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.

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@kernel.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;
+
+	/* 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

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@kernel.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
>

On Fri, Dec 01, 2023 at 05:57:35PM +0100, Christian Brauner wrote:
> 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.

Will do.

> > +/**
> > + * 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.

Yes, that's all correct.