Re: [PATCH] Introduce v3 namespaced file capabilities

[Eric W. Biederman]

Serge,

Is there any change of a Signed-off-by on this patch?  Otherwise I don't
think we can merge it.

Eric

"Serge E. Hallyn" <serge@xxxxxxxxxx> writes:

> Root in a non-initial user ns cannot be trusted to write a traditional
> security.capability xattr.  If it were allowed to do so, then any
> unprivileged user on the host could map his own uid to root in a private
> namespace, write the xattr, and execute the file with privilege on the
> host.
>
> However supporting file capabilities in a user namespace is very
> desirable.  Not doing so means that any programs designed to run with
> limited privilege must continue to support other methods of gaining and
> dropping privilege.  For instance a program installer must detect
> whether file capabilities can be assigned, and assign them if so but set
> setuid-root otherwise.  The program in turn must know how to drop
> partial capabilities, and do so only if setuid-root.
>
> This patch introduces v3 of the security.capability xattr.  It builds a
> vfs_ns_cap_data struct by appending a uid_t rootid to struct
> vfs_cap_data.  This is the absolute uid_t (that is, the uid_t in user
> namespace which mounted the filesystem, usually init_user_ns) of the
> root id in whose namespaces the file capabilities may take effect.
>
> When a task asks to write a v2 security.capability xattr, if it is
> privileged with respect to the userns which mounted the filesystem, then
> nothing should change.  Otherwise, the kernel will transparently rewrite
> the xattr as a v3 with the appropriate rootid.  Subsequently, any task
> executing the file which has the noted kuid as its root uid, or which is
> in a descendent user_ns of such a user_ns, will run the file with
> capabilities.
>
> Similarly when asking to read file capabilities, a v3 capability will
> be presented as v2 if it applies to the caller's namespace.
>
> If a task writes a v3 security.capability, then it can provide a uid for
> the xattr so long as the uid is valid in its own user namespace, and it
> is privileged with CAP_SETFCAP over its namespace.  The kernel will
> translate that rootid to an absolute uid, and write that to disk.  After
> this, a task in the writer's namespace will not be able to use those
> capabilities (unless rootid was 0), but a task in a namespace where the
> given uid is root will.
>
> Only a single security.capability xattr may exist at a time for a given
> file.  A task may overwrite an existing xattr so long as it is
> privileged over the inode.  Note this is a departure from previous
> semantics, which required privilege to remove a security.capability
> xattr.  This check can be re-added if deemed useful.
>
> This allows a simple setcap/setxattr to work, should allow tar to work,
> and should allow us to support tar in one namespace and untar in another
> while preserving the capability, without risking leaking privilege into
> a parent namespace.
>
> A patch to linux-test-project adding a new set of tests for this
> functionality is in the nsfscaps branch at github.com/hallyn/ltp
>
> Changelog:
>    Nov 02 2016: fix invalid check at refuse_fcap_overwrite()
>    Nov 07 2016: convert rootid from and to fs user_ns
>    (From ebiederm: mar 28 2017)
>      commoncap.c: fix typos - s/v4/v3
>      get_vfs_caps_from_disk: clarify the fs_ns root access check
>      nsfscaps: change the code split for cap_inode_setxattr()
>    Apr 09 2017:
>        don't return v3 cap for caps owned by current root.
>       return a v2 cap for a true v2 cap in non-init ns
>    Apr 18 2017:
>       . Change the flow of fscap writing to support s_user_ns writing.
>       . Remove refuse_fcap_overwrite().  The value of the previous
>         xattr doesn't matter.
