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Message-ID: <20231201-seide-famos-74e8c23ee2cc@brauner>
Date: Fri, 1 Dec 2023 17:57:35 +0100
From: Christian Brauner <brauner@...nel.org>
To: "Seth Forshee (DigitalOcean)" <sforshee@...nel.org>
Cc: Serge Hallyn <serge@...lyn.com>, Paul Moore <paul@...l-moore.com>,
Eric Paris <eparis@...hat.com>,
James Morris <jmorris@...ei.org>,
Alexander Viro <viro@...iv.linux.org.uk>,
Miklos Szeredi <miklos@...redi.hu>,
Amir Goldstein <amir73il@...il.com>,
linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
linux-security-module@...r.kernel.org, audit@...r.kernel.org,
linux-unionfs@...r.kernel.org
Subject: Re: [PATCH 06/16] capability: provide a helper for converting
vfs_caps to xattr for userspace
On Wed, Nov 29, 2023 at 03:50:24PM -0600, Seth Forshee (DigitalOcean) wrote:
> cap_inode_getsecurity() implements a handful of policies for capability
> xattrs read by userspace:
>
> - It returns EINVAL if the on-disk capability is in v1 format.
>
> - It masks off all bits in magic_etc except for the version and
> VFS_CAP_FLAGS_EFFECTIVE.
>
> - v3 capabilities are converted to v2 format if the rootid returned to
> userspace would be 0 or if the rootid corresponds to root in an
> ancestor user namespace.
>
> - It returns EOVERFLOW for a v3 capability whose rootid does not map to
> a valid id in current_user_ns() or to root in an ancestor namespace.
Nice. Precise and clear, please just drop these bullet points into the
kernel-doc of that function.
>
> These policies must be maintained when converting vfs_caps to an xattr
> for userspace. Provide a vfs_caps_to_user_xattr() helper which will
> enforce these policies.
>
> Signed-off-by: Seth Forshee (DigitalOcean) <sforshee@...nel.org>
> ---
> include/linux/capability.h | 4 +++
> security/commoncap.c | 68 ++++++++++++++++++++++++++++++++++++++++++++++
> 2 files changed, 72 insertions(+)
>
> diff --git a/include/linux/capability.h b/include/linux/capability.h
> index cdd7d2d8855e..c0bd9447685b 100644
> --- a/include/linux/capability.h
> +++ b/include/linux/capability.h
> @@ -218,6 +218,10 @@ int vfs_caps_to_xattr(struct mnt_idmap *idmap,
> struct user_namespace *dest_userns,
> const struct vfs_caps *vfs_caps,
> void *data, int size);
> +int vfs_caps_to_user_xattr(struct mnt_idmap *idmap,
> + struct user_namespace *dest_userns,
> + const struct vfs_caps *vfs_caps,
> + void *data, int size);
>
> /* audit system wants to get cap info from files as well */
> int get_vfs_caps_from_disk(struct mnt_idmap *idmap,
> diff --git a/security/commoncap.c b/security/commoncap.c
> index ef37966f3522..c645330f83a0 100644
> --- a/security/commoncap.c
> +++ b/security/commoncap.c
> @@ -789,6 +789,74 @@ int vfs_caps_to_xattr(struct mnt_idmap *idmap,
> return ret;
> }
>
> +/**
> + * vfs_caps_to_user_xattr - convert vfs_caps to caps xattr for userspace
> + *
> + * @idmap: idmap of the mount the inode was found from
> + * @dest_userns: user namespace for ids in xattr data
> + * @vfs_caps: source vfs_caps data
> + * @data: destination buffer for rax xattr caps data
> + * @size: size of the @data buffer
> + *
> + * Converts a kernel-interrnal capability into the raw security.capability
> + * xattr format. Includes permission checking and v2->v3 conversion as
> + * appropriate.
> + *
> + * If the xattr is being read or written through an idmapped mount the
> + * idmap of the vfsmount must be passed through @idmap. This function
> + * will then take care to map the rootid according to @idmap.
> + *
> + * Return: On success, return 0; on error, return < 0.
> + */
> +int vfs_caps_to_user_xattr(struct mnt_idmap *idmap,
> + struct user_namespace *dest_userns,
> + const struct vfs_caps *vfs_caps,
> + void *data, int size)
> +{
> + struct vfs_ns_cap_data *ns_caps = data;
> + bool is_v3;
> + u32 magic;
> +
> + /* Preserve previous behavior of returning EINVAL for v1 caps */
> + if ((vfs_caps->magic_etc & VFS_CAP_REVISION_MASK) == VFS_CAP_REVISION_1)
> + return -EINVAL;
> +
> + size = __vfs_caps_to_xattr(idmap, dest_userns, vfs_caps, data, size);
> + if (size < 0)
> + return size;
> +
> + magic = vfs_caps->magic_etc &
> + (VFS_CAP_REVISION_MASK | VFS_CAP_FLAGS_EFFECTIVE);
> + ns_caps->magic_etc = cpu_to_le32(magic);
> +
> + /*
> + * If this is a v3 capability with a valid, non-zero rootid, return
> + * the v3 capability to userspace. A v3 capability with a rootid of
> + * 0 will be converted to a v2 capability below for compatibility
> + * with old userspace.
> + */
> + is_v3 = (vfs_caps->magic_etc & VFS_CAP_REVISION_MASK) == VFS_CAP_REVISION_3;
> + if (is_v3) {
> + uid_t rootid = le32_to_cpu(ns_caps->rootid);
> + if (rootid != (uid_t)-1 && rootid != (uid_t)0)
> + return size;
> + }
> +
> + if (!rootid_owns_currentns(vfs_caps->rootid))
> + return -EOVERFLOW;
For a v2 cap that we read vfs_caps->rootid will be vfsuid 0, right?
So that means we're guaranteed to resolve that in the initial user
namespace. IOW, rootid_owns_currentns() will indeed work with a pure v2
cap. Ok. Just making sure that I understand that this won't cause
EOVERFLOW for v2. But you would've likely seen that in tests right away.
> +
> + /* This comes from a parent namespace. Return as a v2 capability. */
> + if (is_v3) {
> + magic = VFS_CAP_REVISION_2 |
> + (vfs_caps->magic_etc & VFS_CAP_FLAGS_EFFECTIVE);
> + ns_caps->magic_etc = cpu_to_le32(magic);
> + ns_caps->rootid = cpu_to_le32(0);
> + size = XATTR_CAPS_SZ_2;
> + }
> +
> + return size;
> +}
> +
> /**
> * get_vfs_caps_from_disk - retrieve vfs caps from disk
> *
>
> --
> 2.43.0
>
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