linux-kernel - Re: [PATCH] x86/syscall: Avoid memcpy() for ia32 syscall_get

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Message-ID: <20240711214450.GG27299@noisy.programming.kicks-ass.net>
Date: Thu, 11 Jul 2024 23:44:50 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Kees Cook <kees@...nel.org>
Cc: Thomas Gleixner <tglx@...utronix.de>,
	Mirsad Todorovac <mtodorovac69@...il.com>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	Daniel Sneddon <daniel.sneddon@...ux.intel.com>,
	Arnd Bergmann <arnd@...db.de>, Brian Gerst <brgerst@...il.com>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
	Peter Collingbourne <pcc@...gle.com>, linux-kernel@...r.kernel.org,
	linux-hardening@...r.kernel.org
Subject: Re: [PATCH] x86/syscall: Avoid memcpy() for ia32
 syscall_get_arguments()

On Mon, Jul 08, 2024 at 01:22:06PM -0700, Kees Cook wrote:
> Modern (fortified) memcpy() prefers to avoid writing (or reading) beyond
> the end of the addressed destination (or source) struct member:
> 
> In function ‘fortify_memcpy_chk’,
>     inlined from ‘syscall_get_arguments’ at ./arch/x86/include/asm/syscall.h:85:2,
>     inlined from ‘populate_seccomp_data’ at kernel/seccomp.c:258:2,
>     inlined from ‘__seccomp_filter’ at kernel/seccomp.c:1231:3:
> ./include/linux/fortify-string.h:580:25: error: call to ‘__read_overflow2_field’ declared with attribute warning: detected read beyond size of field (2nd parameter); maybe use struct_group()? [-Werror=attribute-warning]
>   580 |                         __read_overflow2_field(q_size_field, size);
>       |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 
> As already done for x86_64 and compat mode, do not use memcpy() to
> extract syscall arguments from struct pt_regs but rather just perform
> direct assignments. Binary output differences are negligible, and actually
> ends up using less stack space:
> 
> -       sub    $0x84,%esp
> +       sub    $0x6c,%esp
> 
> and less text size:
> 
>    text    data     bss     dec     hex filename
>   10794     252       0   11046    2b26 gcc-32b/kernel/seccomp.o.stock
>   10714     252       0   10966    2ad6 gcc-32b/kernel/seccomp.o.after
> 
> Reported-by: Mirsad Todorovac <mtodorovac69@...il.com>
> Closes: https://lore.kernel.org/lkml/9b69fb14-df89-4677-9c82-056ea9e706f5@gmail.com/
> Signed-off-by: Kees Cook <kees@...nel.org>
> ---
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Borislav Petkov <bp@...en8.de>
> Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> Cc: x86@...nel.org
> Cc: "H. Peter Anvin" <hpa@...or.com>
> Cc: Daniel Sneddon <daniel.sneddon@...ux.intel.com>
> Cc: Arnd Bergmann <arnd@...db.de>
> Cc: Brian Gerst <brgerst@...il.com>
> Cc: Josh Poimboeuf <jpoimboe@...nel.org>
> Cc: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
> Cc: Peter Collingbourne <pcc@...gle.com>
> ---
>  arch/x86/include/asm/syscall.h | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/syscall.h b/arch/x86/include/asm/syscall.h
> index 2fc7bc3863ff..7c488ff0c764 100644
> --- a/arch/x86/include/asm/syscall.h
> +++ b/arch/x86/include/asm/syscall.h
> @@ -82,7 +82,12 @@ static inline void syscall_get_arguments(struct task_struct *task,
>  					 struct pt_regs *regs,
>  					 unsigned long *args)
>  {
> -	memcpy(args, &regs->bx, 6 * sizeof(args[0]));
> +	args[0] = regs->bx;
> +	args[1] = regs->cx;
> +	args[2] = regs->dx;
> +	args[3] = regs->si;
> +	args[4] = regs->di;
> +	args[5] = regs->bp;
>  }

Just for my education on things foritfy; would something like:

void syscall_get_arguments(struct pt_regs *regs, unsigned long args[6])
{
        memcpy(args, (typeof(args))&regs->bx, 6*sizeof(args[0]));
}

work?