Re: [RFC] x86: restrict pid namespaces to 32 or 64 bit syscalls

From: H. Peter Anvin
Date: Fri Aug 12 2011 - 16:11:36 EST

On 08/12/2011 10:03 AM, Vasiliy Kulikov wrote:
> This patch allows x86-64 systems with 32 bit syscalls support to lock a
> pid namespace to 32 or 64 bitness syscalls/tasks. By denying rarely
> used compatibility syscalls it reduces an attack surface for 32 bit
> containers.
> The new sysctl is introduced, abi.bitness_locked. If set to 1, it locks
> all tasks inside of current pid namespace to the bitness of init task
> (pid_ns->child_reaper). After that:
> 1) a task trying to do a syscall of other bitness would get a signal as
> if the corresponding syscall is not enabled (IDT entry/MSR is not
> initialized).
> 2) loading ELF binaries of another bitness is prohibited (as if the
> corresponding CONFIG_BINFMT_*=N).
> If there is any task which differs in bitness, the lockup fails.
> In this patch version the lockup is handled by sysctl. In the future I
> plan to do it via prctl() to handle situations of container root
> compromize. For now, the lockup can be configured by init scripts,
> which parse /etc/sysctl.conf and set the sysctl variable. But if
> /sbin/init is compromized, the malicious code would gain a possibility
> to do arbitrary syscalls. So, it should be possible to lockup the
> container before the init execution.
> ( The asm stubs for denied syscalls might be buggy, if so - please
> ignore them :) it is just a PoC. )

NAK on this in its current form, as it breaks the upcoming x32 ABI.
Selection by ABI needs to be more specific.

However, I have to question the value of this... if this is enabled in
the system as a whole (as opposed to compiled out) it seems kind of
pointless... if there are bugs we need to deal with them anyway.

> Qestions/thoughts:
> The patch adds a check in syscalls code. Is it a significant
> slowdown for fast syscalls? If so, probably it worth moving the check
> into scheduler code and enabling/disabling corresponding interrupt/MSRs
> on each task switch?

*YOU* are the person who needs to answer that question by providing
measurements. Quite frankly I suspect checks in the syscall code *or*
task switching MSRs are going to be unacceptable from a performance
point of view.

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