linux-hardening - Re: [PATCH v2 07/14] x86: Use an opaque type for functions not callable from C

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Date:   Thu, 26 Aug 2021 15:11:55 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     X86 ML <x86@...nel.org>, Kees Cook <keescook@...omium.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        linux-hardening@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH v2 07/14] x86: Use an opaque type for functions not
 callable from C

On Thu, Aug 26, 2021 at 9:54 AM Andy Lutomirski <luto@...nel.org> wrote:
>
> On 8/23/21 10:13 AM, Sami Tolvanen wrote:
> > The kernel has several assembly functions that are not directly callable
> > from C. Use an opaque type for these function prototypes to make misuse
> > harder, and to avoid the need to annotate references to these functions
> > for Clang's Control-Flow Integrity (CFI).
>
> You have:
>
> typedef const u8 *asm_func_t;
>
> This is IMO a bit confusing.  asm_func_t like this is an *address* of a
> function, not a function.
>
> To be fair, C is obnoxious, but I think this will lead to more confusion
> than is idea.  For example:
>
> > -extern void __fentry__(void);
> > +DECLARE_ASM_FUNC_SYMBOL(__fentry__);
>
> Okay, __fentry__ is the name of a symbol, and the expression __fentry__
> is a pointer (or an array that decays to a pointer, thanks C), which is
> at least somewhat sensible.  But:
>
> > -extern void (*paravirt_iret)(void);
> > +extern asm_func_t paravirt_iret;
>
> Now paravirt_iret is a global variable that points to an asm func.  I
> bet people will read this wrong and, worse, type it wrong.
>
> I think that there a couple ways to change this that would be a bit nicer.
>
> 1. typedef const u8 asm_func_t[];
>
> This is almost nice, but asm_func_t will still be accepted as a function
> argument, and the automatic decay rules will probably be confusing.
>
> 2. Rename asm_func_t to asm_func_ptr.  Then it's at least a bit more clear.
>
> 3. Use an incomplete struct:
>
> struct asm_func;
>
> typedef struct asm_func asm_func;
>
> extern asm_func some_func;
>
> void *get_ptr(void)
> {
>     return &some_func;
> }
>
> No macros required, and I think it's quite hard to misuse this by
> accident.  asm_func can't be passed as an argument or used as a variable
> because it has incomplete type, and there are no arrays so the decay
> rules aren't in effect.

I considered using an incomplete struct, but that would require an
explicit '&' when we take the address of these symbols, which I
thought would be unnecessary churn. Unless you strongly prefer this
one, I'll go with option 2 and rename asm_func_t to asm_func_ptr in
v3.

Sami