Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753368AbcDZV7e (ORCPT ); Tue, 26 Apr 2016 17:59:34 -0400 Received: from mail-wm0-f50.google.com ([74.125.82.50]:38160 "EHLO mail-wm0-f50.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753192AbcDZV7c (ORCPT ); Tue, 26 Apr 2016 17:59:32 -0400 MIME-Version: 1.0 In-Reply-To: <1461345993-17526-2-git-send-email-serge.hallyn@ubuntu.com> References: <1461345993-17526-1-git-send-email-serge.hallyn@ubuntu.com> <1461345993-17526-2-git-send-email-serge.hallyn@ubuntu.com> Date: Tue, 26 Apr 2016 14:59:30 -0700 X-Google-Sender-Auth: MMPa56JUBfXCIQxnVAEjQixANFI Message-ID: Subject: Re: [PATCH 1/1] simplified security.nscapability xattr From: Kees Cook To: "Serge E. Hallyn" Cc: LKML , Linux API , Linux Containers , "Eric W. Biederman" , "Andrew G. Morgan" , Andy Lutomirski , James Morris Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 9790 Lines: 271 On Fri, Apr 22, 2016 at 10:26 AM, wrote: > From: Serge Hallyn > > 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 > --- > 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 > - > +#include > > #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