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evm: permit only valid security.evm xattrs to be updated

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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 <roberto.sassu@polito.it>
Signed-off-by: Mimi Zohar <zohar@us.ibm.com>
This commit is contained in:
Mimi Zohar 2011-05-12 18:33:20 -04:00
parent 24e0198efe
commit 7102ebcd65
2 changed files with 69 additions and 14 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

6
Documentation/kernel-parameters.txt
View File

@ -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]

77
security/integrity/evm/evm_main.c
View File

@ -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;
}
/**