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

Eric W. Biederman ebiederm at xmission.com
Thu May 28 15:03:28 UTC 2015

Serge Hallyn <serge.hallyn at ubuntu.com> writes:

> Quoting Andy Lutomirski (luto at amacapital.net):
>> On Fri, May 22, 2015 at 10:39 AM, Eric W. Biederman
>> <ebiederm at xmission.com> wrote:
>> > 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

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 },

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)


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