From 8f3b3a6e1b16d7a682fdc054750e26376afa8234 Mon Sep 17 00:00:00 2001 From: Miguel Ojeda Date: Wed, 24 Jul 2024 18:14:55 +0200 Subject: [PATCH] x86/rust: support MITIGATION_RETPOLINE Support `MITIGATION_RETPOLINE` by enabling the target features that Clang does. The existing target feature being enabled was a leftover from our old `rust` branch, and it is not enough: the target feature `retpoline-external-thunk` only implies `retpoline-indirect-calls`, but not `retpoline-indirect-branches` (see LLVM's `X86.td`), unlike Clang's flag of the same name `-mretpoline-external-thunk` which does imply both (see Clang's `lib/Driver/ToolChains/Arch/X86.cpp`). Without this, `objtool` would complain if enabled for Rust, e.g.: rust/core.o: warning: objtool: _R...escape_default+0x13: indirect jump found in RETPOLINE build In addition, change the comment to note that LLVM is the one disabling jump tables when retpoline is enabled, thus we do not need to use `-Zno-jump-tables` for Rust here -- see commit c58f2166ab39 ("Introduce the "retpoline" x86 mitigation technique ...") [1]: The goal is simple: avoid generating code which contains an indirect branch that could have its prediction poisoned by an attacker. In many cases, the compiler can simply use directed conditional branches and a small search tree. LLVM already has support for lowering switches in this way and the first step of this patch is to disable jump-table lowering of switches and introduce a pass to rewrite explicit indirectbr sequences into a switch over integers. As well as a live example at [2]. These should be eventually enabled via `-Ctarget-feature` when `rustc` starts recognizing them (or via a new dedicated flag) [3]. Cc: Daniel Borkmann Link: https://github.com/llvm/llvm-project/commit/c58f2166ab3987f37cb0d7815b561bff5a20a69a [1] Link: https://godbolt.org/z/G4YPr58qG [2] Link: https://github.com/rust-lang/rust/issues/116852 [3] Signed-off-by: Miguel Ojeda --- arch/x86/Makefile | 2 +- scripts/generate_rust_target.rs | 7 +++++++ 2 files changed, 8 insertions(+), 1 deletion(-) diff --git a/arch/x86/Makefile b/arch/x86/Makefile index 801fd85c3ef693..e8214bff1aebd2 100644 --- a/arch/x86/Makefile +++ b/arch/x86/Makefile @@ -220,7 +220,7 @@ ifdef CONFIG_MITIGATION_RETPOLINE KBUILD_CFLAGS += $(RETPOLINE_CFLAGS) # Additionally, avoid generating expensive indirect jumps which # are subject to retpolines for small number of switch cases. - # clang turns off jump table generation by default when under + # LLVM turns off jump table generation by default when under # retpoline builds, however, gcc does not for x86. This has # only been fixed starting from gcc stable version 8.4.0 and # onwards, but not for older ones. See gcc bug #86952. diff --git a/scripts/generate_rust_target.rs b/scripts/generate_rust_target.rs index 641b713a033a7f..44952f0a3aacf1 100644 --- a/scripts/generate_rust_target.rs +++ b/scripts/generate_rust_target.rs @@ -164,7 +164,14 @@ fn main() { ); let mut features = "-3dnow,-3dnowa,-mmx,+soft-float".to_string(); if cfg.has("MITIGATION_RETPOLINE") { + // The kernel uses `-mretpoline-external-thunk` (for Clang), which Clang maps to the + // target feature of the same name plus the other two target features in + // `clang/lib/Driver/ToolChains/Arch/X86.cpp`. These should be eventually enabled via + // `-Ctarget-feature` when `rustc` starts recognizing them (or via a new dedicated + // flag); see https://github.com/rust-lang/rust/issues/116852. features += ",+retpoline-external-thunk"; + features += ",+retpoline-indirect-branches"; + features += ",+retpoline-indirect-calls"; } ts.push("features", features); ts.push("llvm-target", "x86_64-linux-gnu");