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From: "Andy Lutomirski" <luto@kernel.org>
To: "Sami Tolvanen" <samitolvanen@google.com>
Cc: "the arch/x86 maintainers" <x86@kernel.org>,
	"Kees Cook" <keescook@chromium.org>,
	"Josh Poimboeuf" <jpoimboe@redhat.com>,
	"Peter Zijlstra (Intel)" <peterz@infradead.org>,
	"Nathan Chancellor" <nathan@kernel.org>,
	"Nick Desaulniers" <ndesaulniers@google.com>,
	"Sedat Dilek" <sedat.dilek@gmail.com>,
	linux-hardening@vger.kernel.org,
	"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
	clang-built-linux <clang-built-linux@googlegroups.com>
Subject: Re: [PATCH v2 07/14] x86: Use an opaque type for functions not callable from C
Date: Thu, 26 Aug 2021 16:23:50 -0700	[thread overview]
Message-ID: <959b3e65-97c9-4e00-87c5-f4a31c6739de@www.fastmail.com> (raw)
In-Reply-To: <CABCJKueE=_WCLkVDmPss-Bo-1VWaX7W6SSNx33rQCO+eEv_Lzg@mail.gmail.com>



On Thu, Aug 26, 2021, at 3:11 PM, Sami Tolvanen wrote:
> On Thu, Aug 26, 2021 at 9:54 AM Andy Lutomirski <luto@kernel.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
> 

  reply	other threads:[~2021-08-26 23:24 UTC|newest]

Thread overview: 29+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-23 17:13 [PATCH v2 00/14] x86: Add support for Clang CFI Sami Tolvanen
2021-08-23 17:13 ` [PATCH v2 01/14] objtool: Add CONFIG_CFI_CLANG support Sami Tolvanen
2021-08-23 17:13 ` [PATCH v2 02/14] objtool: Add ASM_STACK_FRAME_NON_STANDARD Sami Tolvanen
2021-08-23 17:13 ` [PATCH v2 03/14] linkage: Add DECLARE_ASM_FUNC_SYMBOL Sami Tolvanen
2021-08-23 17:13 ` [PATCH v2 04/14] ftrace: Use an opaque type for functions not callable from C Sami Tolvanen
2021-08-23 17:13 ` [PATCH v2 05/14] lkdtm: Disable UNSET_SMEP with CFI Sami Tolvanen
2021-08-23 17:13 ` [PATCH v2 06/14] lkdtm: Use an opaque type for lkdtm_rodata_do_nothing Sami Tolvanen
2021-08-23 17:13 ` [PATCH v2 07/14] x86: Use an opaque type for functions not callable from C Sami Tolvanen
2021-08-26 16:54   ` Andy Lutomirski
2021-08-26 22:11     ` Sami Tolvanen
2021-08-26 23:23       ` Andy Lutomirski [this message]
2021-08-26 23:45         ` Sami Tolvanen
2021-08-23 17:13 ` [PATCH v2 08/14] x86/extable: Do not mark exception callback as CFI Sami Tolvanen
2021-08-26 16:56   ` Andy Lutomirski
2021-08-30 19:57     ` Sami Tolvanen
2021-08-23 17:13 ` [PATCH v2 09/14] x86/purgatory: Disable CFI Sami Tolvanen
2021-08-23 17:13 ` [PATCH v2 10/14] x86, relocs: Ignore __typeid__ relocations Sami Tolvanen
2021-08-23 17:13 ` [PATCH v2 11/14] x86, module: " Sami Tolvanen
2021-08-23 17:13 ` [PATCH v2 12/14] x86, cpu: Use LTO for cpu.c with CFI Sami Tolvanen
2021-08-23 17:13 ` [PATCH v2 13/14] x86, kprobes: Fix optprobe_template_func type mismatch Sami Tolvanen
2021-08-23 17:13 ` [PATCH v2 14/14] x86, build: Allow CONFIG_CFI_CLANG to be selected Sami Tolvanen
2021-08-23 17:16 ` [PATCH v2 00/14] x86: Add support for Clang CFI Tom Stellard
2021-08-23 17:20   ` Sami Tolvanen
2021-08-24 17:26     ` Tom Stellard
2021-08-24 17:30       ` Sami Tolvanen
2021-08-24 19:46 ` Peter Zijlstra
2021-08-25 15:49   ` Sami Tolvanen
2021-08-26 11:43     ` Peter Zijlstra
2021-08-26 21:52       ` Sami Tolvanen

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