Re: [PATCH 1/1] simplified security.nscapability xattr

Kees Cook
2016-04-26T21:59:34Z

On Fri, Apr 22, 2016 at 10:26 AM, <serge.hallyn@ubuntu.com> wrote:
> From: Serge Hallyn <serge.hallyn@ubuntu.com>
>
> This can only be set by root in his own namespace, and will
> only be respected by namespaces with that same root kuid
> mapped as root, or namespaces descended from it.
>
> This allows a simple setxattr to work, allows tar/untar to
> work, and allows us to tar in one namespace and untar in
> another while preserving the capability, without risking
> leaking privilege into a parent namespace.

The concept seems sensible to me. Various notes below...

>
> Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
> ---
> include/linux/capability.h | 5 ++-
> include/uapi/linux/capability.h | 18 ++++++++
> include/uapi/linux/xattr.h | 3 ++
> security/commoncap.c | 91 +++++++++++++++++++++++++++++++++++++--
> 4 files changed, 112 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/capability.h b/include/linux/capability.h
> index 00690ff..cf533ff 100644
> --- a/include/linux/capability.h
> +++ b/include/linux/capability.h
> @@ -13,7 +13,7 @@
> #define _LINUX_CAPABILITY_H
>
> #include <uapi/linux/capability.h>
> -
> +#include <linux/uidgid.h>
>
> #define _KERNEL_CAPABILITY_VERSION _LINUX_CAPABILITY_VERSION_3
> #define _KERNEL_CAPABILITY_U32S _LINUX_CAPABILITY_U32S_3
> @@ -31,6 +31,9 @@ struct cpu_vfs_cap_data {
> kernel_cap_t inheritable;
> };
>
> +#define NS_CAPS_VERSION(x) (x & 0xFF)
> +#define NS_CAPS_FLAGS(x) ((x >> 8) & 0xFF)
> +
> #define _USER_CAP_HEADER_SIZE (sizeof(struct __user_cap_header_struct))
> #define _KERNEL_CAP_T_SIZE (sizeof(kernel_cap_t))
>
> diff --git a/include/uapi/linux/capability.h b/include/uapi/linux/capability.h
> index 12c37a1..f0b4a66 100644
> --- a/include/uapi/linux/capability.h
> +++ b/include/uapi/linux/capability.h
> @@ -62,6 +62,9 @@ typedef struct __user_cap_data_struct {
> #define VFS_CAP_U32_2 2
> #define XATTR_CAPS_SZ_2 (sizeof(__le32)*(1 + 2*VFS_CAP_U32_2))
>
> +/* version number for security.nscapability xattrs hdr->hdr_info */
> +#define VFS_NS_CAP_REVISION 1
> +
> #define XATTR_CAPS_SZ XATTR_CAPS_SZ_2
> #define VFS_CAP_U32 VFS_CAP_U32_2
> #define VFS_CAP_REVISION VFS_CAP_REVISION_2
> @@ -74,6 +77,21 @@ struct vfs_cap_data {
> } data[VFS_CAP_U32];
> };
>
> +#define VFS_NS_CAP_EFFECTIVE 0x1
> +/*
> + * 32-bit hdr_info contains
> + * 16 leftmost: reserved
> + * next 8: flags (only VFS_NS_CAP_EFFECTIVE so far)
> + * last 8: version
> + */
> +struct vfs_ns_cap_data {
> + __le32 magic_etc;
> + struct {
> + __le32 permitted; /* Little endian */
> + __le32 inheritable; /* Little endian */
> + } data[VFS_CAP_U32];
> +};

This is identical to vfs_cap_data. Is there a reason not to re-use the
existing one?