> ---
>  fs/xattr.c                      |  30 ++++-
>  include/linux/capability.h      |   5 +-
>  include/linux/security.h        |   2 +
>  include/uapi/linux/capability.h |  22 +++-
>  security/commoncap.c            | 237 ++++++++++++++++++++++++++++++++++++----
>  5 files changed, 268 insertions(+), 28 deletions(-)
>
> diff --git a/fs/xattr.c b/fs/xattr.c
> index 7e3317c..75cc65a 100644
> --- a/fs/xattr.c
> +++ b/fs/xattr.c
> @@ -170,12 +170,29 @@ int __vfs_setxattr_noperm(struct dentry *dentry, const char *name,
>  		const void *value, size_t size, int flags)
>  {
>  	struct inode *inode = dentry->d_inode;
> -	int error = -EAGAIN;
> +	int error;
> +	void *wvalue = NULL;
> +	size_t wsize = 0;
>  	int issec = !strncmp(name, XATTR_SECURITY_PREFIX,
>  				   XATTR_SECURITY_PREFIX_LEN);
>  
> -	if (issec)
> +	if (issec) {
>  		inode->i_flags &= ~S_NOSEC;
> +
> +		if (!strcmp(name, "security.capability")) {
> +			error = cap_setxattr_convert_nscap(dentry, value, size,
> +					&wvalue, &wsize);
> +			if (error < 0)
> +				return error;
> +			if (wvalue) {
> +				value = wvalue;
> +				size = wsize;
> +			}
> +		}
> +	}
> +
> +	error = -EAGAIN;
> +
>  	if (inode->i_opflags & IOP_XATTR) {
>  		error = __vfs_setxattr(dentry, inode, name, value, size, flags);
>  		if (!error) {
> @@ -184,8 +201,10 @@ int __vfs_setxattr_noperm(struct dentry *dentry, const char *name,
>  						     size, flags);
>  		}
>  	} else {
> -		if (unlikely(is_bad_inode(inode)))
> -			return -EIO;
> +		if (unlikely(is_bad_inode(inode))) {
> +			error = -EIO;
> +			goto out;
> +		}
>  	}
>  	if (error == -EAGAIN) {
>  		error = -EOPNOTSUPP;
> @@ -200,10 +219,11 @@ int __vfs_setxattr_noperm(struct dentry *dentry, const char *name,
>  		}
>  	}
>  
> +out:
> +	kfree(wvalue);
>  	return error;
>  }
>  
> -
>  int
>  vfs_setxattr(struct dentry *dentry, const char *name, const void *value,
>  		size_t size, int flags)
> diff --git a/include/linux/capability.h b/include/linux/capability.h
> index 6ffb67e..b973433 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
> @@ -248,4 +248,7 @@ extern bool ptracer_capable(struct task_struct *tsk, struct user_namespace *ns);
>  /* audit system wants to get cap info from files as well */
>  extern int get_vfs_caps_from_disk(const struct dentry *dentry, struct cpu_vfs_cap_data *cpu_caps);
>  
> +extern int cap_setxattr_convert_nscap(struct dentry *dentry, const void *value,
> +		size_t size, void **wvalue, size_t *wsize);
> +
>  #endif /* !_LINUX_CAPABILITY_H */
> diff --git a/include/linux/security.h b/include/linux/security.h
> index 96899fa..bd49cc1 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -86,6 +86,8 @@ extern int cap_inode_setxattr(struct dentry *dentry, const char *name,
>  extern int cap_inode_removexattr(struct dentry *dentry, const char *name);
>  extern int cap_inode_need_killpriv(struct dentry *dentry);
>  extern int cap_inode_killpriv(struct dentry *dentry);
> +extern int cap_inode_getsecurity(struct inode *inode, const char *name,
> +				 void **buffer, bool alloc);
>  extern int cap_mmap_addr(unsigned long addr);
>  extern int cap_mmap_file(struct file *file, unsigned long reqprot,
>  			 unsigned long prot, unsigned long flags);
> diff --git a/include/uapi/linux/capability.h b/include/uapi/linux/capability.h
