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evm: prohibit userspace writing 'security.evm' HMAC value

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Calculating the 'security.evm' HMAC value requires access to the
EVM encrypted key.  Only the kernel should have access to it.  This
patch prevents userspace tools(eg. setfattr, cp --preserve=xattr)
from setting/modifying the 'security.evm' HMAC value directly.

Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
Cc: <stable@vger.kernel.org>
This commit is contained in:
Mimi Zohar 2014-05-11 00:05:23 -04:00
parent 14503eb994
commit 2fb1c9a4f2
1 changed files with 10 additions and 2 deletions
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12
security/integrity/evm/evm_main.c
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@ -300,12 +300,20 @@ 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
*
* Updating 'security.evm' requires CAP_SYS_ADMIN privileges and that
* the current value is valid.
* Before allowing the 'security.evm' protected xattr to be updated,
* verify the existing value is valid. As only the kernel should have
* access to the EVM encrypted key needed to calculate the HMAC, prevent
* userspace from writing HMAC value. Writing 'security.evm' requires
* requires CAP_SYS_ADMIN privileges.
*/
int evm_inode_setxattr(struct dentry *dentry, const char *xattr_name,
const void *xattr_value, size_t xattr_value_len)
{
const struct evm_ima_xattr_data *xattr_data = xattr_value;
if ((strcmp(xattr_name, XATTR_NAME_EVM) == 0)
&& (xattr_data->type == EVM_XATTR_HMAC))
return -EPERM;
return evm_protect_xattr(dentry, xattr_name, xattr_value,
xattr_value_len);
}