| Message ID | 20180509202811.29875-2-mjg59@google.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On Wed, 9 May 2018, Matthew Garrett wrote: > Signed-off-by: Matthew Garrett <mjg59@google.com> > --- > > Updated to add support for locking down the list, and also to forbid adding > the same xattr twice. > > Documentation/ABI/testing/evm | 13 +++ > security/integrity/evm/Kconfig | 11 ++ > security/integrity/evm/evm_crypto.c | 2 +- > security/integrity/evm/evm_main.c | 6 +- > security/integrity/evm/evm_secfs.c | 153 ++++++++++++++++++++++++++-- > 5 files changed, 175 insertions(+), 10 deletions(-) Reviewed-by: James Morris <james.morris@microsoft.com>
On Wed, 2018-05-09 at 13:28 -0700, Matthew Garrett wrote: > +/** > + * evm_write_xattrs - write() for <securityfs>/evm_xattrs > + * @file: file pointer, not actually used > + * @buf: where to get the data from > + * @count: bytes sent > + * @ppos: where to start > + * > + * Returns number of bytes written or error code, as appropriate > + */ > +static ssize_t evm_write_xattrs(struct file *file, const char __user *buf, > + size_t count, loff_t *ppos) > +{ > + int len, err; > + struct xattr_list *xattr, *tmp; > + > + if (!capable(CAP_SYS_ADMIN) || evm_xattrs_locked) > + return -EPERM; > + > + if (*ppos != 0) > + return -EINVAL; > + > + if (count > XATTR_NAME_MAX) > + return -E2BIG; > + > + xattr = kmalloc(sizeof(struct xattr_list), GFP_KERNEL); > + if (!xattr) > + return -ENOMEM; > + > + xattr->name = memdup_user_nul(buf, count); Up to now, the set of protected EVM xattrs was in the security domain. The current code permits any string in any domain. If that is the intention, there needs to be an explanation of the security implications of this change at least in the patch description. > + if (IS_ERR(xattr->name)) { > + err = PTR_ERR(xattr->name); > + kfree(xattr); > + return err; > + } > + > + /* Remove any trailing newline */ > + len = strlen(xattr->name); > + if (xattr->name[len-1] == '\n') > + xattr->name[len-1] = '\0'; > + > + if (strcmp(xattr->name, ".") == 0) { > + evm_xattrs_locked = 1; > + err = count; Please update the file mode bits of <securityfs>/evm_xattrs. > + goto out; > + } > + > + /* Guard against races in evm_read_xattrs */ > + mutex_lock(&xattr_list_mutex); > + list_for_each_entry(tmp, &evm_config_xattrnames, list) { > + if (strcmp(xattr->name, tmp->name) == 0) { > + err = -EEXIST; > + mutex_unlock(&xattr_list_mutex); > + goto out; > + } > + } > + list_add_tail_rcu(&xattr->list, &evm_config_xattrnames); > + mutex_unlock(&xattr_list_mutex); > + > + return count; > +out: > + kfree(xattr->name); > + kfree(xattr); > + return err; > +} > + > +static const struct file_operations evm_xattr_ops = { > + .read = evm_read_xattrs, > + .write = evm_write_xattrs, > +}; > + > +static int evm_init_xattrs(void) > +{ > + evm_xattrs = securityfs_create_file("evm_xattrs", 0440, NULL, NULL, > + &evm_xattr_ops); The mode bits should reflect the status of the current status of evm_xattrs. Initially it would be writeable, but later it might change to read-only. Should "evm_xattrs" be defined directly in the securityfs directory or in a subdirectory similar to ima? It will be difficult later on to move "evm_xattrs" to a subdirectory once applications start reading/writing to it. What would the subdirectory be called? Mimi
On Wed, 2018-05-09 at 13:28 -0700, Matthew Garrett wrote: > +/** > + * evm_write_xattrs - write() for <securityfs>/evm_xattrs > + * @file: file pointer, not actually used > + * @buf: where to get the data from > + * @count: bytes sent > + * @ppos: where to start > + * > + * Returns number of bytes written or error code, as appropriate > + */ > +static ssize_t evm_write_xattrs(struct file *file, const char __user *buf, > + size_t count, loff_t *ppos) > +{ > + int len, err; > + struct xattr_list *xattr, *tmp; > + > + if (!capable(CAP_SYS_ADMIN) || evm_xattrs_locked) > + return -EPERM; > + > + if (*ppos != 0) > + return -EINVAL; > + > + if (count > XATTR_NAME_MAX) > + return -E2BIG; > + > + xattr = kmalloc(sizeof(struct xattr_list), GFP_KERNEL); > + if (!xattr) > + return -ENOMEM; > + > + xattr->name = memdup_user_nul(buf, count); > + if (IS_ERR(xattr->name)) { > + err = PTR_ERR(xattr->name); > + kfree(xattr); > + return err; > + } > + > + /* Remove any trailing newline */ > + len = strlen(xattr->name); > + if (xattr->name[len-1] == '\n') > + xattr->name[len-1] = '\0'; > + > + if (strcmp(xattr->name, ".") == 0) { > + evm_xattrs_locked = 1; > + err = count; > + goto out; > + } > + > + /* Guard against races in evm_read_xattrs */ > + mutex_lock(&xattr_list_mutex); > + list_for_each_entry(tmp, &evm_config_xattrnames, list) { > + if (strcmp(xattr->name, tmp->name) == 0) { > + err = -EEXIST; > + mutex_unlock(&xattr_list_mutex); > + goto out; > + } > + } > + list_add_tail_rcu(&xattr->list, &evm_config_xattrnames); > + mutex_unlock(&xattr_list_mutex); > + > + return count; > +out: > + kfree(xattr->name); > + kfree(xattr); > + return err; > +} Shouldn't new xattrs be audited/logged? Mimi
On Thu, May 10, 2018 at 9:42 PM Mimi Zohar <zohar@linux.vnet.ibm.com> wrote: > On Wed, 2018-05-09 at 13:28 -0700, Matthew Garrett wrote: > > + xattr->name = memdup_user_nul(buf, count); > Up to now, the set of protected EVM xattrs was in the security domain. > The current code permits any string in any domain. If that is the > intention, there needs to be an explanation of the security > implications of this change at least in the patch description. Restricting it to security makes sense for now - if it turns out that it seems useful to use other attributes then we can talk about changing that later. > > + if (strcmp(xattr->name, ".") == 0) { > > + evm_xattrs_locked = 1; > > + err = count; > Please update the file mode bits of <securityfs>/evm_xattrs. Ok. > Should "evm_xattrs" be defined directly in the securityfs directory or > in a subdirectory similar to ima? It will be difficult later on to > move "evm_xattrs" to a subdirectory once applications start > reading/writing to it. What would the subdirectory be called? Yeah, that's tricky - the obvious directory would be evm, but that's already in use. integrity makes sense, but then ima should also be under it.
On Fri, 2018-05-11 at 12:51 -0700, Matthew Garrett wrote: > > Should "evm_xattrs" be defined directly in the securityfs directory or > > in a subdirectory similar to ima? It will be difficult later on to > > move "evm_xattrs" to a subdirectory once applications start > > reading/writing to it. What would the subdirectory be called? > > Yeah, that's tricky - the obvious directory would be evm, but that's > already in use. integrity makes sense, but then ima should also be under it. I haven't tried, but maybe we could use softlinks (eg. <securityfs>/ima ==> <securityfs>/integrity/ima)? And then there are namespaces ... Mimi
diff --git a/Documentation/ABI/testing/evm b/Documentation/ABI/testing/evm index d12cb2eae9ee..fa31df7fd30b 100644 --- a/Documentation/ABI/testing/evm +++ b/Documentation/ABI/testing/evm @@ -57,3 +57,16 @@ Description: dracut (via 97masterkey and 98integrity) and systemd (via core/ima-setup) have support for loading keys at boot time. + +What: security/evm_xattrs +Date: April 2018 +Contact: Matthew Garrett <mjg59@google.com> +Description: + Shows the set of extended attributes used to calculate or + validate the EVM signature, and allows additional attributes + to be added at runtime. Any signatures generated after + additional attributes are added (and on files posessing those + additional attributes) will only be valid if the same + additional attributes are configured on system boot. Writing + a single period (.) will lock the xattr list from any further + modification. diff --git a/security/integrity/evm/Kconfig b/security/integrity/evm/Kconfig index e825e0ae78e7..54adb3f9ad1d 100644 --- a/security/integrity/evm/Kconfig +++ b/security/integrity/evm/Kconfig @@ -42,6 +42,17 @@ config