Remove pointer authentication from fibers

This commit removes the pointer authentication instructions from our
fiber implementation. There's more rationale in bytecodealliance#12778 for this but the
basic problems are:

* Android uses the "A" key to sign return addresses, but the "A" key
  differs between threads. This causes resuming fibers across threads to
  crash.
* Rust does not provide the ability to know when pointer authentication
  is enabled, nor does it have a way of enabling pointer authentication
  for generated code on stable.
* We can't currently find reference documentation for which key should
  be used ABI-wise for each target, nor why the Android "A" key is
  changing across threads in the same process.

Overall it seems like we're a bit too far out on a limb trying to enable
this. In the future if all Rust code, except this inline assembly, uses
pointer authentication we'll be in a good spot to reevaluate this
perhaps. Until that day though it seems premature to enable this.

Closes bytecodealliance#12778

Author: alexcrichton

crates/fiber/src/lib.rs

@@ -438,4 +438,18 @@ mod tests {
         assert!(FiberStack::new(usize::MAX, true).is_err());
         assert!(FiberStack::new(usize::MAX, false).is_err());
     }
+
+    #[test]
+    fn cross_thread_fiber() {
+        let fiber = Fiber::<(), (), ()>::new(fiber_stack(1024 * 1024), move |_, s| {
+            s.suspend(());
+        })
+        .unwrap();
+        assert!(fiber.resume(()).is_err());
+        std::thread::spawn(move || {
+            assert!(fiber.resume(()).is_ok());
+        })
+        .join()
+        .unwrap();
+    }
 }

crates/fiber/src/stackswitch/aarch64.rs

@@ -7,31 +7,9 @@
 //
 // Also at this time this file is heavily based off the x86_64 file, so you'll
 // probably want to read that one as well.
-//
-// Finally, control flow integrity hardening has been applied to the code using
-// the Pointer Authentication (PAuth) and Branch Target Identification (BTI)
-// technologies from the Arm instruction set architecture:
-// * All callable functions start with either the `BTI c` or `PACIASP`/`PACIBSP`
-//   instructions
-// * Return addresses are signed and authenticated using the stack pointer
-//   value as a modifier (similarly to the salt in a HMAC operation); the
-//   `DW_CFA_AARCH64_negate_ra_state` DWARF operation (aliased with the
-//   `.cfi_window_save` assembler directive) informs an unwinder about this
 
 use core::arch::naked_asm;
 
-cfg_if::cfg_if! {
-    if #[cfg(target_vendor = "apple")] {
-        macro_rules! paci1716 { () => ("pacib1716\n"); }
-        macro_rules! pacisp { () => ("pacibsp\n"); }
-        macro_rules! autisp { () => ("autibsp\n"); }
-    } else {
-        macro_rules! paci1716 { () => ("pacia1716\n"); }
-        macro_rules! pacisp { () => ("paciasp\n"); }
-        macro_rules! autisp { () => ("autiasp\n"); }
-    }
-}
-
 #[inline(never)] // FIXME(rust-lang/rust#148307)
 pub(crate) unsafe extern "C" fn wasmtime_fiber_switch(top_of_stack: *mut u8) {
     unsafe { wasmtime_fiber_switch_(top_of_stack) }
@@ -42,10 +20,6 @@ unsafe extern "C" fn wasmtime_fiber_switch_(top_of_stack: *mut u8 /* x0 */) {
     naked_asm!(concat!(
         "
             .cfi_startproc
-        ",
-        pacisp!(),
-        "
-            .cfi_window_save
             // Save all callee-saved registers on the stack since we're
             // assuming they're clobbered as a result of the stack switch.
             stp x29, x30, [sp, -16]!
@@ -80,10 +54,6 @@ unsafe extern "C" fn wasmtime_fiber_switch_(top_of_stack: *mut u8 /* x0 */) {
             ldp x25, x26, [sp], 16
             ldp x27, x28, [sp], 16
             ldp x29, x30, [sp], 16
-        ",
-        autisp!(),
-        "
-            .cfi_window_save
             ret
             .cfi_endproc
         ",
@@ -133,45 +103,14 @@ pub(crate) unsafe fn wasmtime_fiber_init(
             x20: entry_point as *mut u8,
             x21: entry_arg0,
             x22: wasmtime_fiber_switch_ as *mut u8,
-
-            // We set up the newly initialized fiber, so that it resumes
-            // execution from wasmtime_fiber_start(). As a result, we need a
-            // signed address of this function because `wasmtime_fiber_switch`
-            // ends with a `auti{a,b}sp` instruction. There are 2 requirements:
-            // * We would like to use an instruction that is executed as a no-op
-            //   by processors that do not support PAuth, so that the code is
-            //   backward-compatible and there is no duplication; `PACIA1716` is
-            //   a suitable one.
-            // * The fiber stack pointer value that is used by the signing
-            //   operation must match the value when the pointer is
-            //   authenticated inside wasmtime_fiber_switch(), which is 16 bytes
-            //   below the `top_of_stack` which will be `sp` at the time of the
-            //   `auti{a,b}sp`.
-            //
-            // TODO: Use the PACGA instruction to authenticate the saved register
-            // state, which avoids creating signed pointers to
-            // wasmtime_fiber_start(), and provides wider coverage.
-            lr: paci1716(wasmtime_fiber_start as *mut u8, top_of_stack.sub(16)),
+            lr: wasmtime_fiber_start as *mut u8,
 
             last_sp: initial_stack.cast(),
             ..InitialStack::default()
         });
     }
 }
 
-/// Signs `r17` with the value in `r16` using either `paci{a,b}1716` depending
-/// on the platform.
-fn paci1716(mut r17: *mut u8, r16: *mut u8) -> *mut u8 {
-    unsafe {
-        core::arch::asm!(
-            paci1716!(),
-            inout("x17") r17,
-            in("x16") r16,
-        );
-        r17
-    }
-}
-
 // See the x86_64 file for more commentary on what these CFI directives are
 // doing. Like over there note that the relative offsets to registers here
 // match the frame layout in `wasmtime_fiber_switch`.
@@ -188,7 +127,6 @@ unsafe extern "C" fn wasmtime_fiber_start() -> ! {
             0x23, 0xa0, 0x1  /* DW_OP_plus_uconst 0xa0 */
         .cfi_rel_offset x30, -0x08
         .cfi_rel_offset x29, -0x10
-        .cfi_window_save
         .cfi_rel_offset x28, -0x18
         .cfi_rel_offset x27, -0x20
         .cfi_rel_offset x26, -0x28