[BOLT][binary-analysis] Add initial pac-ret gadget scanner

bolt/docs/BinaryAnalysis.md

@@ -9,9 +9,182 @@ analyses implemented in the BOLT libraries.
 
 ## Which binary analyses are implemented?
 
-At the moment, no binary analyses are implemented.
+* [Security scanners](#security-scanners)
+  * [pac-ret analysis](#pac-ret-analysis)
 
-The goal is to make it easy using a plug-in framework to add your own analyses.
+### Security scanners
+
+For the past 25 years, a large numbers of exploits have been built and used in
+the wild to undermine computer security. The majority of these exploits abuse
+memory vulnerabilities in programs, see evidence from
+[Microsoft](https://youtu.be/PjbGojjnBZQ?si=oCHCa0SHgaSNr6Gr&t=836),
+[Chromium](https://www.chromium.org/Home/chromium-security/memory-safety/) and
+[Android](https://security.googleblog.com/2021/01/data-driven-security-hardening-in.html).
+
+It is not surprising therefore, that a large number of mitigations have been
+added to instruction sets and toolchains to make it harder to build an exploit
+using a memory vulnerability. Examples are: stack canaries, stack clash,
+pac-ret, shadow stacks, arm64e, and many more.
+
+These mitigations guarantee a so-called "security property" on the binaries they
+produce. For example, for stack canaries, the security property is roughly that
+a canary is located on the stack between the set of saved registers and the set
+of local variables. For pac-ret, it is roughly that either the return address is
+never stored/retrieved to/from memory; or, there are no writes to the register
+containing the return address between an instruction authenticating it and a
+return instruction using it.
+
+From time to time, however, a bug gets found in the implementation of such
+mitigations in toolchains. Also, code that is written in assembler by hand
+requires the developer to ensure these security properties by hand.
+
+In short, it is sometimes found that a few places in the binary code are not
+protected as well as expected given the requested mitigations. Attackers could
+make use of those places (sometimes called gadgets) to circumvent the protection
+that the mitigation should give.
+
+One of the reasons that such gadgets, or holes in the mitigation implementation,
+exist is that typically the amount of testing and verification for these
+security properties is limited to checking results on specific examples.
+
+In comparison, for testing functional correctness, or for testing performance,
+toolchain and software in general typically get tested with large test suites
+and benchmarks. In contrast, this typically does not get done for testing the
+security properties of binary code.
+
+Unlike functional correctness where compilation errors result in test failures,
+and performance where speed and size differences are measurable, broken security
+properties cannot be easily observed using existing testing and benchmarking
+tools.
+
+The security scanners implemented in `llvm-bolt-binary-analysis` aim to enable
+the testing of security hardening in arbitrary programs and not just specific
+examples.
+
+
+#### pac-ret analysis
+
+`pac-ret` protection is a security hardening scheme implemented in compilers
+such as GCC and Clang, using the command line option
+`-mbranch-protection=pac-ret`. This option is enabled by default on most widely
+used Linux distributions.
+
+The hardening scheme mitigates
+[Return-Oriented Programming (ROP)](https://llsoftsec.github.io/llsoftsecbook/#return-oriented-programming)
+attacks by making sure that return addresses are only ever stored to memory with
+a cryptographic hash, called a
+["Pointer Authentication Code" (PAC)](https://llsoftsec.github.io/llsoftsecbook/#pointer-authentication),
+in the upper bits of the pointer. This makes it substantially harder for
+attackers to divert control flow by overwriting a return address with a
+different value.
+
+The hardening scheme relies on compilers producing appropriate code sequences when
+processing return addresses, especially when these are stored to and retrieved
+from memory.
+
+The `pac-ret` binary analysis can be invoked using the command line option
+`--scanners=pac-ret`. It makes `llvm-bolt-binary-analysis` scan through the
+provided binary, checking each function for the following security property:
+
+> For each procedure and exception return instruction, the destination register
+> must have one of the following properties:
+>
+> 1. be immutable within the function, or
+> 2. the last write to the register must be by an authenticating instruction. This
+>    includes combined authentication and return instructions such as `RETAA`.
+
+##### Example 1
+
+For example, a typical non-pac-ret-protected function looks as follows:
+
+```
+        stp     x29, x30, [sp, #-0x10]!
+        mov     x29, sp
+        bl      g@PLT
+        add     x0, x0, #0x3
+        ldp     x29, x30, [sp], #0x10
+        ret
+```
+
+The return instruction `ret` implicitly uses register `x30` as the address to
+return to. Register `x30` was last written by instruction `ldp`, which is not an
+authenticating instruction. `llvm-bolt-binary-analysis --scanners=pac-ret` will
+report this as follows:
+
+```
+GS-PACRET: non-protected ret found in function f1, basic block .LBB00, at address 10310
+  The return instruction is     00010310:       ret # pacret-gadget: pac-ret-gadget<Ret:MCInstBBRef<BB:.LBB00:6>, Overwriting:[MCInstBBRef<BB:.LBB00:5> ]>
+  The 1 instructions that write to the return register after any authentication are:
+  1.     0001030c:      ldp     x29, x30, [sp], #0x10
+  This happens in the following basic block:
+    000102fc:   stp     x29, x30, [sp, #-0x10]!
+    00010300:   mov     x29, sp
+    00010304:   bl      g@PLT
+    00010308:   add     x0, x0, #0x3
+    0001030c:   ldp     x29, x30, [sp], #0x10
+    00010310:   ret # pacret-gadget: pac-ret-gadget<Ret:MCInstBBRef<BB:.LBB00:6>, Overwriting:[MCInstBBRef<BB:.LBB00:5> ]>
+```
+
+The exact format of how `llvm-bolt-binary-analysis` reports this is expected to
+evolve over time.
+
+##### Example 2: multiple "last-overwriting" instructions
+
+A simple example that shows how there can be a set of "last overwriting"
+instructions of a register follows:
+
+```
+        paciasp
+        stp     x29, x30, [sp, #-0x10]!
+        ldp     x29, x30, [sp], #0x10
+        cbnz    x0, 1f
+        autiasp
+1:
+        ret
+```
+
+This will produce the following diagnostic:
+
+```
+GS-PACRET: non-protected ret found in function f_crossbb1, basic block .Ltmp0, at address 102dc
+  The return instruction is     000102dc:       ret # pacret-gadget: pac-ret-gadget<Ret:MCInstBBRef<BB:.Ltmp0:0>, Overwriting:[MCInstBBRef<BB:.LFT0:0> MCInstBBRef<BB:.LBB00:2> ]>
+  The 2 instructions that write to the return register after any authentication are:
+  1.     000102d0:      ldp     x29, x30, [sp], #0x10
+  2.     000102d8:      autiasp
+```
+
+(Yes, this diagnostic could be improved because the second "overwriting"
+instruction, `autiasp`, is an authenticating instruction...)
+
+##### Known false positives or negatives
+
+The following are current known cases of false positives:
+
+1. Not handling "no-return" functions. See issue
+   [#115154](https://github.com/llvm/llvm-project/issues/115154) for details and
+   pointers to open PRs to fix this.
+2. Not recognizing that a move of a properly authenticated value between registers,
+   results in the destination register having a properly authenticated value.
+   For example, the scanner currently produces a false negative for the following
+   code sequence:
+   ```
+        autiasp
+        mov     x16, x30
+        ret     x16
+   ```
+
+The following are current known cases of false negatives:
+
+1. Not handling functions for which the CFG cannot be reconstructed by BOLT. The
+   plan is to implement support for this, picking up the implementation from the
+   [prototype branch](
+   https://github.com/llvm/llvm-project/compare/main...kbeyls:llvm-project:bolt-gadget-scanner-prototype).
+
+BOLT cannot currently handle functions with `cfi_negate_ra_state` correctly,
+i.e. any binaries built with `-mbranch-protection=pac-ret`. The scanner is meant
+to be used on specifically such binaries, so this is a major limitation! Work is
+going on in PR [#120064](https://github.com/llvm/llvm-project/pull/120064) to
+fix this.
 
