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    Subject[PATCH v5 03/12] evm: Refuse EVM_ALLOW_METADATA_WRITES only if an HMAC key is loaded
    Date
    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.

    Cc: stable@vger.kernel.org # 4.16.x
    Fixes: ae1ba1676b88e ("EVM: Allow userland to permit modification of EVM-protected metadata")
    Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
    ---
    Documentation/ABI/testing/evm | 5 +++--
    security/integrity/evm/evm_secfs.c | 4 ++--
    2 files changed, 5 insertions(+), 4 deletions(-)

    diff --git a/Documentation/ABI/testing/evm b/Documentation/ABI/testing/evm
    index 3c477ba48a31..eb6d70fd6fa2 100644
    --- a/Documentation/ABI/testing/evm
    +++ b/Documentation/ABI/testing/evm
    @@ -49,8 +49,9 @@ 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.
    + Note that once an HMAC key has been loaded, it will no longer
    + be possible to enable metadata modification and, if it is
    + already enabled, it will be disabled.

    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 bbc85637e18b..197a4b83e534 100644
    --- a/security/integrity/evm/evm_secfs.c
    +++ b/security/integrity/evm/evm_secfs.c
    @@ -81,10 +81,10 @@ static ssize_t evm_write_key(struct file *file, const char __user *buf,
    return -EINVAL;

    /* Don't allow a request to freshly enable metadata writes if
    - * keys are loaded.
    + * an HMAC key is loaded.
    */
    if ((i & EVM_ALLOW_METADATA_WRITES) &&
    - ((evm_initialized & EVM_KEY_MASK) != 0) &&
    + ((evm_initialized & EVM_INIT_HMAC) != 0) &&
    !(evm_initialized & EVM_ALLOW_METADATA_WRITES))
    return -EPERM;

    --
    2.26.2
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