[llvm] [BOLT] Gadget scanner: detect authentication oracles (PR #135663)

[Kristof Beyls via llvm-commits]

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@@ -717,6 +740,457 @@ SrcSafetyAnalysis::create(BinaryFunction &BF,
                                                        RegsToTrackInstsFor);
 }
 
+/// A state representing which registers are safe to be used as the destination
+/// operand of an authentication instruction.
+///
+/// Similar to SrcState, it is the responsibility of the analysis to take
+/// register aliasing into account.
+///
+/// Depending on the implementation, it may be possible that an authentication
+/// instruction returns an invalid pointer on failure instead of terminating
+/// the program immediately (assuming the program will crash as soon as that
+/// pointer is dereferenced). To prevent brute-forcing the correct signature,
+/// it should be impossible for an attacker to test if a pointer is correctly
+/// signed - either the program should be terminated on authentication failure
+/// or it should be impossible to tell whether authentication succeeded or not.
+///
+/// For that reason, a restricted set of operations is allowed on any register
+/// containing a value derived from the result of an authentication instruction
+/// until that register is either wiped or checked not to contain a result of a
+/// failed authentication.
+///
+/// Specifically, the safety property for a register is computed by iterating
+/// the instructions in backward order: the source register Xn of an instruction
+/// Inst is safe if at least one of the following is true:
+/// * Inst checks if Xn contains the result of a successful authentication and
+///   terminates the program on failure. Note that Inst can either naturally
+///   dereference Xn (load, branch, return, etc. instructions) or be the first
+///   instruction of an explicit checking sequence.
+/// * Inst performs safe address arithmetic AND both source and result
+///   registers, as well as any temporary registers, must be safe after
+///   execution of Inst (temporaries are not used on AArch64 and thus not
+///   currently supported/allowed).
+///   See MCPlusBuilder::analyzeAddressArithmeticsForPtrAuth for the details.
+/// * Inst fully overwrites Xn with an unrelated value.
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kbeyls wrote:

Just wanted to say that I think this is a great explanation in this comment, thank you for making the documentation this clear :)

https://github.com/llvm/llvm-project/pull/135663