From 66dbc325afcef909043c30e90930a36823fc734c Mon Sep 17 00:00:00 2001 From: Mimi Zohar Date: Tue, 15 Mar 2011 16:12:09 -0400 Subject: evm: re-release EVM protects a file's security extended attributes(xattrs) against integrity attacks. This patchset provides the framework and an initial method. The initial method maintains an HMAC-sha1 value across the security extended attributes, storing the HMAC value as the extended attribute 'security.evm'. Other methods of validating the integrity of a file's metadata will be posted separately (eg. EVM-digital-signatures). While this patchset does authenticate the security xattrs, and cryptographically binds them to the inode, coming extensions will bind other directory and inode metadata for more complete protection. To help simplify the review and upstreaming process, each extension will be posted separately (eg. IMA-appraisal, IMA-appraisal-directory). For a general overview of the proposed Linux integrity subsystem, refer to Dave Safford's whitepaper: http://downloads.sf.net/project/linux-ima/linux-ima/Integrity_overview.pdf. EVM depends on the Kernel Key Retention System to provide it with a trusted/encrypted key for the HMAC-sha1 operation. The key is loaded onto the root's keyring using keyctl. Until EVM receives notification that the key has been successfully loaded onto the keyring (echo 1 > /evm), EVM can not create or validate the 'security.evm' xattr, but returns INTEGRITY_UNKNOWN. Loading the key and signaling EVM should be done as early as possible. Normally this is done in the initramfs, which has already been measured as part of the trusted boot. For more information on creating and loading existing trusted/encrypted keys, refer to Documentation/keys-trusted-encrypted.txt. A sample dracut patch, which loads the trusted/encrypted key and enables EVM, is available from http://linux-ima.sourceforge.net/#EVM. Based on the LSMs enabled, the set of EVM protected security xattrs is defined at compile. EVM adds the following three calls to the existing security hooks: evm_inode_setxattr(), evm_inode_post_setxattr(), and evm_inode_removexattr. To initialize and update the 'security.evm' extended attribute, EVM defines three calls: evm_inode_post_init(), evm_inode_post_setattr() and evm_inode_post_removexattr() hooks. To verify the integrity of a security xattr, EVM exports evm_verifyxattr(). Changelog v7: - Fixed URL in EVM ABI documentation Changelog v6: (based on Serge Hallyn's review) - fix URL in patch description - remove evm_hmac_size definition - use SHA1_DIGEST_SIZE (removed both MAX_DIGEST_SIZE and evm_hmac_size) - moved linux include before other includes - test for crypto_hash_setkey failure - fail earlier for invalid key - clear entire encrypted key, even on failure - check xattr name length before comparing xattr names Changelog: - locking based on i_mutex, remove evm_mutex - using trusted/encrypted keys for storing the EVM key used in the HMAC-sha1 operation. - replaced crypto hash with shash (Dmitry Kasatkin) - support for additional methods of verifying the security xattrs (Dmitry Kasatkin) - iint not allocated for all regular files, but only for those appraised - Use cap_sys_admin in lieu of cap_mac_admin - Use __vfs_setxattr_noperm(), without permission checks, from EVM Signed-off-by: Mimi Zohar Acked-by: Serge Hallyn --- Documentation/ABI/testing/evm | 23 +++++++++++++++++++++++ 1 file changed, 23 insertions(+) create mode 100644 Documentation/ABI/testing/evm (limited to 'Documentation') diff --git a/Documentation/ABI/testing/evm b/Documentation/ABI/testing/evm new file mode 100644 index 000000000000..8374d4557e5d --- /dev/null +++ b/Documentation/ABI/testing/evm @@ -0,0 +1,23 @@ +What: security/evm +Date: March 2011 +Contact: Mimi Zohar +Description: + EVM protects a file's security extended attributes(xattrs) + against integrity attacks. The initial method maintains an + HMAC-sha1 value across the extended attributes, storing the + value as the extended attribute 'security.evm'. + + EVM depends on the Kernel Key Retention System to provide it + with a trusted/encrypted key for the HMAC-sha1 operation. + The key is loaded onto the root's keyring using keyctl. Until + EVM receives notification that the key has been successfully + loaded onto the keyring (echo 1 > /evm), EVM + can not create or validate the 'security.evm' xattr, but + returns INTEGRITY_UNKNOWN. Loading the key and signaling EVM + should be done as early as possible. Normally this is done + in the initramfs, which has already been measured as part + of the trusted boot. For more information on creating and + loading existing trusted/encrypted keys, refer to: + Documentation/keys-trusted-encrypted.txt. (A sample dracut + patch, which loads the trusted/encrypted key and enables + EVM, is available from http://linux-ima.sourceforge.net/#EVM.) -- cgit v1.2.3 From 7102ebcd65c1cdb5d5a87c7c5cf7a46f5afb0cac Mon Sep 17 00:00:00 2001 From: Mimi Zohar Date: Thu, 12 May 2011 18:33:20 -0400 Subject: evm: permit only valid security.evm xattrs to be updated In addition to requiring CAP_SYS_ADMIN permission to modify/delete security.evm, prohibit invalid security.evm xattrs from changing, unless in fixmode. This patch prevents inadvertent 'fixing' of security.evm to reflect offline modifications. Changelog v7: - rename boot paramater 'evm_mode' to 'evm' Reported-by: Roberto Sassu Signed-off-by: Mimi