EVM_EXTRA_SMACK_XATTRS additional info to the calculation, requires existing EVM labeled file systems to be relabeled. +config EVM_ADD_XATTRS + bool "Add additional EVM extended attributes at runtime" + depends on EVM + default n + help + Allow userland to provide additional xattrs for HMAC calculation. + + When this option is enabled, root can add additional xattrs to the + list used by EVM by writing them into + /sys/kernel/security/evm_xattrs. + config EVM_LOAD_X509 bool "Load an X509 certificate onto the '.evm' trusted keyring" depends on EVM && INTEGRITY_TRUSTED_KEYRING diff --git a/security/integrity/evm/evm_crypto.c b/security/integrity/evm/evm_crypto.c index caeea20670cc..494da5fcc092 100644 --- a/security/integrity/evm/evm_crypto.c +++ b/security/integrity/evm/evm_crypto.c @@ -208,7 +208,7 @@ static int evm_calc_hmac_or_hash(struct dentry *dentry, return PTR_ERR(desc); error = -ENODATA; - list_for_each_entry(xattr, &evm_config_xattrnames, list) { + list_for_each_entry_rcu(xattr, &evm_config_xattrnames, list) { bool is_ima = false; if (strcmp(xattr->name, XATTR_NAME_IMA) == 0) diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c index dd2415c55982..f049af2cc037 100644 --- a/security/integrity/evm/evm_main.c +++ b/security/integrity/evm/evm_main.c @@ -35,7 +35,7 @@ static const char * const integrity_status_msg[] = { }; int evm_hmac_attrs; -static struct xattr_list evm_config_default_xattrnames[] __ro_after_init = { +static struct xattr_list evm_config_default_xattrnames[] = { #ifdef CONFIG_SECURITY_SELINUX {.name = XATTR_NAME_SELINUX}, #endif @@ -98,7 +98,7 @@ static int evm_find_protected_xattrs(struct dentry *dentry) if (!(inode->i_opflags & IOP_XATTR)) return -EOPNOTSUPP; - list_for_each_entry(xattr, &evm_config_xattrnames, list) { + list_for_each_entry_rcu(xattr, &evm_config_xattrnames, list) { error = __vfs_getxattr(dentry, inode, xattr->name, NULL, 0); if (error < 0) { if (error == -ENODATA) @@ -225,7 +225,7 @@ static int evm_protected_xattr(const char *req_xattr_name) struct xattr_list *xattr; namelen = strlen(req_xattr_name); - list_for_each_entry(xattr, &evm_config_xattrnames, list) { + list_for_each_entry_rcu(xattr, &evm_config_xattrnames, list) { if ((strlen(xattr->name) == namelen) && (strncmp(req_xattr_name, xattr->name, namelen) == 0)) { found = 1; diff --git a/security/integrity/evm/evm_secfs.c b/security/integrity/evm/evm_secfs.c index feba03bbedae..8dc11c0ac8a0 100644 --- a/security/integrity/evm/evm_secfs.c +++ b/security/integrity/evm/evm_secfs.c @@ -17,10 +17,17 @@ #include <linux/uaccess.h> #include <linux/module.h> +#include <linux/mutex.h> #include "evm.h" static struct dentry *evm_init_tpm; +#ifdef CONFIG_EVM_ADD_XATTRS +static struct dentry *evm_xattrs; +static DEFINE_MUTEX(xattr_list_mutex); +static int evm_xattrs_locked; +#endif + /** * evm_read_key - read() for <securityfs>/evm * @@ -107,13 +114,147 @@ static const struct file_operations evm_key_ops = { .write = evm_write_key, }; -int __init evm_init_secfs(void) +#ifdef CONFIG_EVM_ADD_XATTRS +/** + * evm_read_xattrs - read() for <securityfs>/evm_xattrs + * + * @filp: file pointer, not actually used + * @buf: where to put the result + * @count: maximum to send along + * @ppos: where to start + * + * Returns number of bytes read or error code, as appropriate + */ +static ssize_t evm_read_xattrs(struct file *filp, char __user *buf, + size_t count, loff_t *ppos) +{ + char *temp; + int offset = 0; + ssize_t rc, size = 0; + struct xattr_list *xattr; + + if (*ppos != 0) + return 0; + + rc = mutex_lock_interruptible(&xattr_list_mutex); + if (rc) + return -ERESTARTSYS; + + list_for_each_entry(xattr, &evm_config_xattrnames, list) + size += strlen(xattr->name) + 1; + + temp = kmalloc(size + 1, GFP_KERNEL); + if (!temp) + return -ENOMEM; + + list_for_each_entry(xattr, &evm_config_xattrnames, list) { + sprintf(temp + offset, "%s\n", xattr->name); + offset += strlen(xattr->name) + 1; + } + + mutex_unlock(&xattr_list_mutex); + rc = simple_read_from_buffer(buf, count, ppos, temp, strlen(temp)); + + return rc; +} + +/** + * evm_write_xattrs - write() for <securityfs>/evm_xattrs + * @file: file pointer, not actually used + * @buf: where to get the data from + * @count: bytes sent + * @ppos: where to start + * + * Returns number of bytes written or error code, as appropriate + */ +static ssize_t evm_write_xattrs(struct file *file, const char __user *buf, + size_t count, loff_t *ppos) +{ + int len, err; + struct xattr_list *xattr, *tmp; + + if (!capable(CAP_SYS_ADMIN) || evm_xattrs_locked) + return -EPERM; + + if (*ppos != 0) + return -EINVAL; + + if (count > XATTR_NAME_MAX) + return -E2BIG; + + xattr = kmalloc(sizeof(struct xattr_list), GFP_KERNEL); + if (!xattr) + return -ENOMEM; + + xattr->name = memdup_user_nul(buf, count); + if (IS_ERR(xattr->name)) { + err = PTR_ERR(xattr->name); + kfree(xattr); + return err; + } + + /* Remove any trailing newline */ + len = strlen(xattr->name); + if (xattr->name[len-1] == '\n') + xattr->name[len-1] = '\0'; + + if (strcmp(xattr->name, ".") == 0) { + evm_xattrs_locked = 1; + err = count; + goto out; + } + + /* Guard against races in evm_read_xattrs */ + mutex_lock(&xattr_list_mutex); + list_for_each_entry(tmp, &evm_config_xattrnames, list) { + if (strcmp(xattr->name, tmp->name) == 0) { + err = -EEXIST; + mutex_unlock(&xattr_list_mutex); + goto out; + } + } + list_add_tail_rcu(&xattr->list, &evm_config_xattrnames); + mutex_unlock(&xattr_list_mutex); + + return count; +out: + kfree(xattr->name); + kfree(xattr); + return err; +} + +static const struct file_operations evm_xattr_ops = { + .read = evm_read_xattrs, + .write = evm_write_xattrs, +}; + +static int evm_init_xattrs(void) +{ + evm_xattrs = securityfs_create_file("evm_xattrs", 0440, NULL, NULL, + &evm_xattr_ops); + if (!evm_xattrs || IS_ERR(evm_xattrs)) + return -EFAULT; + + return 0; +} +#else +static int evm_init_xattrs(void) { - int error = 0; + return 0; +} +#endif - evm_init_tpm = securityfs_create_file("evm", S_IRUSR | S_IRGRP, - NULL, NULL, &evm_key_ops); +int __init evm_init_secfs(void) +{ + evm_init_tpm = securityfs_create_file("evm", 0440, NULL, NULL, + &evm_key_ops); if (!evm_init_tpm || IS_ERR(evm_init_tpm)) - error = -EFAULT; - return error; + return -EFAULT; + + if (evm_init_xattrs()) { + securityfs_remove(evm_init_tpm); + return -EFAULT; + } + + return 0; }
Sites may wish to provide additional metadata alongside files in order to make more fine-grained security decisions[1]. The security of this is enhanced if this metadata is protected, something that EVM makes possible. However, the kernel cannot know about the set of extended attributes that local admins may wish to protect, and hardcoding this policy in the kernel makes it difficult to change over time and less convenient for distributions to enable. This patch adds a new /sys/kernel/security/evm_xattrs node, which can be read to obtain the current set of EVM-protected extended attributes or written to in order to add new entries. Extending this list will not change the validity of any existing signatures provided that the file in question does not have any of the additional extended attributes - missing xattrs are skipped when calculating the EVM hash. [1] For instance, a package manager could install information about the package uploader in an additional extended attribute. Local LSM policy could then be associated with that extended attribute in order to restrict the privileges available to packages from less trusted uploaders. Signed-off-by: Matthew Garrett <mjg59@google.com> --- Updated to add support for locking down the list, and also to forbid adding the same xattr twice. Documentation/ABI/testing/evm | 13 +++ security/integrity/evm/Kconfig | 11 ++ security/integrity/evm/evm_crypto.c | 2 +- security/integrity/evm/evm_main.c | 6 +- security/integrity/evm/evm_secfs.c | 153 ++++++++++++++++++++++++++-- 5 files changed, 175 insertions(+), 10 deletions(-)