Re: Getting rid of invalid SYSCALL RSP under Xen?

[Andrew Cooper]

On 27/07/15 00:27, Andy Lutomirski wrote:
>
>>>>> For SYSRET, I think the way to go is to force Xen to always use the
>>>>> syscall slow path.  Instead, Xen could hook into
>>>>> syscall_return_via_sysret or even right before the opportunistic
>>>>> sysret stuff.  Then we could remove the USERGS_SYSRET hooks entirely.
>>>>>
>>>>> Would this work?
>>>> None of the opportunistic sysret stuff makes sense under Xen.  The path
>>>> will inevitably end up in xen_iret making a hypercall.  Short circuiting
>>>> all of this seems like a good idea, especially if it allows for the
>>>> removal of the UERGS_SYSRET.
>>> Doesn't Xen decide what to do based on VGCF_IN_SYSCALL?  Maybe Xen
>>> should have its own opportunistic VGCF_IN_SYSCALL logic.
>> VGCF_in_syscall affects whether the extra r11/rcx get restored or not,
>> as the hypercall itself is implemented using syscall.  As the extra
>> r11/rcx (and rax for that matter) are unconditionally saved in the
>> hypercall stub, I can't see anything Linux could usefully do,
>> opportunistically speaking.
> Xen does:
>
> /* %rbx: struct vcpu, interrupts disabled */
> restore_all_guest:
>         ASSERT_INTERRUPTS_DISABLED
>         RESTORE_ALL
>         testw $TRAP_syscall,4(%rsp)
>         jz    iret_exit_to_guest
>
>         /* Don't use SYSRET path if the return address is not canonical. */
>         movq  8(%rsp),%rcx
>         sarq  $47,%rcx
>         incl  %ecx
>         cmpl  $1,%ecx
>         ja    .Lforce_iret
>
>         cmpw  $FLAT_USER_CS32,16(%rsp)# CS
>         movq  8(%rsp),%rcx            # RIP
>         movq  24(%rsp),%r11           # RFLAGS
>         movq  32(%rsp),%rsp           # RSP
>         je    1f
>         sysretq
> 1:      sysretl
>
> That's essentially the same thing as opportunistic sysret.  If Linux
> stops setting VGCF_in_syscall, though, I think we'll bypass that code,
> which will hurt performance.  Whether this should be fixed in the
> hypervisor or in the guest kernel hooks, I don't know, but it would be
> easy to have a very simple xen_opportunistic_sysret path that checks
> rcx==rip and r11==rflags and, if so, sets VGCF_in_syscall.

I see your point.  I didn't intend to suggest that Linux should stop
setting VGCF_in_syscall, as it is the only entity which knows whether it
is safe to clobber rcx/r11 in user context.

Having said this, Xen could certainly do its own opportunistic sysret
calculations as well.  There are a number of issues in the Xen sysret
code which I plan to fix in due course, and I will see about making this
adjustment.

>
>>> Hmm, maybe some of this would be easier to think about if, rather than
>>> having a paravirt op, we could have:
>>>
>>> ALTERNATIVE "", "jmp xen_pop_things_and_iret", X86_FEATURE_XEN
>>>
>>> Or just IF_XEN("jmp ...");
>>>
>>> As a practical matter, x86_64 has native and Xen -- I don't think
>>> there's any other paravirt platform that needs the asm hooks.
>> It would certainly seem so.  A careful use of IF_XEN() or two would make
>> the code far clearer to read, and to drop the hooks.
>>
> Want to add an IF_XEN macro?

I currently have a blocker bug against the impending Xen 4.6 release
which is higher on my todo list, but I will look into this as soon as I can.

~Andrew
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