linux-kernel - Re: [PATCH 11/14] llvm: kCFI pointer stuff

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Message-ID: <CAADnVQKQED2pit_DcpDWPuueHH3RLXe4pB++tU888EU=8UrNpA@mail.gmail.com>
Date: Sun, 29 Sep 2024 10:53:05 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>, 
	alyssa.milburn@...el.com, scott.d.constable@...el.com, 
	Joao Moreira <joao@...rdrivepizza.com>, Andrew Cooper <andrew.cooper3@...rix.com>, 
	Josh Poimboeuf <jpoimboe@...nel.org>, "Jose E. Marchesi" <jose.marchesi@...cle.com>, 
	"H.J. Lu" <hjl.tools@...il.com>, Nick Desaulniers <ndesaulniers@...gle.com>, 
	Sami Tolvanen <samitolvanen@...gle.com>, Nathan Chancellor <nathan@...nel.org>, ojeda@...nel.org, 
	Kees Cook <kees@...nel.org>
Subject: Re: [PATCH 11/14] llvm: kCFI pointer stuff

On Fri, Sep 27, 2024 at 12:50 PM Peter Zijlstra <peterz@...radead.org> wrote:
>
> Quick hack to extend the Clang-kCFI function meta-data (u32 hash) with
> a u8 bitmask of pointer arguments. This should really be under a new
> compiler flag, dependent on both x86_64 and kCFI.
>
> Per the comment, the bitmask represents the register based arguments
> as the first 6 bits and bit 6 is used to cover all stack based
> arguments. The high bit is used for invalid values.
>
> The purpose is to put a store dependency on the set registers, thereby
> blocking speculation paths that would otherwise expoit their value.
>
>
> Note1:
>
> This implementation simply sets the bit for any pointer type. A better
> implementation would only set the bit for any argument that is
> dereferenced in the function body.
>
> This better implementation would also capture things like:
>
>   void foo(unsigned long addr, void *args)
>   {
>     u32 t = *(u32 *)addr;
>     bar(t, args);
>   }
>
> Which, in contrast to the implementation below, would set bit0 while
> leaving bit1 unset -- the exact opposite of this implementation.
>
> Notably, addr *is* dereferenced, even though it is not a pointer on
> entry, while args is a pointer, but is not derefereced but passed on
> to bar -- if bar uses it, it gets to deal with it.
>
> Note2:
>
> Do we want to make this a u32 to keep room for all registers? AFAICT
> the current use is only concerned with the argument registers and
> those are limited to 6 for the C ABI, but custom (assembly) functions
> could use things outside of that.
>
> ---
> diff --git a/llvm/lib/Target/X86/X86AsmPrinter.cpp b/llvm/lib/Target/X86/X86AsmPrinter.cpp
> index 73c745062096..42dcbc40ab4b 100644
> --- a/llvm/lib/Target/X86/X86AsmPrinter.cpp
> +++ b/llvm/lib/Target/X86/X86AsmPrinter.cpp
> @@ -143,11 +143,28 @@ void X86AsmPrinter::EmitKCFITypePadding(const MachineFunction &MF,
>    // one. Otherwise, just pad with nops. The X86::MOV32ri instruction emitted
>    // in X86AsmPrinter::emitKCFITypeId is 5 bytes long.
>    if (HasType)
> -    PrefixBytes += 5;
> +    PrefixBytes += 7;
>
>    emitNops(offsetToAlignment(PrefixBytes, MF.getAlignment()));
>  }
>
> +static uint8_t getKCFIPointerArgs(const Function &F)
> +{
> +  uint8_t val = 0;
> +
> +  if (F.isVarArg())
> +    return 0x7f;
> +
> +  for (int i = 0; i < F.arg_size() ; i++) {
> +    Argument *A = F.getArg(i);
> +    Type *T = A->getType();
> +    if (T->getTypeID() == Type::PointerTyID)
> +      val |= 1 << std::min(i, 6);
> +  }
> +
> +  return val;
> +}
> +
>  /// emitKCFITypeId - Emit the KCFI type information in architecture specific
>  /// format.
>  void X86AsmPrinter::emitKCFITypeId(const MachineFunction &MF) {
> @@ -183,6 +200,26 @@ void X86AsmPrinter::emitKCFITypeId(const MachineFunction &MF) {
>                                .addReg(X86::EAX)
>                                .addImm(MaskKCFIType(Type->getZExtValue())));
>
> +  // Extend the kCFI meta-data with a byte that has a bit set for each argument
> +  // register that contains a pointer. Specifically for x86_64, which has 6
> +  // argument registers:
> +  //
> +  //   bit0 - rdi
> +  //   bit1 - rsi
> +  //   bit2 - rdx
> +  //   bit3 - rcx
> +  //   bit4 - r8
> +  //   bit5 - r9
> +  //
> +  // bit6 will denote any pointer on stack (%rsp), and all 7 bits set will
> +  // indicate vararg or any other 'unknown' configuration. Leaving bit7 for
> +  // error states.
> +  //
> +  // XXX: should be conditional on some new x86_64 specific 'bhi' argument.
> +  EmitAndCountInstruction(MCInstBuilder(X86::MOV8ri)
> +                 .addReg(X86::AL)
> +                 .addImm(getKCFIPointerArgs(F)));

If I'm reading this correctly it will be an 8-bit move which
doesn't clear upper bits.
If consumer is in assembly it's ok-ish,
but it's an argument to __bhi_args_foo functions,
so should be properly zero extended per call convention.