> index 49bc062..fd4f87d 100644
> --- a/include/uapi/linux/capability.h
> +++ b/include/uapi/linux/capability.h
> @@ -60,9 +60,13 @@ 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))
>  
> -#define XATTR_CAPS_SZ           XATTR_CAPS_SZ_2
> -#define VFS_CAP_U32             VFS_CAP_U32_2
> -#define VFS_CAP_REVISION	VFS_CAP_REVISION_2
> +#define VFS_CAP_REVISION_3	0x03000000
> +#define VFS_CAP_U32_3           2
> +#define XATTR_CAPS_SZ_3         (sizeof(__le32)*(2 + 2*VFS_CAP_U32_3))
> +
> +#define XATTR_CAPS_SZ           XATTR_CAPS_SZ_3
> +#define VFS_CAP_U32             VFS_CAP_U32_3
> +#define VFS_CAP_REVISION	VFS_CAP_REVISION_3
>  
>  struct vfs_cap_data {
>  	__le32 magic_etc;            /* Little endian */
> @@ -72,6 +76,18 @@ struct vfs_cap_data {
>  	} data[VFS_CAP_U32];
>  };
>  
> +/*
> + * same as vfs_cap_data but with a rootid at the end
> + */
> +struct vfs_ns_cap_data {
> +	__le32 magic_etc;
> +	struct {
> +		__le32 permitted;    /* Little endian */
> +		__le32 inheritable;  /* Little endian */
> +	} data[VFS_CAP_U32];
> +	__le32 rootid;
> +};
> +
>  #ifndef __KERNEL__
>  
>  /*
> diff --git a/security/commoncap.c b/security/commoncap.c
> index 78b3783..8abb9bf 100644
> --- a/security/commoncap.c
> +++ b/security/commoncap.c
> @@ -332,6 +332,179 @@ int cap_inode_killpriv(struct dentry *dentry)
>  	return error;
>  }
>  
> +static bool rootid_owns_currentns(kuid_t kroot)
> +{
> +	struct user_namespace *ns;
> +
> +	if (!uid_valid(kroot))
> +		return false;
> +
> +	for (ns = current_user_ns(); ; ns = ns->parent) {
> +		if (from_kuid(ns, kroot) == 0)
> +			return true;
> +		if (ns == &init_user_ns)
> +			break;
> +	}
> +
> +	return false;
> +}
> +
> +/*
> + * getsecurity: We are called for security.* before any attempt to read the
> + * xattr from the inode itself.
> + *
> + * This gives us a chance to read the on-disk value and convert it.  If we
> + * return -EOPNOTSUPP, then vfs_getxattr() will call the i_op handler.
> + *
> + * Note we are not called by vfs_getxattr_alloc(), but that is only called
> + * by the integrity subsystem, which really wants the unconverted values -
> + * so that's good.
> + */
> +int cap_inode_getsecurity(struct inode *inode, const char *name, void **buffer,
> +			  bool alloc)
> +{
> +	int size, ret;
> +	kuid_t kroot;
> +	uid_t root, mappedroot;
> +	char *tmpbuf = NULL;
> +	struct vfs_ns_cap_data *nscap;
> +	struct dentry *dentry;
> +	struct user_namespace *fs_ns;
> +
> +	if (strcmp(name, "capability") != 0)
> +		return -EOPNOTSUPP;
> +
> +	dentry = d_find_alias(inode);
> +	if (!dentry)
> +		return -EINVAL;
> +
> +	size = sizeof(struct vfs_ns_cap_data);
> +	ret = vfs_getxattr_alloc(dentry, "security.capability",
> +				 &tmpbuf, size, GFP_NOFS);
> +
> +	if (ret < 0)
> +		return ret;
> +
> +	fs_ns = inode->i_sb->s_user_ns;
> +	if (ret == sizeof(struct vfs_cap_data)) {
> +		/* If this is sizeof(vfs_cap_data) then we're ok with the
> +		 * on-disk value, so return that.  */
> +		if (alloc)
> +			*buffer = tmpbuf;
> +		else
> +			kfree(tmpbuf);
> +		return ret;
> +	} else if (ret != sizeof(struct vfs_ns_cap_data)) {
> +		kfree(tmpbuf);
> +		return -EINVAL;
> +	}
> +
> +	nscap = (struct vfs_ns_cap_data *) tmpbuf;
