Quoting Stefan Berger (Stefan Bergerstefanb@xxxxxxxxxxxxxxxxxx):
> er.kernel.org>
> X-Mailing-List: linux-kernel@xxxxxxxxxxxxxxx
> Content-Length: 19839
> Lines: 700
> X-UID: 24770
> Status: RO
>
> From: Stefan Berger <stefanb@xxxxxxxxxxxxxxxxxx>
>
> This patch enables security.capability in user namespaces but also
> takes a more general approach to enabling extended attributes in user
> namespaces.
>
> The following rules describe the approach using security.foo as a
> 'user namespace enabled' extended attribute:
>
> Reading of extended attributes:
>
> 1a) Reading security.foo from a user namespace will read
> security.foo@uid=<uid> of the parent user namespace instead with uid
> being the mapping of root in that parent user namespace. An
> exception is if root is mapped to uid 0 on the host, and in this case
> we will read security.foo directly.
> --> reading security.foo will read security.foo@uid=1000 for uid
> mapping of root to 1000.
>
> 1b) If security.foo@uid=<uid> is not available, the security.foo of the
> parent namespace is tried to be read. This procedure is repeated up to
> the init user namespace. This step only applies for reading of extended
> attributes and provides the same behavior as older system where the
> host's extended attributes applied to user namespaces.
>
> 2) All security.foo@uid=<uid> with valid uid mapping in the user namespace
> can be read. The uid within the user namespace will be mapped to the
> corresponding uid on the host and that uid will be used in the name of
> the extended attribute.
> -> reading security.foo@uid=1 will read security.foo@uid=1001 for uid
> mapping of root to 1000, size of at least 2.
>
> All security.foo@uid=<uid> can be read (by root) on the host with values
> of <uid> also being subject to checking for valid mappings.
>
> 3) No other security.foo* can be read.
>
> The same rules for reading apply to writing and removing of user
> namespace enabled extended attributes.
>
> When listing extended attributes of a file, only those are presented
> to the user namespace that have a valid mapping. Besides that, names
> of the extended attributes are adjusted to represent the mapping.
> This means that if root is mapped to uid 1000 on the host, the
> security.foo@uid=1000 will be listed as security.foo in the user
> namespace, security.foo@uid=1001 becomes security.foo@uid=1 and so on.
>
> Signed-off-by: Stefan Berger <stefanb@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Serge Hallyn <serge@xxxxxxxxxx>
> Reviewed-by: Serge Hallyn <serge@xxxxxxxxxx>
> ---
> fs/xattr.c | 509 +++++++++++++++++++++++++++++++++++++++++++++--
> security/commoncap.c | 36 +++-
> security/selinux/hooks.c | 9 +-
> 3 files changed, 523 insertions(+), 31 deletions(-)
>
> diff --git a/fs/xattr.c b/fs/xattr.c
> index 464c94b..eacad9e 100644
> --- a/fs/xattr.c
> +++ b/fs/xattr.c
> @@ -133,20 +133,440 @@ xattr_permission(struct inode *inode, const char *name, int mask)
> return inode_permission(inode, mask);
> }
>
> +/*
> + * A list of extended attributes that are supported in user namespaces
> + */
> +static const char *const userns_xattrs[] = {
> + XATTR_NAME_CAPS,
> + NULL
> +};
> +
> +/*
> + * xattrs_is_userns_supported - Check whether an xattr is supported in userns
> + *
> + * @name: full name of the extended attribute
> + * @prefix: do a prefix match (true) or a full match (false)
> + *
> + * This function returns < 0 if not supported, an index into userns_xattrs[]
> + * otherwise.
> + */
> +static int
> +xattr_is_userns_supported(const char *name, int prefix)
> +{
> + int i;
> +
> + if (!name)
> + return -1;
> +
> + for (i = 0; userns_xattrs[i]; i++) {
> + if (prefix) {
> + if (!strncmp(userns_xattrs[i], name,
> + strlen(userns_xattrs[i])))
> + return i;
I think you here need to also check that the next char is either
'\0' or '.' (or maybe '@')
> + } else {
> + if (!strcmp(userns_xattrs[i], name))
> + return i;
> + }
> + }
> + return -1;
> +}
> +
> +/*
> + * xattr_write_uid - print a string in the format of "%s@uid=%u", which
> + * includes a prefix string
> + *
> + * @uid: the uid
> + * @prefix: prefix string; may be NULL
> + *
> + * This function returns a buffer with the string, or a NULL pointer in
> + * case of out-of-memory error.
> + */
> +static char *
> +xattr_write_uid(uid_t uid, const char *prefix)
> +{
> + size_t buflen;
> + char *buffer;
> +
> + buflen = sizeof("@uid=") - 1 + sizeof("4294967295") - 1 + 1;
> + if (prefix)
> + buflen += strlen(prefix);
> +
> + buffer = kmalloc(buflen, GFP_KERNEL);
> + if (!buffer)
> + return NULL;
> +
> + if (uid == 0)
> + *buffer = 0;
Do you need to print out the prefix here?
> + else
> + sprintf(buffer, "%s@uid=%u",
> + (prefix) ? prefix : "",
> + uid);
> +
> + return buffer;
> +}
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