On Thu, May 28, 2015 at 10:03:28AM -0500, Eric W. Biederman wrote:
> Serge Hallyn <serge.hallyn@xxxxxxxxxx> writes:
>
> > Quoting Andy Lutomirski (luto@xxxxxxxxxxxxxx):
> >> On Fri, May 22, 2015 at 10:39 AM, Eric W. Biederman
> >> <ebiederm@xxxxxxxxxxxx> 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
> "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.
> 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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