Re: [PATCH v4 02/17] x86/entry/64/compat: Fix bad fast syscall arg failure path

Andy Lutomirski
2015-06-30T16:04:32Z

On Jun 30, 2015 3:59 AM, "Borislav Petkov" <bp@alien8.de> wrote:
>
> On Mon, Jun 29, 2015 at 12:33:34PM -0700, Andy Lutomirski wrote:
> > diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S
> > index bb187a6a877c..efe0b1e499fa 100644
> > --- a/arch/x86/entry/entry_64_compat.S
> > +++ b/arch/x86/entry/entry_64_compat.S
> > @@ -425,8 +425,39 @@ cstar_tracesys:
> > END(entry_SYSCALL_compat)
> >
> > ia32_badarg:
> > - ASM_CLAC
> > - movq $-EFAULT, RAX(%rsp)
> > + /*
> > + * So far, we've entered kernel mode, set AC, turned on IRQs, and
> > + * saved C regs except r8-r11. We haven't done any of the other
> > + * standard entry work, though. We want to bail, but we shouldn't
> > + * treat this as a syscall entry since we don't even know what the
> > + * args are. Instead, treat this as a non-syscall entry, finish
> > + * the entry work, and immediately exit after setting AX = -EFAULT.
> > + *
> > + * We're really just being polite here. Killing the task outright
> > + * would be a reasonable action, too. Given that the only valid
> > + * way to have gotten here is through the vDSO, and we already know
> > + * that the stack pointer is bad, the task isn't going to survive
> > + * for long no matter what we do.
>
> You mean something like
>
> force_sig_info(SIGSEGV, &si, current);
>
> ?
>
> I'd say we do it and not noodle unnecessarily with zeroing out pt_regs
> if the task is going to die anyway. IOW, make it die faster. :)

That's even more complicated. To send a signal, we need valid pt_regs
when we save our context in do_signal. I was thinking do_exit.

In any case, we can certainly change the behavior here, but I did it
this way at the beginning of the series because this patch mostly
preserves existing behavior while fixing what is arguably a bug and
because without something like this we'll have context tracking issues
later in the series.

My eventual goal is to try to turn entry_SYSENTER_32 into a minimal
piece of asm that pushes regs and calls a C function like
do_SYSENTER_32. Once that happens, we can just write:

enter_from_user_mode();

local_irq_enable();

if (__get_user(...) != 0) {
fail();
}

pt_regs->whatever = the thing we just read;

... and do the syscall

Given the current state of the asm, I think I'd rather get farther
along in the cleanups before trying to change behavior here.

--Andy

>
> > + */
> > +
> > + ASM_CLAC /* undo STAC */
> > + movq $-EFAULT, RAX(%rsp) /* return -EFAULT if possible */
> > +
> > + /* Fill in the rest of pt_regs */
> > + xorl %eax, %eax
> > + movq %rax, R11(%rsp)
> > + movq %rax, R10(%rsp)
> > + movq %rax, R9(%rsp)
> > + movq %rax, R8(%rsp)
> > + SAVE_EXTRA_REGS
> > +
> > + /* Turn IRQs back off. */
> > + DISABLE_INTERRUPTS(CLBR_NONE)
> > + TRACE_IRQS_OFF
> > +
> > + /* And exit again. */
> > + jmp retint_user
> > +
> > ia32_ret_from_sys_call:
> > xorl %eax, %eax /* Do not leak kernel information */
> > movq %rax, R11(%rsp)
> > --
> > 2.4.3
>
> --
> Regards/Gruss,
> Boris.
>
> ECO tip #101: Trim your mails when you reply.
> --