evm: Check size of security.evm before using it

This patch checks the size for the EVM_IMA_XATTR_DIGSIG and
EVM_XATTR_PORTABLE_DIGSIG types to ensure that the algorithm is read from
the buffer returned by vfs_getxattr_alloc().

Cc: stable@vger.kernel.org # 4.19.x
Fixes: 5feeb61183dde ("evm: Allow non-SHA1 digital signatures")
Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>

diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
index 6ae00fe..76d1914 100644 (file)
--- a/security/integrity/evm/evm_main.c
+++ b/security/integrity/evm/evm_main.c
@@ -184,6 +184,12 @@ static enum integrity_status evm_verify_hmac(struct dentry *dentry,
                break;
        case EVM_IMA_XATTR_DIGSIG:
        case EVM_XATTR_PORTABLE_DIGSIG:
+               /* accept xattr with non-empty signature field */
+               if (xattr_len <= sizeof(struct signature_v2_hdr)) {
+                       evm_status = INTEGRITY_FAIL;
+                       goto out;
+               }
+
                hdr = (struct signature_v2_hdr *)xattr_data;
                digest.hdr.algo = hdr->hash_algo;
                rc = evm_calc_hash(dentry, xattr_name, xattr_value,