linux-kernel - Re: [RFC PATCH v1 0/2] Add capabilities file to sysfs

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Message-ID: <7d1d7682-dd19-1b8a-ee5a-50fe6479589b@schaufler-ca.com>
Date:   Mon, 27 Dec 2021 14:22:41 -0800
From:   Casey Schaufler <casey@...aufler-ca.com>
To:     Francis Laniel <flaniel@...ux.microsoft.com>,
        linux-kernel@...r.kernel.org
Cc:     Serge Hallyn <serge@...lyn.com>,
        linux-security-module@...r.kernel.org,
        Casey Schaufler <casey@...aufler-ca.com>
Subject: Re: [RFC PATCH v1 0/2] Add capabilities file to sysfs

On 12/27/2021 12:54 PM, Francis Laniel wrote:
> Hi.
>
>
> First, I hope you are fine and the same for your relatives.
>
> Capabilities are used to check if a thread has the right to perform a given
> action [1].
> For example, a thread with CAP_BPF set can use the bpf() syscall.
>
> Capabilities are used in the container world.
> In terms of code, several projects related to container maintain code where the
> capabilities are written alike include/uapi/linux/capability.h [2][3][4][5].
> For these projects, their codebase should be updated when a new capability is
> added to the kernel.
> Some other projects rely on <sys/capability.h> [6].
> In this case, this header file should reflect the capabilities offered by the
> kernel.
>
> So, in this series, I added a new file to sysfs: /sys/kernel/capabilities.

This should be /sys/kernel/security/capabilities.

> The goal of this file is to be used by "container world" software to know kernel
> capabilities at run time instead of compile time.
>
> The underlying kernel attribute is read-only and its content is the capability
> number associated with the capability name:
> root@...amd64:~# cat /sys/kernel/capabilities
> 0       CAP_CHOWN
> 1       CAP_DAC_OVERRIDE
> ...
> 39      CAP_BPF
>
> The kernel already exposes the last capability number under:
> /proc/sys/kernel/cap_last_cap
> So, I think there should not be any issue exposing all the capabilities it
> offers.
> If there is any, please share it as I do not want to introduce issue with this
> series.
>
> Also, if you see any way to improve this series please share it as it would
> increase this contribution quality.
>
> Francis Laniel (2):
>    capability: Add cap_strings.
>    kernel/ksysfs.c: Add capabilities attribute.
>
>   include/uapi/linux/capability.h |  1 +
>   kernel/capability.c             | 45 +++++++++++++++++++++++++++++++++
>   kernel/ksysfs.c                 | 18 +++++++++++++
>   3 files changed, 64 insertions(+)
>
>
> Best regards and thank you in advance for your reviews.
> ---
> [1] man capabilities
> [2] https://github.com/containerd/containerd/blob/1a078e6893d07fec10a4940a5664fab21d6f7d1e/pkg/cap/cap_linux.go#L135
> [3] https://github.com/moby/moby/commit/485cf38d48e7111b3d1f584d5e9eab46a902aabc#diff-2e04625b209932e74c617de96682ed72fbd1bb0d0cb9fb7c709cf47a86b6f9c1
> moby relies on containerd code.
> [4] https://github.com/syndtr/gocapability/blob/42c35b4376354fd554efc7ad35e0b7f94e3a0ffb/capability/enum.go#L47
> [5] https://github.com/opencontainers/runc/blob/00f56786bb220b55b41748231880ba0e6380519a/libcontainer/capabilities/capabilities.go#L12
> runc relies on syndtr package.
> [6] https://github.com/containers/crun/blob/fafb556f09e6ffd4690c452ff51856b880c089f1/src/libcrun/linux.c#L35
>