Implement Spectre mitigations for table accesses and br_tables. (#4092)

Currently, we have partial Spectre mitigation: we protect heap accesses
with dynamic bounds checks. Specifically, we guard against errant
accesses on the misspeculated path beyond the bounds-check conditional
branch by adding a conditional move that is also dependent on the
bounds-check condition. This data dependency on the condition is not
speculated and thus will always pick the "safe" value (in the heap case,
a NULL address) on the misspeculated path, until the pipeline flushes
and recovers onto the correct path.

This PR uses the same technique both for table accesses -- used to
implement Wasm tables -- and for jumptables, used to implement Wasm
`br_table` instructions.

In the case of Wasm tables, the cmove picks the table base address on
the misspeculated path. This is equivalent to reading the first table
entry. This prevents loads of arbitrary data addresses on the
misspeculated path.

In the case of `br_table`, the cmove picks index 0 on the misspeculated
path. This is safer than allowing a branch to an address loaded from an
index under misspeculation (i.e., it preserves control-flow integrity
even under misspeculation).

The table mitigation is controlled by a Cranelift setting, on by
default. The br_table mitigation is always on, because it is part of the
single lowering pseudoinstruction. In both cases, the impact should be
minimal: a single extra cmove in a (relatively) rarely-used operation.

The table mitigation is architecture-independent (happens during
legalization); the br_table mitigation has been implemented for both x64
and aarch64. (I don't know enough about s390x to implement this
confidently there, but would happily review a PR to do the same on that
platform.)

cranelift/codegen/src/isa/aarch64/mod.rs

@@ -181,7 +181,7 @@ mod test {
     use super::*;
     use crate::cursor::{Cursor, FuncCursor};
     use crate::ir::types::*;
-    use crate::ir::{AbiParam, ExternalName, Function, InstBuilder, Signature};
+    use crate::ir::{AbiParam, ExternalName, Function, InstBuilder, JumpTableData, Signature};
     use crate::isa::CallConv;
     use crate::settings;
     use crate::settings::Configurable;
@@ -294,4 +294,79 @@ mod test {
 
         assert_eq!(code, &golden[..]);
     }
+
+    #[test]
+    fn test_br_table() {
+        let name = ExternalName::testcase("test0");
+        let mut sig = Signature::new(CallConv::SystemV);
+        sig.params.push(AbiParam::new(I32));
+        sig.returns.push(AbiParam::new(I32));
+        let mut func = Function::with_name_signature(name, sig);
+
+        let bb0 = func.dfg.make_block();
+        let arg0 = func.dfg.append_block_param(bb0, I32);
+        let bb1 = func.dfg.make_block();
+        let bb2 = func.dfg.make_block();
+        let bb3 = func.dfg.make_block();
+
+        let mut pos = FuncCursor::new(&mut func);
+
+        pos.insert_block(bb0);
+        let mut jt_data = JumpTableData::new();
+        jt_data.push_entry(bb1);
+        jt_data.push_entry(bb2);
+        let jt = pos.func.create_jump_table(jt_data);
+        pos.ins().br_table(arg0, bb3, jt);
+
+        pos.insert_block(bb1);
+        let v1 = pos.ins().iconst(I32, 1);
+        pos.ins().return_(&[v1]);
+
+        pos.insert_block(bb2);
+        let v2 = pos.ins().iconst(I32, 2);
+        pos.ins().return_(&[v2]);
+
+        pos.insert_block(bb3);
+        let v3 = pos.ins().iconst(I32, 3);
+        pos.ins().return_(&[v3]);
+
+        let mut shared_flags_builder = settings::builder();
+        shared_flags_builder.set("opt_level", "none").unwrap();
+        shared_flags_builder.set("enable_verifier", "true").unwrap();
+        let shared_flags = settings::Flags::new(shared_flags_builder);
+        let isa_flags = aarch64_settings::Flags::new(&shared_flags, aarch64_settings::builder());
+        let backend = AArch64Backend::new_with_flags(
+            Triple::from_str("aarch64").unwrap(),
+            shared_flags,
+            isa_flags,
+        );
+        let result = backend
+            .compile_function(&mut func, /* want_disasm = */ false)
+            .unwrap();
+        let code = result.buffer.data();
+
+        //   0:   7100081f        cmp     w0, #0x2
+        //   4:   54000102        b.cs    0x24  // b.hs, b.nlast
+        //   8:   9a8023e9        csel    x9, xzr, x0, cs  // cs = hs, nlast
+        //   c:   10000088        adr     x8, 0x1c
+        //  10:   b8a95909        ldrsw   x9, [x8, w9, uxtw #2]
+        //  14:   8b090108        add     x8, x8, x9
+        //  18:   d61f0100        br      x8
+        //  1c:   00000010        udf     #16
+        //  20:   00000018        udf     #24
+        //  24:   d2800060        mov     x0, #0x3                        // #3
+        //  28:   d65f03c0        ret
+        //  2c:   d2800020        mov     x0, #0x1                        // #1
+        //  30:   d65f03c0        ret
+        //  34:   d2800040        mov     x0, #0x2                        // #2
+        //  38:   d65f03c0        ret
+
+        let golden = vec![
+            31, 8, 0, 113, 2, 1, 0, 84, 233, 35, 128, 154, 136, 0, 0, 16, 9, 89, 169, 184, 8, 1, 9,
+            139, 0, 1, 31, 214, 16, 0, 0, 0, 24, 0, 0, 0, 96, 0, 128, 210, 192, 3, 95, 214, 32, 0,
+            128, 210, 192, 3, 95, 214, 64, 0, 128, 210, 192, 3, 95, 214,
+        ];
+
+        assert_eq!(code, &golden[..]);
+    }
 }