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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
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