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authorRoberto Sassu <roberto.sassu@huawei.com>2021-05-14 17:27:44 +0200
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2021-07-14 16:55:46 +0200
commit53124265fca84159ac4b411f1ea43eba010f974e (patch)
tree1fdbd3c926e6c8cf6e6c4eabe737b2a29f23dbe5
parent7b84c7d7e2c2a2ceb10a8ba1460c2a859c633400 (diff)
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evm: Refuse EVM_ALLOW_METADATA_WRITES only if an HMAC key is loaded
commit 9acc89d31f0c94c8e573ed61f3e4340bbd526d0c upstream. EVM_ALLOW_METADATA_WRITES is an EVM initialization flag that can be set to temporarily disable metadata verification until all xattrs/attrs necessary to verify an EVM portable signature are copied to the file. This flag is cleared when EVM is initialized with an HMAC key, to avoid that the HMAC is calculated on unverified xattrs/attrs. Currently EVM unnecessarily denies setting this flag if EVM is initialized with a public key, which is not a concern as it cannot be used to trust xattrs/attrs updates. This patch removes this limitation. Fixes: ae1ba1676b88e ("EVM: Allow userland to permit modification of EVM-protected metadata") Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com> Cc: stable@vger.kernel.org # 4.16.x Signed-off-by: Mimi Zohar <zohar@linux.ibm.com> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
-rw-r--r--Documentation/ABI/testing/evm26
-rw-r--r--security/integrity/evm/evm_secfs.c8
2 files changed, 28 insertions, 6 deletions
diff --git a/Documentation/ABI/testing/evm b/Documentation/ABI/testing/evm
index 3c477ba48a31..2243b72e4110 100644
--- a/Documentation/ABI/testing/evm
+++ b/Documentation/ABI/testing/evm
@@ -49,8 +49,30 @@ Description:
modification of EVM-protected metadata and
disable all further modification of policy
- Note that once a key has been loaded, it will no longer be
- possible to enable metadata modification.
+ Echoing a value is additive, the new value is added to the
+ existing initialization flags.
+
+ For example, after::
+
+ echo 2 ><securityfs>/evm
+
+ another echo can be performed::
+
+ echo 1 ><securityfs>/evm
+
+ and the resulting value will be 3.
+
+ Note that once an HMAC key has been loaded, it will no longer
+ be possible to enable metadata modification. Signaling that an
+ HMAC key has been loaded will clear the corresponding flag.
+ For example, if the current value is 6 (2 and 4 set)::
+
+ echo 1 ><securityfs>/evm
+
+ will set the new value to 3 (4 cleared).
+
+ Loading an HMAC key is the only way to disable metadata
+ modification.
Until key loading has been signaled EVM can not create
or validate the 'security.evm' xattr, but returns
diff --git a/security/integrity/evm/evm_secfs.c b/security/integrity/evm/evm_secfs.c
index cfc3075769bb..a7042ae90b9e 100644
--- a/security/integrity/evm/evm_secfs.c
+++ b/security/integrity/evm/evm_secfs.c
@@ -80,12 +80,12 @@ static ssize_t evm_write_key(struct file *file, const char __user *buf,
if (!i || (i & ~EVM_INIT_MASK) != 0)
return -EINVAL;
- /* Don't allow a request to freshly enable metadata writes if
- * keys are loaded.
+ /*
+ * Don't allow a request to enable metadata writes if
+ * an HMAC key is loaded.
*/
if ((i & EVM_ALLOW_METADATA_WRITES) &&
- ((evm_initialized & EVM_KEY_MASK) != 0) &&
- !(evm_initialized & EVM_ALLOW_METADATA_WRITES))
+ (evm_initialized & EVM_INIT_HMAC) != 0)
return -EPERM;
if (i & EVM_INIT_HMAC) {