> +	root = le32_to_cpu(nscap->rootid);
> +	kroot = make_kuid(fs_ns, root);
> +
> +	/* If the root kuid maps to a valid uid in current ns, then return
> +	 * this as a nscap. */
> +	mappedroot = from_kuid(current_user_ns(), kroot);
> +	if (mappedroot != (uid_t)-1 && mappedroot != (uid_t)0) {
> +		if (alloc) {
> +			*buffer = tmpbuf;
> +			nscap->rootid = cpu_to_le32(mappedroot);
> +		} else
> +			kfree(tmpbuf);
> +		return size;
> +	}
> +
> +	if (!rootid_owns_currentns(kroot)) {
> +		kfree(tmpbuf);
> +		return -EOPNOTSUPP;
> +	}
> +
> +	/* This comes from a parent namespace.  Return as a v2 capability */
> +	size = sizeof(struct vfs_cap_data);
> +	if (alloc) {
> +		*buffer = kmalloc(size, GFP_ATOMIC);
> +		if (*buffer) {
> +			struct vfs_cap_data *cap = *buffer;
> +			__le32 nsmagic, magic;
> +			magic = VFS_CAP_REVISION_2;
> +			nsmagic = le32_to_cpu(nscap->magic_etc);
> +			if (nsmagic & VFS_CAP_FLAGS_EFFECTIVE)
> +				magic |= VFS_CAP_FLAGS_EFFECTIVE;
> +			memcpy(&cap->data, &nscap->data, sizeof(__le32) * 2 * VFS_CAP_U32);
> +			cap->magic_etc = cpu_to_le32(magic);
> +		}
> +	}
> +	kfree(tmpbuf);
> +	return size;
> +}
> +
> +static kuid_t rootid_from_xattr(const void *value, size_t size,
> +				struct user_namespace *task_ns)
> +{
> +	const struct vfs_ns_cap_data *nscap = value;
> +	uid_t rootid = 0;
> +
> +	if (size == XATTR_CAPS_SZ_3)
> +		rootid = le32_to_cpu(nscap->rootid);
> +
> +	return make_kuid(task_ns, rootid);
> +}
> +
> +/*
> + * User requested a write of security.capability.
> + *
> + * If all is ok, we return 0.  If the capability needs to be converted,
> + * wvalue will be allocated (and needs to be freed) with the new value.
> + * On error, return < 0.
> + */
> +int cap_setxattr_convert_nscap(struct dentry *dentry, const void *value, size_t size,
> +				    void **wvalue, size_t *wsize)
> +{
> +	struct vfs_ns_cap_data *nscap;
> +	uid_t nsrootid;
> +	const struct vfs_cap_data *cap = value;
> +	__u32 magic, nsmagic;
> +	struct inode *inode = d_backing_inode(dentry);
> +	struct user_namespace *task_ns = current_user_ns(),
> +		*fs_ns = inode->i_sb->s_user_ns;
> +	kuid_t rootid;
> +
> +	if (!value)
> +		return -EINVAL;
> +	if (size != XATTR_CAPS_SZ_2 && size != XATTR_CAPS_SZ_3)
> +		return -EINVAL;
> +	if (!capable_wrt_inode_uidgid(inode, CAP_SETFCAP))
> +		return -EPERM;
> +	if (size == XATTR_CAPS_SZ_2)
> +		if (ns_capable(inode->i_sb->s_user_ns, CAP_SETFCAP))
> +			// user is privileged, just write the v2
> +			return 0;
> +
> +	rootid = rootid_from_xattr(value, size, task_ns);
> +	if (!uid_valid(rootid))
> +		return -EINVAL;
> +
> +	nsrootid = from_kuid(fs_ns, rootid);
> +	if (nsrootid == -1)
> +		return -EINVAL;
> +
> +	*wsize = sizeof(struct vfs_ns_cap_data);
> +	nscap = kmalloc(*wsize, GFP_ATOMIC);
> +	if (!nscap)
> +		return -ENOMEM;
> +	nscap->rootid = cpu_to_le32(nsrootid);
> +	nsmagic = VFS_CAP_REVISION_3;
> +	magic = le32_to_cpu(cap->magic_etc);
> +	if (magic & VFS_CAP_FLAGS_EFFECTIVE)
> +		nsmagic |= VFS_CAP_FLAGS_EFFECTIVE;
> +	nscap->magic_etc = cpu_to_le32(nsmagic);
> +	memcpy(&nscap->data, &cap->data, sizeof(__le32) * 2 * VFS_CAP_U32);
> +
> +	*wvalue = nscap;
> +	return 0;
> +}
> +
>  /*
>   * Calculate the new process capability sets from the capability sets attached
>   * to a file.
