[compiler-rt] [libcxxabi] [libunwind] [runtimes][PAC] Harden unwinding when possible (#138571) (PR #143230)

[Anton Korobeynikov via llvm-commits]

asl wrote:

> The fact I'm worried about is whether implicit signing and authentication on accesses to `__ptrauth`-qualified fields may introduce signing or authentication oracles usable by an attacker, since many values stored to these fields are initially non-signed. This is possibly mitigated by the fact that all these fields use address diversity with distinct integer discriminators and/or the original values are taken from read-only memory. On the other hand, discriminator computation, auth / sign intrinsic and load / store to memory are currently three separate operations when accessing a `__ptrauth`-qualified field, thus spilling of intermediate values to the stack is possible. Furthermore, even if the non-signed value originates from a read-only memory, this is not expressed in LLVM IR terms, thus the optimization pipeline may transform sensitive instruction sequences in an unsafe way.

I think we will need to revise / reinvestigate this issue after this PR and see if we can move forward with more robust approach.

https://github.com/llvm/llvm-project/pull/143230