On Fri, 2018-04-13 at 15:52 -0700, Matthew Garrett wrote: > 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. Once EVM is enabled (by writing > to /sys/kernel/security/evm), further writes are blocked. This should > make no difference to security considerations around EVM - even if an > attacker is able to get additional extended attributes added to this > list, it will simply result in existing signatures failing to validate. Up to now, the security.evm HMAC was calculated based on the existing set of protected xattrs, but did not require all of the xattrs to exist. Here, after adding a new xattr to the set of protected xattrs, the HMAC/signature verification of existing security.evm xattrs should continue to validate, just as it currently does with non-existent xattrs. Writing an existing, currently non-existent or new xattrs would cause the signature/HMAC to be re-calculate and written out as an HMAC. Only the new EVM portable and immutable signatures would result in signature verification failures. Although evm_config_xattrnames is not currently defined as a const, it should have been. Including additional xattrs via a securityfs file, limits how the memory for the entire list of xattrs and the pointer to that list can be protected. Does this extra list of xattrs need to be run time or build time configurable? If it's build time configurable you'd be able to use __ro_after_init. For run time configurable, perhaps the proposed "post-init read-only memory" (https://lwn.net/Articles/750215/) could be used. Mimi > > [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@xxxxxxxxxx> > --- > Documentation/ABI/testing/evm | 16 +++++ > security/integrity/evm/evm.h | 3 +- > security/integrity/evm/evm_main.c | 4 +- > security/integrity/evm/evm_secfs.c | 111 +++++++++++++++++++++++++++-- > 4 files changed, 128 insertions(+), 6 deletions(-) > > diff --git a/Documentation/ABI/testing/evm b/Documentation/ABI/testing/evm > index d12cb2eae9ee..5f60c263462c 100644 > --- a/Documentation/ABI/testing/evm > +++ b/Documentation/ABI/testing/evm > @@ -57,3 +57,19 @@ 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@xxxxxxxxxx> > +Description: > + Shows the set of extended attributes used to calculate or > + validate the EVM signature, and allows additional attributes > + to be added at runtime. Adding additional extended attributes > + will result in any existing signatures generated without the > + additional attributes becoming invalid, and any signatures > + generated after additional attributes are added will only be > + valid if the same additional attributes are configured on > + system boot. > + > + This list cannot be modified after EVM has been enabled at > + runtime. > \ No newline at end of file > diff --git a/security/integrity/evm/evm.h b/security/integrity/evm/evm.h > index 45c4a89c02ff..b134358860d2 100644 > --- a/security/integrity/evm/evm.h > +++ b/security/integrity/evm/evm.h > @@ -40,7 +40,8 @@ extern struct crypto_shash *hmac_tfm; > extern struct crypto_shash *hash_tfm; > > /* List of EVM protected security xattrs */ > -extern char *evm_config_xattrnames[]; > +extern char **evm_config_xattrnames; > +extern char **evm_config_extra_xattrnames; > > int evm_init_key(void); > int evm_update_evmxattr(struct dentry *dentry, > diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c > index 9ea9c19a545c..debc8f836a9c 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; > > -char *evm_config_xattrnames[] = { > +char *evm_config_default_xattrnames[] = { > #ifdef CONFIG_SECURITY_SELINUX > XATTR_NAME_SELINUX, > #endif > @@ -56,6 +56,8 @@ char *evm_config_xattrnames[] = { > XATTR_NAME_CAPS, > NULL > }; > +char **evm_config_xattrnames = evm_config_default_xattrnames; > +char **evm_config_extra_xattrnames; > > static int evm_fixmode; > static int __init evm_set_fixmode(char *str) > diff --git a/security/integrity/evm/evm_secfs.c b/security/integrity/evm/evm_secfs.c > index feba03bbedae..2db800076728 100644 > --- a/security/integrity/evm/evm_secfs.c > +++ b/security/integrity/evm/evm_secfs.c > @@ -20,6 +20,7 @@ > #include "evm.h" > > static struct dentry *evm_init_tpm; > +static struct dentry *evm_xattrs; > > /** > * evm_read_key - read() for <securityfs>/evm > @@ -107,13 +108,115 @@ static const struct file_operations evm_key_ops = { > .write = evm_write_key, > }; > > -int __init evm_init_secfs(void) > +/** > + * 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; > + char **xattr; > + int offset = 0; > + ssize_t rc, size = 0; > + > + if (*ppos != 0) > + return 0; > + > + for (xattr = evm_config_xattrnames; *xattr != NULL; xattr++) > + size += strlen(*xattr) + 1; > + > + temp = kmalloc(size + 1, GFP_KERNEL); > + if (!temp) > + return -ENOMEM; > + > + for (xattr = evm_config_xattrnames; *xattr != NULL; xattr++) { > + sprintf(temp + offset, "%s\n", *xattr); > + offset += strlen(*xattr) + 1; > + } > + > + 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 error = 0; > + bool populate = false; > + int len, entries = 0; > + char **xattr, **temp; > + > + if (!capable(CAP_SYS_ADMIN) || (evm_initialized & EVM_SETUP_COMPLETE)) > + return -EPERM; > + > + if (*ppos != 0) > + return -EINVAL; > + > + for (xattr = evm_config_xattrnames; *xattr != NULL; xattr++) > + entries++; > + > + if (!evm_config_extra_xattrnames) > + populate = true; > + > + temp = krealloc(evm_config_extra_xattrnames, > + (entries + 2) * sizeof(char *), > + GFP_KERNEL); > + > + if (!temp) > + return -ENOMEM; > > + evm_config_extra_xattrnames = temp; > + > + if (populate) { > + memcpy(evm_config_extra_xattrnames, evm_config_xattrnames, > + entries * sizeof(char *)); > + evm_config_xattrnames = evm_config_extra_xattrnames; > + } > + > + evm_config_extra_xattrnames[entries] = memdup_user_nul(buf, count); > + evm_config_extra_xattrnames[entries + 1] = NULL; > + > + /* Remove any trailing newline */ > + len = strlen(evm_config_extra_xattrnames[entries]); > + if (evm_config_extra_xattrnames[entries][len-1] == '\n') > + evm_config_extra_xattrnames[entries][len-1] = '\0'; > + return count; > +} > + > +static const struct file_operations evm_xattr_ops = { > + .read = evm_read_xattrs, > + .write = evm_write_xattrs, > +}; > + > +int __init evm_init_secfs(void) > +{ > evm_init_tpm = securityfs_create_file("evm", S_IRUSR | S_IRGRP, > NULL, NULL, &evm_key_ops); > if (!evm_init_tpm || IS_ERR(evm_init_tpm)) > - error = -EFAULT; > - return error; > + return -EFAULT; > + > + evm_xattrs = securityfs_create_file("evm_xattrs", S_IRUSR | S_IRGRP, > + NULL, NULL, &evm_xattr_ops); > + if (!evm_xattrs || IS_ERR(evm_xattrs)) { > + securityfs_remove(evm_init_tpm); > + return -EFAULT; > + } > + > + return 0; > }
