Re: [PATCH] x86/syscall: Avoid memcpy() for ia32 syscall_get_arguments()

[Peter Zijlstra]

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@xxxxxxxxx>
> Closes: https://lore.kernel.org/lkml/9b69fb14-df89-4677-9c82-056ea9e706f5@xxxxxxxxx/
> Signed-off-by: Kees Cook <kees@xxxxxxxxxx>
> ---
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Cc: x86@xxxxxxxxxx
> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> Cc: Daniel Sneddon <daniel.sneddon@xxxxxxxxxxxxxxx>
> Cc: Arnd Bergmann <arnd@xxxxxxxx>
> Cc: Brian Gerst <brgerst@xxxxxxxxx>
> Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> Cc: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
> Cc: Peter Collingbourne <pcc@xxxxxxxxxx>
> ---
>  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?