Seed cipher lists and version bounds from the system crypto policy

BUILDING.md

@@ -126,6 +126,41 @@ distributed libcrypto. See [docs/SymbolVersioning.md](docs/SymbolVersioning.md)
 for detailed information about symbol versioning, version evolution, and CI
 integration.
 
+### System Crypto Policies (opt-in)
+
+On Linux distributions that ship the `crypto-policies` framework (for example
+Amazon Linux 2023 and Fedora), AWS-LC can be built to seed newly created
+`SSL_CTX` objects from the system-wide OpenSSL back-end policy file. This is
+disabled by default; enable it with `-DENABLE_CRYPTO_POLICIES=ON`. It is aimed at
+distribution packagers who want AWS-LC to honor the operator-selected system TLS
+policy without per-application code changes.
+
+When enabled, `SSL_CTX_new` reads
+`/etc/crypto-policies/back-ends/opensslcnf.config` after applying AWS-LC's
+built-in defaults and applies the `CipherString`, `Ciphersuites`, and
+`TLS`/`DTLS` `MinProtocol`/`MaxProtocol` directives. This is best-effort: a
+missing or malformed file, or a directive AWS-LC does not support, is ignored,
+and consumers may still override any setting afterward. The `@SECLEVEL=N` prefix
+in `CipherString` is parsed and dropped because AWS-LC does not implement
+OpenSSL security levels.
+
+A `MinProtocol` naming a version AWS-LC does not have is the exception: the floor
+rises to the policy's `MaxProtocol`. Ignoring the directive would leave AWS-LC's
+built-in floor of TLS 1.0, which is below any floor the policy can ask for, so
+the context would offer the versions the policy forbids. A `MinProtocol` older
+than TLS 1.0, such as `SSLv3`, keeps the built-in floor, which is already
+stricter.
+
+AWS-LC reads the file once per process, as OpenSSL reads `openssl.cnf`, so a
+policy change takes effect only in processes started afterward. A read that fails
+is retried on the next `SSL_CTX_new`, so a policy file that appears later is
+picked up.
+
+`AWSLC_CRYPTO_POLICY_FILE` names the policy file, at build time with
+`-DAWSLC_CRYPTO_POLICY_FILE=/path/to/file` and at run time as an environment
+variable, which wins. The environment is ignored in set-uid and set-gid
+processes.
+
 ### Other Build Options
 
 See [CMake's documentation](https://cmake.org/cmake/help/v3.4/manual/cmake-variables.7.html)

docs/porting/configuration-differences.md

@@ -34,6 +34,34 @@ The following table contains the differences in libssl configuration options AWS
 * **Aside from and `SSL_MODE_AUTO_RETRY` being "ON" by default in OpenSSL, everything is "OFF" by default in OpenSSL.**
 * Each “**Context Flag”** has a link that provides more details on the flag’s functionality and our decision behind it (WIP)
 
+### System Crypto Policies Seeding (opt-in)
+
+By default AWS-LC ignores all system configuration files. When built with
+`-DENABLE_CRYPTO_POLICIES=ON` (off by default), AWS-LC seeds each newly created
+`SSL_CTX` from the system-wide `crypto-policies` OpenSSL back-end file at
+`/etc/crypto-policies/back-ends/opensslcnf.config`, which is shipped by Amazon
+Linux 2023 and Fedora. Seeding happens inside `SSL_CTX_new`, after the built-in
+defaults are applied and before the context is returned, so a consumer that
+subsequently calls the relevant setters overrides the seeded values.
+
+The directives applied are `CipherString`, `Ciphersuites`, `TLS.MinProtocol`,
+`TLS.MaxProtocol`, `DTLS.MinProtocol`, and `DTLS.MaxProtocol`. Seeding is
+best-effort: a missing or malformed file, or a directive AWS-LC does not support,
+is ignored rather than fatal. A directive AWS-LC cannot satisfy is skipped and
+the built-in default stands.
+
+A `MinProtocol` naming a version AWS-LC does not have is the exception: the floor
+rises to the policy's `MaxProtocol`. The built-in floor of TLS 1.0 sits below any
+floor a policy can ask for, so skipping the directive would leave the context
+offering the versions the policy forbids. A `MinProtocol` older than TLS 1.0, such
+as `SSLv3`, keeps the built-in floor.
+
+A context created from one of the legacy
+version-locked methods, such as `TLSv1_2_method`, keeps its single pinned version
+and takes no protocol bounds from the policy. See
+[BUILDING.md](../../BUILDING.md) for the build flag and the
+`AWSLC_CRYPTO_POLICY_FILE` override.
+
 
 <table border=0 cellspacing=0 cellpadding=0
  style='border-collapse:collapse'>

docs/porting/functionality-differences.md

@@ -15,6 +15,14 @@ libssl is the portion of OpenSSL which supports TLS. AWS-LC does not have suppor
 
 **If you have a valid use case for any missing functionality or if anything is not clarified in our documentation, feel free to [cut an issue](https://github.com/aws/aws-lc/issues/new?assignees=&labels=&projects=&template=general-issue.md&title=) or create a PR to let us know.**
 
+Note on the opt-in `-DENABLE_CRYPTO_POLICIES` build (off by default): when AWS-LC
+seeds an `SSL_CTX` from the system `crypto-policies` OpenSSL back-end, the
+`@SECLEVEL=N` prefix of the `CipherString` directive is parsed and ignored
+because AWS-LC has no security levels. Only the remaining cipher list is applied,
+so the key-size and hash constraints implied by a security level are not
+enforced. See
+[configuration-differences.md](configuration-differences.md) for details.
+
 ### libssl No-ops
 
 <table border=0 cellspacing=0 cellpadding=0