linux-hardening - Re: [RFC PATCH v2 20/21] x86: Add support for CONFIG_CFI

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Date:   Mon, 16 May 2022 09:39:02 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     David Laight <David.Laight@...lab.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "x86@...nel.org" <x86@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Joao Moreira <joao@...rdrivepizza.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        "linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "llvm@...ts.linux.dev" <llvm@...ts.linux.dev>
Subject: Re: [RFC PATCH v2 20/21] x86: Add support for CONFIG_CFI_CLANG

On Mon, May 16, 2022 at 1:32 AM David Laight <David.Laight@...lab.com> wrote:
>
> From: Sami Tolvanen
> > Sent: 13 May 2022 21:22
> >
> > With CONFIG_CFI_CLANG, the compiler injects a type preamble
> > immediately before each function and a check to validate the target
> > function type before indirect calls:
> >
> >   ; type preamble
> >   __cfi_function:
> >     int3
> >     int3
> >     mov <id>, %eax
>
> Interesting - since this code can't be executed there is no
> point adding an instruction 'prefix' to the 32bit constant.

The reason to embed the type into an instruction is to avoid the need
to special case objtool's instruction decoder.

> >     int3
> >     int3
> >   function:
> >     ...
> >   ; indirect call check
> >     cmpl    <id>, -6(%r11)
> >     je      .Ltmp1
> >     ud2
> >   .Ltmp1:
> >     call    __x86_indirect_thunk_r11
> >
> > Define the __CFI_TYPE helper macro for manual type annotations in
> > assembly code, add error handling for the CFI ud2 traps, and allow
> > CONFIG_CFI_CLANG to be selected on x86_64.
> >
> ...
> > +
> > +     /*
> > +      * The compiler generates the following instruction sequence
> > +      * for indirect call checks:
> > +      *
> > +      *   cmpl    <id>, -6(%reg)     ; 7 bytes
>
> If the <id> is between -128 and 127 then an 8bit constant
> (sign extended) might be used.
> Possibly the compiler forces the assembler to generate the
> long form.
>
> There could also be a REX prefix.
> That will break any code that tries to use %reg.

The compiler always generates this specific instruction sequence.

> > +      *   je      .Ltmp1             ; 2 bytes
> > +      *   ud2                        ; <- addr
> > +      *   .Ltmp1:
> > +      *
> > +      * Both the type and the target address can be decoded from the
> > +      * cmpl instruction.
> > +      */
> > +     if (copy_from_kernel_nofault(buffer, (void *)regs->ip - 9, MAX_INSN_SIZE))
> > +             return;
> > +     if (insn_decode_kernel(&insn, buffer))
> > +             return;
> > +     if (insn.opcode.value != 0x81 || X86_MODRM_REG(insn.modrm.value) != 7)
> > +             return;
>
> Since you are looking for a very specific opcode why bother
> calling insn_decode_kernel() - just check for the required (masked)
> byte values.

Because I need to decode both the immediate value and the register
from that instruction.

> > +
> > +     *type = insn.immediate.value;
> > +
> > +     offset = insn_get_modrm_rm_off(&insn, regs);
>
> Given the expected instruction, isn't that -6 ??

No, this is the register offset.

> > +     if (offset < 0)
> > +             return;
> > +
> > +     *target = *(unsigned long *)((void *)regs + offset);
>
> WTF is that calculating??

It's reading the register value from pt_regs.

Sami