> +
> #ifndef __KERNEL__
>
> /*
> diff --git a/include/uapi/linux/xattr.h b/include/uapi/linux/xattr.h
> index 1590c49..67c80ab 100644
> --- a/include/uapi/linux/xattr.h
> +++ b/include/uapi/linux/xattr.h
> @@ -68,6 +68,9 @@
> #define XATTR_CAPS_SUFFIX "capability"
> #define XATTR_NAME_CAPS XATTR_SECURITY_PREFIX XATTR_CAPS_SUFFIX
>
> +#define XATTR_NS_CAPS_SUFFIX "nscapability"
> +#define XATTR_NAME_NS_CAPS XATTR_SECURITY_PREFIX XATTR_NS_CAPS_SUFFIX
> +
> #define XATTR_POSIX_ACL_ACCESS "posix_acl_access"
> #define XATTR_NAME_POSIX_ACL_ACCESS XATTR_SYSTEM_PREFIX XATTR_POSIX_ACL_ACCESS
> #define XATTR_POSIX_ACL_DEFAULT "posix_acl_default"

Are these documented anywhere in Documentation/ or in man pages? This
seems like it'd need a man-page update too.

> diff --git a/security/commoncap.c b/security/commoncap.c
> index 48071ed..8f3f34a 100644
> --- a/security/commoncap.c
> +++ b/security/commoncap.c
> @@ -313,6 +313,10 @@ int cap_inode_need_killpriv(struct dentry *dentry)
> if (!inode->i_op->getxattr)
> return 0;
>
> + error = inode->i_op->getxattr(dentry, XATTR_NAME_NS_CAPS, NULL, 0);
> + if (error > 0)
> + return 1;
> +
> error = inode->i_op->getxattr(dentry, XATTR_NAME_CAPS, NULL, 0);
> if (error <= 0)
> return 0;

I think this might be more readable if the getxattr calls were
standardized (one returns 1, the other returns 0, with inverted tests
-- hard to read). IIUC, if either returns > 0, return 1, otherwise
return 0.

> @@ -330,11 +334,17 @@ int cap_inode_need_killpriv(struct dentry *dentry)
> int cap_inode_killpriv(struct dentry *dentry)
> {
> struct inode *inode = d_backing_inode(dentry);
> + int ret1, ret2;
>
> if (!inode->i_op->removexattr)
> return 0;
>
> - return inode->i_op->removexattr(dentry, XATTR_NAME_CAPS);
> + ret1 = inode->i_op->removexattr(dentry, XATTR_NAME_CAPS);
> + ret2 = inode->i_op->removexattr(dentry, XATTR_NAME_NS_CAPS);
> +
> + if (ret1 != 0)
> + return ret1;
> + return ret2;
> }
>
> /*
> @@ -438,6 +448,65 @@ int get_vfs_caps_from_disk(const struct dentry *dentry, struct cpu_vfs_cap_data
> return 0;
> }
>
> +int get_vfs_ns_caps_from_disk(const struct dentry *dentry, struct cpu_vfs_cap_data *cpu_caps)
> +{
> + struct inode *inode = d_backing_inode(dentry);
> + unsigned i;
> + u32 magic_etc;
> + ssize_t size;
> + struct vfs_ns_cap_data nscap;
> + bool foundroot = false;
> + struct user_namespace *ns;
> +
> + memset(cpu_caps, 0, sizeof(struct cpu_vfs_cap_data));
> +
> + if (!inode || !inode->i_op->getxattr)
> + return -ENODATA;
> +
> + /* verify that current or ancestor userns root owns this file */
> + for (ns = current_user_ns(); ; ns = ns->parent) {
> + if (from_kuid(ns, dentry->d_inode->i_uid) == 0) {
> + foundroot = true;
> + break;
> + }
> + if (ns == &init_user_ns)
> + break;
> + }
> + if (!foundroot)
> + return -ENODATA;
> +
> + size = inode->i_op->getxattr((struct dentry *)dentry, XATTR_NAME_NS_CAPS,
> + &nscap, sizeof(nscap));
> + if (size == -ENODATA || size == -EOPNOTSUPP)
> + /* no data, that's ok */
> + return -ENODATA;
> + if (size < 0)
> + return size;
> + if (size != sizeof(nscap))
> + return -EINVAL;
> +
> + magic_etc = le32_to_cpu(nscap.magic_etc);
> +
> + if (NS_CAPS_VERSION(magic_etc) != VFS_NS_CAP_REVISION)
> + return -EINVAL;
> +
> + cpu_caps->magic_etc = VFS_CAP_REVISION_2;
> + if (NS_CAPS_FLAGS(magic_etc) & VFS_NS_CAP_EFFECTIVE)
> + cpu_caps->magic_etc |= VFS_CAP_FLAGS_EFFECTIVE;
> + /* copy the entry */
> + CAP_FOR_EACH_U32(i) {
> + if (i >= VFS_CAP_U32_2)
> + break;
> + cpu_caps->permitted.cap[i] = le32_to_cpu(nscap.data[i].permitted);
> + cpu_caps->inheritable.cap[i] = le32_to_cpu(nscap.data[i].inheritable);
> + }
> +
> + cpu_caps->permitted.cap[CAP_LAST_U32] &= CAP_LAST_U32_VALID_MASK;
> + cpu_caps->inheritable.cap[CAP_LAST_U32] &= CAP_LAST_U32_VALID_MASK;
> +
> + return 0;
> +}
> +
> /*
> * Attempt to get the on-exec apply capability sets for an executable file from
> * its xattrs and, if present, apply them to the proposed credentials being
> @@ -456,11 +525,13 @@ static int get_file_caps(struct linux_binprm *bprm, bool *effective, bool *has_c
> if (bprm->file->f_path.mnt->mnt_flags & MNT_NOSUID)
> return 0;
>
> - rc = get_vfs_caps_from_disk(bprm->file->f_path.dentry, &vcaps);
> + rc = get_vfs_ns_caps_from_disk(bprm->file->f_path.dentry, &vcaps);
> + if (rc == -ENODATA)
> + rc = get_vfs_caps_from_disk(bprm->file->f_path.dentry, &vcaps);