> @@ -385,7 +558,10 @@ int get_vfs_caps_from_disk(const struct dentry *dentry, struct cpu_vfs_cap_data
>  	__u32 magic_etc;
>  	unsigned tocopy, i;
>  	int size;
> -	struct vfs_cap_data caps;
> +	struct vfs_ns_cap_data data, *nscaps = &data;
> +	struct vfs_cap_data *caps = (struct vfs_cap_data *) &data;
> +	kuid_t rootkuid;
> +	struct user_namespace *fs_ns = inode->i_sb->s_user_ns;
>  
>  	memset(cpu_caps, 0, sizeof(struct cpu_vfs_cap_data));
>  
> @@ -393,18 +569,20 @@ int get_vfs_caps_from_disk(const struct dentry *dentry, struct cpu_vfs_cap_data
>  		return -ENODATA;
>  
>  	size = __vfs_getxattr((struct dentry *)dentry, inode,
> -			      XATTR_NAME_CAPS, &caps, XATTR_CAPS_SZ);
> +			      XATTR_NAME_CAPS, &data, XATTR_CAPS_SZ);
>  	if (size == -ENODATA || size == -EOPNOTSUPP)
>  		/* no data, that's ok */
>  		return -ENODATA;
> +
>  	if (size < 0)
>  		return size;
>  
>  	if (size < sizeof(magic_etc))
>  		return -EINVAL;
>  
> -	cpu_caps->magic_etc = magic_etc = le32_to_cpu(caps.magic_etc);
> +	cpu_caps->magic_etc = magic_etc = le32_to_cpu(caps->magic_etc);
>  
> +	rootkuid = make_kuid(fs_ns, 0);
>  	switch (magic_etc & VFS_CAP_REVISION_MASK) {
>  	case VFS_CAP_REVISION_1:
>  		if (size != XATTR_CAPS_SZ_1)
> @@ -416,15 +594,27 @@ int get_vfs_caps_from_disk(const struct dentry *dentry, struct cpu_vfs_cap_data
>  			return -EINVAL;
>  		tocopy = VFS_CAP_U32_2;
>  		break;
> +	case VFS_CAP_REVISION_3:
> +		if (size != XATTR_CAPS_SZ_3)
> +			return -EINVAL;
> +		tocopy = VFS_CAP_U32_3;
> +		rootkuid = make_kuid(fs_ns, le32_to_cpu(nscaps->rootid));
> +		break;
> +
>  	default:
>  		return -EINVAL;
>  	}
> +	/* Limit the caps to the mounter of the filesystem
> +	 * or the more limited uid specified in the xattr.
