUTO_SYS_MASK,  LXC_AUTO_SYS_RW,     "sysfs",                                             "%r/sys",                       "sysfs",    MS_NODEV|MS_NOEXEC|MS_NOSUID,   NULL },
+		{ LXC_AUTO_SYS_MASK,  LXC_AUTO_SYS_RO,     "sysfs",                                             "%r/sys",                       "sysfs",    MS_NODEV|MS_NOEXEC|MS_NOSUID|MS_RDONLY, NULL },
 		{ LXC_AUTO_SYS_MASK,  LXC_AUTO_SYS_MIXED,  "sysfs",                                             "%r/sys",                       "sysfs",    MS_NODEV|MS_NOEXEC|MS_NOSUID,   NULL },
 		{ LXC_AUTO_SYS_MASK,  LXC_AUTO_SYS_MIXED,  "%r/sys",                                            "%r/sys",                       NULL,       MS_BIND,                        NULL },
 		{ LXC_AUTO_SYS_MASK,  LXC_AUTO_SYS_MIXED,  NULL,                                                "%r/sys",                       NULL,       MS_REMOUNT|MS_BIND|MS_RDONLY,   NULL },

fwiw - the first one works, the second one does not due to an apparent
inability to statvfs the origin.

Good to hear.  That confirms there are no other gotchas waiting in the
wings.

Apparently my second suggested patch is buggy due to an invalid source
string.  The source would need to be %r/proc or %r/sysfs to use statvfs
productively.

quoted

Alternately you can read the flags off of the original mount of proc or sysfs.

diff --git a/src/lxc/conf.c b/src/lxc/conf.c
index 9870455b3cae..50ea49973e80 100644
--- a/src/lxc/conf.c
+++ b/src/lxc/conf.c

@@ -712,7 +712,9 @@ static unsigned long add_required_remount_flags(const char *s, const char *d,
        struct statvfs sb;
        unsigned long required_flags = 0;
 
-       if (!(flags & MS_REMOUNT))
+       if (!(flags & MS_REMOUNT) &&
+           (strcmp(s, "proc") != 0) &&
+           (strcmp(s, "sysfs") != 0))
                return flags;
 
        if (!s)

Eric
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