Zohar --- Documentation/kernel-parameters.txt | 6 +++ security/integrity/evm/evm_main.c | 77 ++++++++++++++++++++++++++++++------- 2 files changed, 69 insertions(+), 14 deletions(-) (limited to 'Documentation') diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt index fd248a318211..db97ff1da8c0 100644 --- a/Documentation/kernel-parameters.txt +++ b/Documentation/kernel-parameters.txt @@ -48,6 +48,7 @@ parameter is applicable: EDD BIOS Enhanced Disk Drive Services (EDD) is enabled EFI EFI Partitioning (GPT) is enabled EIDE EIDE/ATAPI support is enabled. + EVM Extended Verification Module FB The frame buffer device is enabled. GCOV GCOV profiling is enabled. HW Appropriate hardware is enabled. @@ -750,6 +751,11 @@ bytes respectively. Such letter suffixes can also be entirely omitted. This option is obsoleted by the "netdev=" option, which has equivalent usage. See its documentation for details. + evm= [EVM] + Format: { "fix" } + Permit 'security.evm' to be updated regardless of + current integrity status. + failslab= fail_page_alloc= fail_make_request=[KNL] diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c index eb07f9d13c24..94d66af07aa4 100644 --- a/security/integrity/evm/evm_main.c +++ b/security/integrity/evm/evm_main.c @@ -37,13 +37,25 @@ char *evm_config_xattrnames[] = { NULL }; +static int evm_fixmode; +static int __init evm_set_fixmode(char *str) +{ + if (strncmp(str, "fix", 3) == 0) + evm_fixmode = 1; + return 0; +} +__setup("evm=", evm_set_fixmode); + /* * evm_verify_hmac - calculate and compare the HMAC with the EVM xattr * * Compute the HMAC on the dentry's protected set of extended attributes - * and compare it against the stored security.evm xattr. (For performance, - * use the previoulsy retrieved xattr value and length to calculate the - * HMAC.) + * and compare it against the stored security.evm xattr. + * + * For performance: + * - use the previoulsy retrieved xattr value and length to calculate the + * HMAC.) + * - cache the verification result in the iint, when available. * * Returns integrity status */ @@ -54,9 +66,10 @@ static enum integrity_status evm_verify_hmac(struct dentry *dentry, struct integrity_iint_cache *iint) { struct evm_ima_xattr_data xattr_data; + enum integrity_status evm_status; int rc; - if (iint->evm_status == INTEGRITY_PASS) + if (iint && iint->evm_status == INTEGRITY_PASS) return iint->evm_status; /* if status is not PASS, try to check again - against -ENOMEM */ @@ -71,18 +84,21 @@ static enum integrity_status evm_verify_hmac(struct dentry *dentry, sizeof xattr_data, GFP_NOFS); if (rc < 0) goto err_out; - iint->evm_status = INTEGRITY_PASS; - return iint->evm_status; + evm_status = INTEGRITY_PASS; + goto out; err_out: switch (rc) { case -ENODATA: /* file not labelled */ - iint->evm_status = INTEGRITY_NOLABEL; + evm_status = INTEGRITY_NOLABEL; break; default: - iint->evm_status = INTEGRITY_FAIL; + evm_status = INTEGRITY_FAIL; } - return iint->evm_status; +out: + if (iint) + iint->evm_status = evm_status; + return evm_status; } static int evm_protected_xattr(const char *req_xattr_name) @@ -157,6 +173,22 @@ static int evm_protect_xattr(struct dentry *dentry, const char *xattr_name, return 0; } +/* + * evm_verify_current_integrity - verify the dentry's metadata integrity + * @dentry: pointer to the affected dentry + * + * Verify and return the dentry's metadata integrity. The exceptions are + * before EVM is initialized or in 'fix' mode. + */ +static enum integrity_status evm_verify_current_integrity(struct dentry *dentry) +{ + struct inode *inode = dentry->d_inode; + + if (!evm_initialized || !S_ISREG(inode->i_mode) || evm_fixmode) + return 0; + return evm_verify_hmac(dentry, NULL, NULL, 0, NULL); +} + /** * evm_inode_setxattr - protect the EVM extended attribute * @dentry: pointer to the affected dentry @@ -164,13 +196,22 @@ static int evm_protect_xattr(struct dentry *dentry, const char *xattr_name, * @xattr_value: pointer to the new extended attribute value * @xattr_value_len: pointer to the new extended attribute value length * - * Prevent 'security.evm' from being modified + * Updating 'security.evm' requires CAP_SYS_ADMIN privileges and that + * the current value is valid. */ int evm_inode_setxattr(struct dentry *dentry, const char *xattr_name, const void *xattr_value, size_t xattr_value_len) { - return evm_protect_xattr(dentry, xattr_name, xattr_value, - xattr_value_len); + + enum integrity_status evm_status; + int ret; + + ret = evm_protect_xattr(dentry, xattr_name, xattr_value, + xattr_value_len); + if (ret) + return ret; + evm_status = evm_verify_current_integrity(dentry); + return evm_status == INTEGRITY_PASS ? 0 : -EPERM; } /** @@ -178,11 +219,19 @@ int evm_inode_setxattr(struct dentry *dentry, const char *xattr_name, * @dentry: pointer to the affected dentry * @xattr_name: pointer to the affected extended attribute name * - * Prevent 'security.evm' from being removed. + * Removing 'security.evm' requires CAP_SYS_ADMIN privileges and that + * the current value is valid. */ int evm_inode_removexattr(struct dentry *dentry, const char *xattr_name) { - return evm_protect_xattr(dentry, xattr_name, NULL, 0); + enum integrity_status evm_status; + int ret; + + ret = evm_protect_xattr(dentry, xattr_name, NULL, 0); + if (ret) + return ret; + evm_status = evm_verify_current_integrity(dentry); + return evm_status == INTEGRITY_PASS ? 0 : -EPERM; } /** -- cgit v1.2.3