This commit modifies the CID configuration API mbedtls_ssl_conf_cid_len()
to allow the configuration of the stack's behaviour when receiving an
encrypted DTLS record with unexpected CID.
Context:
The CID draft does not require that the length of CIDs used for incoming
records must not change in the course of a connection. Since the record
header does not contain a length field for the CID, this means that if
CIDs of varying lengths are used, the CID length must be inferred from
other aspects of the record header (such as the epoch) and/or by means
outside of the protocol, e.g. by coding its length in the CID itself.
Inferring the CID length from the record's epoch is theoretically possible
in DTLS 1.2, but it requires the information about the epoch to be present
even if the epoch is no longer used: That's because one should silently drop
records from old epochs, but not the entire datagrams to which they belong
(there might be entire flights in a single datagram, including a change of
epoch); however, in order to do so, one needs to parse the record's content
length, the position of which is only known once the CID length for the epoch
is known. In conclusion, it puts a significant burden on the implementation
to infer the CID length from the record epoch, which moreover mangles record
processing with the high-level logic of the protocol (determining which epochs
are in use in which flights, when they are changed, etc. -- this would normally
determine when we drop epochs).
Moreover, with DTLS 1.3, CIDs are no longer uniquely associated to epochs,
but every epoch may use a set of CIDs of varying lengths -- in that case,
it's even theoretically impossible to do record header parsing based on
the epoch configuration only.
We must therefore seek a way for standalone record header parsing, which
means that we must either (a) fix the CID lengths for incoming records,
or (b) allow the application-code to configure a callback to implement
an application-specific CID parsing which would somehow infer the length
of the CID from the CID itself.
Supporting multiple lengths for incoming CIDs significantly increases
complexity while, on the other hand, the restriction to a fixed CID length
for incoming CIDs (which the application controls - in contrast to the
lengths of the CIDs used when writing messages to the peer) doesn't
appear to severely limit the usefulness of the CID extension.
Therefore, the initial implementation of the CID feature will require
a fixed length for incoming CIDs, which is what this commit enforces,
in the following way:
In order to avoid a change of API in case support for variable lengths
CIDs shall be added at some point, we keep mbedtls_ssl_set_cid(), which
includes a CID length parameter, but add a new API mbedtls_ssl_conf_cid_len()
which applies to an SSL configuration, and which fixes the CID length that
any call to mbetls_ssl_set_cid() which applies to an SSL context that is bound
to the given SSL configuration must use.
While this creates a slight redundancy of parameters, it allows to
potentially add an API like mbedtls_ssl_conf_cid_len_cb() later which
could allow users to register a callback which dynamically infers the
length of a CID at record header parsing time, without changing the
rest of the API.
The CA callback changes introduce mbedtls_calloc() and
mbedtls_free() to ssl_client2 and ssl_server2, which
wasn't defined unless MBEDTLS_PLATFORM_C was set.
Additional work done as part of merge:
- Run ./tests/scripts/check-generated-files.sh and check in the
resulting changes to programs/ssl/query_config.c
This commit improves hygiene and formatting of macro definitions
throughout the library. Specifically:
- It adds brackets around parameters to avoid unintended
interpretation of arguments, e.g. due to operator precedence.
- It adds uses of the `do { ... } while( 0 )` idiom for macros that
can be used as commands.
Additional changes to temporarily enable running tests:
ssl_srv.c and test_suite_ecdh use mbedtls_ecp_group_load instead of
mbedtls_ecdh_setup
test_suite_ctr_drbg uses mbedtls_ctr_drbg_update instead of
mbedtls_ctr_drbg_update_ret
The previous prototype gave warnings are the strings produced by #cond and
__FILE__ are const, so we shouldn't implicitly cast them to non-const.
While at it modifying most example programs:
- include the header that has the function declaration, so that the definition
can be checked to match by the compiler
- fix whitespace
- make it work even if PLATFORM_C is not defined:
- CHECK_PARAMS is not documented as depending on PLATFORM_C and there is
no reason why it should
- so, remove the corresponding #if defined in each program...
- and add missing #defines for mbedtls_exit when needed
The result has been tested (make all test with -Werror) with the following
configurations:
- full with CHECK_PARAMS with PLATFORM_C
- full with CHECK_PARAMS without PLATFORM_C
- full without CHECK_PARAMS without PLATFORM_C
- full without CHECK_PARAMS with PLATFORM_C
Additionally, it has been manually tested that adding
mbedtls_aes_init( NULL );
near the normal call to mbedtls_aes_init() in programs/aes/aescrypt2.c has the
expected effect when running the program.
The sample programs require an additional handler function of
mbedtls_param_failed() to handle any failed parameter validation checks enabled
by the MBEDTLS_CHECK_PARAMS config.h option.
Previously, command line arguments `psk_slot` and `psk_list_slot`
could be used to indicate the PSA key slots that the example
applications should use to store the PSK(s) provided.
This commit changes this approach to use the utility function
`mbedtls_psa_get_free_key_slot()` to obtain free key slots from
the PSA Crypto implementation automatically, so that users only
need to pass boolean flags `psk_opaque` and `psk_list_opaque`
on the command line to enable / disable PSA-based opaque PSKs.
This commit adds command line parameters `psk_slot` and `psk_list_slot`
to the example application `programs/ssl/ssl_server2`. These have the
following semantics:
- `psk_slot`: The same semantics as for the `ssl_client2` example
application. That is, if a PSK is configured through the use
of the command line parameters `psk` and `psk_identity`, then
`psk_slot=X` can be used to import the PSK into PSA key slot X
and registering it statically with the SSL configuration through
the new API call mbedtls_ssl_conf_hs_opaque().
- `psk_list_slot`: In addition to the static PSK registered in the
the SSL configuration, servers can register a callback for picking
the PSK corresponding to the PSK identity that the client chose.
The `ssl_server2` example application uses such a callback to select
the PSK from a list of PSKs + Identities provided through the
command line parameter `psk_list`, and to register the selected
PSK via `mbedtls_ssl_set_hs_psk()`. In this case, the new parameter
`psk_list_slot=X` has the effect of registering all PSKs provided in
in `psk_list` as PSA keys in the key slots starting from slot `X`,
and having the PSK selection callback register the chosen PSK
through the new API function `mbedtls_ssl_set_hs_psk_opaque()`.
If `MBEDTLS_MEMORY_BUFFER_ALLOC_C` is configured and Mbed TLS'
custom buffer allocator is used for calloc() and free(), the
read buffer used by the server example application is allocated
from the buffer allocator, but freed after the buffer allocator
has been destroyed. If memory backtracing is enabled, this leaves
a memory leak in the backtracing structure allocated for the buffer,
as found by valgrind.
Fixes#2069.
