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SubjectRe: [PATCH v2 0/6] device_cgroup: replace internally whitelist with exception list
Quoting Aristeu Rozanski (aris@redhat.com):
> The original model of device_cgroup is having a whitelist where all the
> allowed devices are listed. The problem with this approach is that is
> impossible to have the case of allowing everything but few devices.
>
> The reason for that lies in the way the whitelist is handled internally:
> since there's only a whitelist, the "all devices" entry would have to be
> removed and replaced by the entire list of possible devices but the ones
> that are being denied. Since dev_t is 32 bits long, representing the allowed
> devices as a bitfield is not memory efficient.
>
> This patch replaces the "whitelist" by a "exceptions" list and the default
> policy is kept as "deny_all" variable in dev_cgroup structure.
>
> The current interface determines that whenever "a" is written to devices.allow
> or devices.deny, the entry masking all devices will be added or removed,
> respectively. This behavior is kept and it's what will determine the default
> policy:
>
> # cat devices.list
> a *:* rwm
> # echo a >devices.deny
> # cat devices.list
> # echo a >devices.allow
> # cat devices.list
> a *:* rwm
>
> The interface is also preserved. For example, if one wants to block only access
> to /dev/null:
> # ls -l /dev/null
> crw-rw-rw- 1 root root 1, 3 Jul 24 16:17 /dev/null
> # echo a >devices.allow
> # echo "c 1:3 rwm" >devices.deny
> # cat /dev/null
> cat: /dev/null: Operation not permitted
> # echo >/dev/null
> bash: /dev/null: Operation not permitted
> # mknod /tmp/null c 1 3
> mknod: /tmp/null: Operation not permitted
> # echo "c 1:3 r" >devices.allow
> # cat /dev/null
> # echo >/dev/null
> bash: /dev/null: Operation not permitted
> # mknod /tmp/null c 1 3
> mknod: /tmp/null: Operation not permitted
> # echo "c 1:3 rw" >devices.allow
> # echo >/dev/null
> # cat /dev/null
> # mknod /tmp/null c 1 3
> mknod: /tmp/null: Operation not permitted
> # echo "c 1:3 rwm" >devices.allow
> # echo >/dev/null
> # cat /dev/null
> # mknod /tmp/null c 1 3
> #
>
> v2:
> - stop using simple_strtoul()
> - fix checkpatch warnings
> - rename deny_all to behavior
> - updated documentation
> - added new files to cgroupfs to better reflect the internal state
>
> Documentation/cgroups/devices.txt | 73 ++++--
> security/device_cgroup.c | 443 +++++++++++++++++++++++---------------
> 2 files changed, 333 insertions(+), 183 deletions(-)
>
> --
> Aristeu

Thanks, Aristeu, very nice.

-serge


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