So nscaps overrides a "regular" file cap? That might seem worth
mentioning in the change log or somewhere.

> if (rc < 0) {
> if (rc == -EINVAL)
> - printk(KERN_NOTICE "%s: get_vfs_caps_from_disk returned %d for %s\n",
> - __func__, rc, bprm->filename);
> + printk(KERN_NOTICE "Invalid argument reading file caps for %s\n",
> + bprm->filename);
> else if (rc == -ENODATA)
> rc = 0;
> goto out;
> @@ -662,6 +733,12 @@ int cap_inode_setxattr(struct dentry *dentry, const char *name,
> return 0;
> }
>
> + if (!strcmp(name, XATTR_NAME_NS_CAPS)) {
> + if (!capable_wrt_inode_uidgid(dentry->d_inode, CAP_SETFCAP))
> + return -EPERM;
> + return 0;
> + }
> +
> if (!strncmp(name, XATTR_SECURITY_PREFIX,
> sizeof(XATTR_SECURITY_PREFIX) - 1) &&
> !capable(CAP_SYS_ADMIN))
> @@ -688,6 +765,12 @@ int cap_inode_removexattr(struct dentry *dentry, const char *name)
> return 0;
> }
>
> + if (!strcmp(name, XATTR_NAME_NS_CAPS)) {
> + if (!capable_wrt_inode_uidgid(dentry->d_inode, CAP_SETFCAP))
> + return -EPERM;
> + return 0;
> + }
> +

This looks like userspace must knowingly be aware that it is in a
namespace and to DTRT instead of it being translated by the kernel
when setxattr is called under !init_user_ns?

> if (!strncmp(name, XATTR_SECURITY_PREFIX,
> sizeof(XATTR_SECURITY_PREFIX) - 1) &&
> !capable(CAP_SYS_ADMIN))
> --
> 1.7.9.5
>

--
Kees Cook
Chrome OS & Brillo Security