Merge pull request #10670 from gilles-peskine-arm/security-md-mention-compiler-4.x

mbedtls: Mention compiler optimization in the threat model

ChangeLog.d/security-advice.txt

@@ -0,0 +1,2 @@
+Security
+   * Added advice about compiler options in SECURITY.md.

SECURITY.md

@@ -107,6 +107,12 @@ model, they need to be mitigated by physical countermeasures.
 
 ### Caveats
 
+#### Compiler-induced side channels
+
+Mbed TLS is mostly written in C. We use standard C except with known compilers, so we do not expect compilers to introduce direct vulnerabilities. However, compilers can introduce [timing side channels](#timing-attacks) in code that was intended to be constant-time. Mbed TLS includes countermeasures to try to prevent this. But given the diversity of compilers, compiler options and target platforms, this prevention may not be complete.
+
+We recommend compiling Mbed TLS with commonly used levels of optimizations, such as `-O2` or `-Os`. We will generally treat exploitable timing side channels as a vulnerability if they appear with a common compiler at a common level of optimization. Higher levels of optimization such as `-O3` or `-Oz` are still likely to be safe but are less scrutinized. We do not recommend using individual options that might introduce data-dependent timing, and we will not try to work around such optimizations if they are not part of a commonly used level.
+
 #### Out-of-scope countermeasures
 
 Mbed TLS has evolved organically and a well defined threat model hasn't always