Re: [PATCH 2/3] [RFC] seccomp: give BPF x32 bit when restoring x32 filter

[Paul Moore <pmoore@redhat.com>]

On Thursday, July 10, 2014 11:38:13 PM Richard Guy Briggs wrote:
> Commit
> 	fca460f hpa@zytor.com 2012-02-19 07:56:26 -0800
> 	x32: Handle the x32 system call flag
> 
> provided a method to multiplex architecture with the syscall number for X32
> calls.
> 
> Commit
> 	8b4b9f2 pmoore@redhat.com 2013-02-15 12:21:43 -0500
> 	x86: remove the x32 syscall bitmask from syscall_get_nr()
> 
> broke audit and potentially other users of syscall_get_nr() which depend on
> that call as named.

Arguably audit is broken anyway by not correctly treating syscall numbers as 
32 bit integers like everyone else.

The commit above, 8b4b9f2, changed syscall_get_nr() so that it returned the 
same syscall number that is used by the architecture's ABI; just like every* 
other architecture in the kernel.

* Admittedly I didn't check every architecture's implementation, but after a 
half dozen I stopped checking as there was a definite trend.

{snip}

> diff --git a/arch/x86/include/asm/syscall.h b/arch/x86/include/asm/syscall.h
> index d6a756a..d58b6be 100644
> --- a/arch/x86/include/asm/syscall.h
> +++ b/arch/x86/include/asm/syscall.h
> @@ -236,6 +236,10 @@ static inline int syscall_get_arch(void)
>  		return AUDIT_ARCH_I386;
>  #endif
>  	/* Both x32 and x86_64 are considered "64-bit". */
> +#ifdef CONFIG_X86_X32_ABI
> +	if (task_pt_regs(current)->orig_ax & __X32_SYSCALL_BIT)
> +		return AUDIT_ARCH_X86_X32;
> +#endif

No.  See my comments above and in other parts of this thread.

>  	return AUDIT_ARCH_X86_64;
>  }
>  #endif	/* CONFIG_X86_32 */
> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> index b35c215..bc18214 100644
> --- a/kernel/seccomp.c
> +++ b/kernel/seccomp.c
> @@ -73,6 +73,12 @@ static void populate_seccomp_data(struct seccomp_data
> *sd)
> 
>  	sd->nr = syscall_get_nr(task, regs);
>  	sd->arch = syscall_get_arch();
> +#ifdef CONFIG_X86_X32_ABI
> +	if (sd->arch == AUDIT_ARCH_X86_X32) {
> +		sd->arch = AUDIT_ARCH_X86_64;
> +		sd->nr |= __X32_SYSCALL_BIT;
> +	}
> +#endif

Once again, I'm not really sure I need to comment further here, but don't 
change syscall_get_nr(), it should return the same syscall number as was used 
by userspace to initiate the syscall.  If you really want to use the new 
AUDIT_ARCH_X86_X32 macro/define, go ahead, but make sure you rewrite it to the 
x86-64 value here so as to not break compatibility with existing seccomp 
filter users.

-- 
paul moore
security and virtualization @ redhat