> +	 */
> +	if (!rootid_owns_currentns(rootkuid))
> +		return -ENODATA;
>  
>  	CAP_FOR_EACH_U32(i) {
>  		if (i >= tocopy)
>  			break;
> -		cpu_caps->permitted.cap[i] = le32_to_cpu(caps.data[i].permitted);
> -		cpu_caps->inheritable.cap[i] = le32_to_cpu(caps.data[i].inheritable);
> +		cpu_caps->permitted.cap[i] = le32_to_cpu(caps->data[i].permitted);
> +		cpu_caps->inheritable.cap[i] = le32_to_cpu(caps->data[i].inheritable);
>  	}
>  
>  	cpu_caps->permitted.cap[CAP_LAST_U32] &= CAP_LAST_U32_VALID_MASK;
> @@ -462,8 +652,8 @@ static int get_file_caps(struct linux_binprm *bprm, bool *effective, bool *has_c
>  	rc = get_vfs_caps_from_disk(bprm->file->f_path.dentry, &vcaps);
>  	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;
> @@ -660,15 +850,16 @@ int cap_bprm_secureexec(struct linux_binprm *bprm)
>  int cap_inode_setxattr(struct dentry *dentry, const char *name,
>  		       const void *value, size_t size, int flags)
>  {
> -	if (!strcmp(name, XATTR_NAME_CAPS)) {
> -		if (!capable(CAP_SETFCAP))
> -			return -EPERM;
> +	/* Ignore non-security xattrs */
> +	if (strncmp(name, XATTR_SECURITY_PREFIX,
> +			sizeof(XATTR_SECURITY_PREFIX) - 1) != 0)
> +		return 0;
> +
> +	// For XATTR_NAME_CAPS the check will be done in __vfs_setxattr_noperm()
> +	if (strcmp(name, XATTR_NAME_CAPS) == 0)
>  		return 0;
> -	}
>  
> -	if (!strncmp(name, XATTR_SECURITY_PREFIX,
> -		     sizeof(XATTR_SECURITY_PREFIX) - 1) &&
> -	    !capable(CAP_SYS_ADMIN))
> +	if (!capable(CAP_SYS_ADMIN))
>  		return -EPERM;
>  	return 0;
>  }
> @@ -686,15 +877,22 @@ int cap_inode_setxattr(struct dentry *dentry, const char *name,
>   */
>  int cap_inode_removexattr(struct dentry *dentry, const char *name)
>  {
> -	if (!strcmp(name, XATTR_NAME_CAPS)) {
> -		if (!capable(CAP_SETFCAP))
> +	/* Ignore non-security xattrs */
> +	if (strncmp(name, XATTR_SECURITY_PREFIX,
> +			sizeof(XATTR_SECURITY_PREFIX) - 1) != 0)
> +		return 0;
> +
> +	if (strcmp(name, XATTR_NAME_CAPS) == 0) {
> +		/* security.capability gets namespaced */
> +		struct inode *inode = d_backing_inode(dentry);
> +		if (!inode)
> +			return -EINVAL;
> +		if (!capable_wrt_inode_uidgid(inode, CAP_SETFCAP))
>  			return -EPERM;
>  		return 0;
>  	}
>  
> -	if (!strncmp(name, XATTR_SECURITY_PREFIX,
> -		     sizeof(XATTR_SECURITY_PREFIX) - 1) &&
> -	    !capable(CAP_SYS_ADMIN))
> +	if (!capable(CAP_SYS_ADMIN))
>  		return -EPERM;
>  	return 0;
>  }
> @@ -1082,6 +1280,7 @@ struct security_hook_list capability_hooks[] = {
>  	LSM_HOOK_INIT(bprm_secureexec, cap_bprm_secureexec),
>  	LSM_HOOK_INIT(inode_need_killpriv, cap_inode_need_killpriv),
>  	LSM_HOOK_INIT(inode_killpriv, cap_inode_killpriv),
> +	LSM_HOOK_INIT(inode_getsecurity, cap_inode_getsecurity),
>  	LSM_HOOK_INIT(mmap_addr, cap_mmap_addr),
>  	LSM_HOOK_INIT(mmap_file, cap_mmap_file),
>  	LSM_HOOK_INIT(task_fix_setuid, cap_task_fix_setuid),