bug#46292: ‘guix environment -C’ fails with Linux 4.19 (Debian)

[Lucas Nussbaum]

Hi Ludovic,

On 18/02/21 at 12:38 +0100, Ludovic Courtès wrote:
> Hi Lucas,
> 
> Lucas Nussbaum <lucas.nussbaum@inria.fr> skribis:
> 
> > This is not due to NFS, but due to the fact that the NFS mount is
> > mounted nosuid (and nodev, probably). I can reproduce it on a local
> > filesystem mounted nosuid.
> >
> > It seems that, when remounting a bind mount which is originally nosuid
> > inside a mount ns, you need to specify explicitely the nosuid option, or
> > else can_change_locked_flags()[1] will return false.
> >
> > [1] https://github.com/torvalds/linux/blame/master/fs/namespace.c#L2480
> >
> > There's a concept of "locked mount flags" that cannot be cleared by a
> > less privileged user (see [2]). Our call to 'mount -o remount' ignores the
> > fact that the filesystem is mounted nosuid (and does not include this
> > flag), so the remount call tries to remove nosuid, and fails.
> >
> > [2] 
> > https://github.com/torvalds/linux/commit/9566d6742852c527bf5af38af5cbb878dad75705
> 
> Ooh, thanks for investigating!
> 
> > This probably needs to be fixed in Guix by fetching the current mount
> > flags and including them in the bind+remount+readonly call.
> > Unfortunately I did not find an easy way to convert mount flags in
> > /proc/$$/mountinfo to flags for the mount syscall...
> 
> I tried grabbing mount options from there and reapplying them to the
> MS_REMOUNT call (patch below).  However, that still doesn’t work:
> 
> --8<---------------cut here---------------start------------->8---
> 14273 mount("/gnu/store/mmhimfwmmidf09jw1plw3aw1g1zn2nkh-bash-static-5.0.16", 
> "/tmp/guix-directory.Plgkgt//gnu/store/mmhimfwmmidf09jw1plw3aw1g1zn2nkh-bash-static-5.0.16",
>  0x236a4b0, MS_RDONLY|MS_REMOUNT|MS_BIND, "rw,nosuid,nodev,relatime") = -1 
> EPERM (Operation not permitted)
> --8<---------------cut here---------------end--------------->8---

That's strange: it worked in my manual tests.

> Interestingly, the ‘mount’ command does not attempt to re-apply the
> original mount options (“nosuid” & co.):
> 
> --8<---------------cut here---------------start------------->8---
> # strace -e mount mount --bind -o ro t m
> mount("/home/lcourtes/t", "/home/lcourtes/m", 0x564dde270cb0, 
> MS_RDONLY|MS_BIND, NULL) = 0
> mount("none", "/home/lcourtes/m", NULL, MS_RDONLY|MS_REMOUNT|MS_BIND, NULL) = 
> -1 EPERM (Operation not permitted)
> mount: /home/lcourtes/m: filesystem was mounted, but any subsequent operation 
> failed: Unknown error 5005.
> +++ exited with 32 +++
> # mount --version
> mount from util-linux 2.33.1 (libmount 2.33.1: selinux, smack, btrfs, 
> namespaces, assert, debug)
> --8<---------------cut here---------------end--------------->8---
> 
> To be continued…

I think that's something I also initially misunderstood as well: mount
-o remount,<flags> essentially means: remount the filesystem with a
fresh set of flags. The set of flags previously configured is completely
ignored.

Lucas