linux-kernel - Re: [PATCH v3] selinux: enable per-file labeling for functionfs

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Message-ID: <CAEjxPJ7-M5OAiTLmOynP36HF6XmJKhH2kTFAGmhg8ohCkZuT8w@mail.gmail.com>
Date: Fri, 29 Aug 2025 08:18:08 -0400
From: Stephen Smalley <stephen.smalley.work@...il.com>
To: Neill Kapron <nkapron@...gle.com>
Cc: Paul Moore <paul@...l-moore.com>, Ondrej Mosnacek <omosnace@...hat.com>, kernel-team@...roid.com, 
	selinux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] selinux: enable per-file labeling for functionfs

On Thu, Aug 28, 2025 at 1:03 PM Neill Kapron <nkapron@...gle.com> wrote:
>
> This patch adds support for genfscon per-file labeling of functionfs
> files as well as support for userspace to apply labels after new
> functionfs endpoints are created.
>
> This allows for separate labels and therefore access control on a
> per-endpoint basis. An example use case would be for the default
> endpoint EP0 used as a restricted control endpoint, and additional
> usb endpoints to be used by other more permissive domains.
>
> It should be noted that if there are multiple functionfs mounts on a
> system, genfs file labels will apply to all mounts, and therefore will not
> likely be as useful as the userspace relabeling portion of this patch -
> the addition to selinux_is_genfs_special_handling().
>
> This patch introduces the functionfs_seclabel policycap to maintain
> existing functionfs genfscon behavior unless explicitly enabled.
>
> Signed-off-by: Neill Kapron <nkapron@...gle.com>

Acked-by: Stephen Smalley <stephen.smalley.work@...il.com>

As before, don't rely on the policy capability bit remaining stable
until Paul merges this patch.
Also, not worth re-spinning IMHO but the changelog below normally goes
after the "---" before
the diffstat so that it doesn't get included in the commit message
since no one cares about
the in-submission changes once the patch is merged.

>
> Changes since v1:
> - Add functionfs_seclabel policycap
> - Move new functionality to the end of existing lists
>
> Changes since v2:
> - Sending as separate patches
> ---
>  security/selinux/hooks.c                   | 8 ++++++--
>  security/selinux/include/policycap.h       | 1 +
>  security/selinux/include/policycap_names.h | 1 +
>  security/selinux/include/security.h        | 6 ++++++
>  4 files changed, 14 insertions(+), 2 deletions(-)
>
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index e474cd7398ef..333bb6cba25e 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -476,7 +476,9 @@ static int selinux_is_genfs_special_handling(struct super_block *sb)
>                 !strcmp(sb->s_type->name, "rootfs") ||
>                 (selinux_policycap_cgroupseclabel() &&
>                  (!strcmp(sb->s_type->name, "cgroup") ||
> -                 !strcmp(sb->s_type->name, "cgroup2")));
> +                 !strcmp(sb->s_type->name, "cgroup2"))) ||
> +               (selinux_policycap_functionfs_seclabel() &&
> +                !strcmp(sb->s_type->name, "functionfs"));
>  }
>
>  static int selinux_is_sblabel_mnt(struct super_block *sb)
> @@ -741,7 +743,9 @@ static int selinux_set_mnt_opts(struct super_block *sb,
>             !strcmp(sb->s_type->name, "binder") ||
>             !strcmp(sb->s_type->name, "bpf") ||
>             !strcmp(sb->s_type->name, "pstore") ||
> -           !strcmp(sb->s_type->name, "securityfs"))
> +           !strcmp(sb->s_type->name, "securityfs") ||
> +           (selinux_policycap_functionfs_seclabel() &&
> +            strcmp(sb->s_type->name, "functionfs")))
>                 sbsec->flags |= SE_SBGENFS;
>
>         if (!strcmp(sb->s_type->name, "sysfs") ||
> diff --git a/security/selinux/include/policycap.h b/security/selinux/include/policycap.h
> index 7405154e6c42..135a969f873c 100644
> --- a/security/selinux/include/policycap.h
> +++ b/security/selinux/include/policycap.h
> @@ -17,6 +17,7 @@ enum {
>         POLICYDB_CAP_NETLINK_XPERM,
>         POLICYDB_CAP_NETIF_WILDCARD,
>         POLICYDB_CAP_GENFS_SECLABEL_WILDCARD,
> +       POLICYDB_CAP_FUNCTIONFS_SECLABEL,
>         __POLICYDB_CAP_MAX
>  };
>  #define POLICYDB_CAP_MAX (__POLICYDB_CAP_MAX - 1)
> diff --git a/security/selinux/include/policycap_names.h b/security/selinux/include/policycap_names.h
> index d8962fcf2ff9..ff8882887651 100644
> --- a/security/selinux/include/policycap_names.h
> +++ b/security/selinux/include/policycap_names.h
> @@ -20,6 +20,7 @@ const char *const selinux_policycap_names[__POLICYDB_CAP_MAX] = {
>         "netlink_xperm",
>         "netif_wildcard",
>         "genfs_seclabel_wildcard",
> +       "functionfs_seclabel",
>  };
>  /* clang-format on */
>
> diff --git a/security/selinux/include/security.h b/security/selinux/include/security.h
> index 7f19972f7922..0f954a40d3fc 100644
> --- a/security/selinux/include/security.h
> +++ b/security/selinux/include/security.h
> @@ -203,6 +203,12 @@ static inline bool selinux_policycap_netlink_xperm(void)
>                 selinux_state.policycap[POLICYDB_CAP_NETLINK_XPERM]);
>  }
>
> +static inline bool selinux_policycap_functionfs_seclabel(void)
> +{
> +       return READ_ONCE(
> +               selinux_state.policycap[POLICYDB_CAP_FUNCTIONFS_SECLABEL]);
> +}
> +
>  struct selinux_policy_convert_data;
>
>  struct selinux_load_state {
> --
> 2.51.0.318.gd7df087d1a-goog
>