Buildroot: incorrect permissons on /dev/shm

[Ben Hutchings <ben.hutchings () essensium com>]

Buildroot is a Linux distribution and system builder for embedded
systems.  Starting in Buildroot 2011.08, its default /etc/fstab
included an entry for /dev/shm with incorrect permissons (sticky bit
not set). (CWE-276)

Buildroot 2017.08 removed this entry for systems using systemd, and it
has never been included for systems using OpenRC.  So this only
affects Buildroot-built systems that use sysvinit, and some older
systems that use systemd.

Ben.

On Thu, Apr 11, 2024 at 05:20:16PM +0200, Ben Hutchings wrote:
> /dev/shm is a world-writable directory, like /tmp, and should also
> have the sticky bit set.  Without this, any user can delete and
> replace another user's files in /dev/shm.
>
> This bug has been present since /dev/shm was added to the skeleton
> /etc/fstab, but appears to have been fixed for systems using systemd
> by commit 76fc9275f14e "system: separate sysv and systemd parts of the
> skeleton" which went into Buildroot 2017.08.
>
> Signed-off-by: Ben Hutchings <ben.hutchings () mind be>
> Fixes: 22fde22e35f98f7830c2f8955465532328348cd1
> ---
>  package/skeleton-init-sysv/skeleton/etc/fstab | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/package/skeleton-init-sysv/skeleton/etc/fstab b/package/skeleton-init-sysv/skeleton/etc/fstab
> index 169054b74f..06c20fe9d5 100644
> --- a/package/skeleton-init-sysv/skeleton/etc/fstab
> +++ b/package/skeleton-init-sysv/skeleton/etc/fstab
> @@ -2,7 +2,7 @@
>  /dev/root    /               ext2    rw,noauto       0       1
>  proc         /proc           proc    defaults        0       0
>  devpts               /dev/pts        devpts  defaults,gid=5,mode=620,ptmxmode=0666   0       0
> -tmpfs                /dev/shm        tmpfs   mode=0777       0       0
> +tmpfs                /dev/shm        tmpfs   mode=1777       0       0
>  tmpfs                /tmp            tmpfs   mode=1777       0       0
>  tmpfs                /run            tmpfs   mode=0755,nosuid,nodev  0       0
>  sysfs                /sys            sysfs   defaults        0       0
> -- 
> 2.39.2

-- 
Ben Hutchings · Senior Embedded Software Engineer, Essensium-Mind · mind.be