[PATCH 3.13 133/198] evm: prohibit userspace writing 'security.evm' HMAC value

From: Kamal Mostafa
Date: Tue Jul 15 2014 - 18:05:26 EST -stable review patch. If anyone has any objections, please let me know.


From: Mimi Zohar <zohar@xxxxxxxxxxxxxxxxxx>

commit 2fb1c9a4f2dbc2f0bd2431c7fa64d0b5483864e4 upstream.

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@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Kamal Mostafa <kamal@xxxxxxxxxxxxx>
security/integrity/evm/evm_main.c | 12 ++++++++++--
1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
index 336b3dd..3c5cbb9 100644
--- a/security/integrity/evm/evm_main.c
+++ b/security/integrity/evm/evm_main.c
@@ -285,12 +285,20 @@ out:
* @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,

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