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" <se...@hallyn.com> 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),
