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

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Message-Id: <959b3e65-97c9-4e00-87c5-f4a31c6739de@www.fastmail.com>
Date:   Thu, 26 Aug 2021 16:23:50 -0700
From:   "Andy Lutomirski" <luto@...nel.org>
To:     "Sami Tolvanen" <samitolvanen@...gle.com>
Cc:     "the arch/x86 maintainers" <x86@...nel.org>,
        "Kees Cook" <keescook@...omium.org>,
        "Josh Poimboeuf" <jpoimboe@...hat.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        "Nathan Chancellor" <nathan@...nel.org>,
        "Nick Desaulniers" <ndesaulniers@...gle.com>,
        "Sedat Dilek" <sedat.dilek@...il.com>,
        linux-hardening@...r.kernel.org,
        "Linux Kernel Mailing List" <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 3:11 PM, Sami Tolvanen wrote:
> 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.
> 

Do you have a sense for how many occurrences there are that would need an &?

> Sami
>