linux-kernel - Re: [tip:x86/vdso] x86/vdso32/syscall.S: Do not load __USER32

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Date:	Thu, 23 Apr 2015 02:07:19 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Brian Gerst <brgerst@...il.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andy Lutomirski <luto@...nel.org>,
	Will Drewry <wad@...omium.org>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Kees Cook <keescook@...omium.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"linux-tip-commits@...r.kernel.org" 
	<linux-tip-commits@...r.kernel.org>
Subject: Re: [tip:x86/vdso] x86/vdso32/syscall.S: Do not load __USER32_DS to %ss

On Thu, Apr 23, 2015 at 1:49 AM, Andy Lutomirski <luto@...capital.net> wrote:
>
> I'm curious whether we can somehow end up in the kernel without a
> sensible SS.  What happens if we have SS = 0?
>
> Try this on for size:
>
> 1. Wine process does syscall
> 2. Context switch to any other task
> 3. Interrupt (software or hardware), which loads SS with ss0, which is
> 0 on x86_64.
> 4. Context switch back to Wine.
> 5. sysretl
>
> Would fixing this be as simple as changing this code in
> arch/x86/kernel/process.c:
>
> __visible DEFINE_PER_CPU_SHARED_ALIGNED(struct tss_struct, cpu_tss) = {
>         .x86_tss = {
>                 .sp0 = TOP_OF_INIT_STACK,
> #ifdef CONFIG_X86_32
>                 .ss0 = __KERNEL_DS,
>
> by moving the ifdef down a line?  Even if that fixed it, it would be
> extremely fragile, but IMO it would be a good change to make
> regardless (i.e. the kernel's SS would be less unpredictable).

Confirmed with KVM on VMX: we can definitely end up in the kernel with SS == 0.

I don't know whether changing ss0 would be a good idea, though.  It
would be cleaner, but it could slow down interrupt processing:
interrupt delivery would have to do an extra GDT load.

Food for thought: wouldn't this mean that we have a bug on sysretq
too?  If we're in the kernel with SS == 0, we do sysretq, and then
user code does a far jump to 32-bit code, then we end up with a bogus
SS.  Maybe we don't care, and reloading SS on every sysretq would
suck.  We could fix it up in a kind of evil way: in do_stack_segment,
we could detect that we had SS == __USER_DS, in which case #SS should
be impossible, and just return without signalling the process.  IRET
would fix up the attributes.

We just might need a stable fix, though -- I wonder if there's any bad
interaction with opportunistic sysret in 4.0.  Maybe we should
benchmark ss0 = __KERNEL_DS and try it after all.

--Andy
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