From mboxrd@z Thu Jan 1 00:00:00 1970 From: ebiederm@xmission.com (Eric W. Biederman) Subject: Re: [PATCH] Introduce v3 namespaced file capabilities Date: Fri, 21 Apr 2017 16:37:00 -0500 Message-ID: <87a879sarn.fsf@xmission.com> References: <20170419164824.GA27843@mail.hallyn.com> Mime-Version: 1.0 Content-Type: text/plain Return-path: In-Reply-To: <20170419164824.GA27843@mail.hallyn.com> (Serge E. Hallyn's message of "Wed, 19 Apr 2017 11:48:24 -0500") Sender: owner-linux-security-module@vger.kernel.org To: "Serge E. Hallyn" Cc: Seth Forshee , lkml , linux-api@vger.kernel.org, linux-security-module@vger.kernel.org, Kees Cook , Andreas Gruenbacher , Andy Lutomirski , "Andrew G. Morgan" List-Id: linux-api@vger.kernel.org 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" 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 > - > +#include > > #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),