 ## How to add your own binary analysis
 

bolt/include/bolt/Core/MCPlusBuilder.h

@@ -27,6 +27,7 @@
 #include "llvm/MC/MCInstrAnalysis.h"
 #include "llvm/MC/MCInstrDesc.h"
 #include "llvm/MC/MCInstrInfo.h"
+#include "llvm/MC/MCRegister.h"
 #include "llvm/Support/Allocator.h"
 #include "llvm/Support/Casting.h"
 #include "llvm/Support/ErrorHandling.h"
@@ -550,6 +551,22 @@ class MCPlusBuilder {
     return Analysis->isReturn(Inst);
   }
 
+  virtual ErrorOr<MCPhysReg> getAuthenticatedReg(const MCInst &Inst) const {
+    llvm_unreachable("not implemented");
+    return getNoRegister();
+  }
+
+  virtual bool isAuthenticationOfReg(const MCInst &Inst,
+                                     MCPhysReg AuthenticatedReg) const {
+    llvm_unreachable("not implemented");
+    return false;
+  }
+
+  virtual ErrorOr<MCPhysReg> getRegUsedAsRetDest(const MCInst &Inst) const {
+    llvm_unreachable("not implemented");
+    return getNoRegister();
+  }
+
   virtual bool isTerminator(const MCInst &Inst) const;
 
   virtual bool isNoop(const MCInst &Inst) const {