Re: [PATCH 17/17] x86: simplify iret stack handling on SYSCALL64 fastpath

[Andy Lutomirski]

On Mon, Aug 11, 2014 at 5:24 AM, Denys Vlasenko <dvlasenk@xxxxxxxxxx> wrote:
> On 08/11/2014 12:42 AM, Andy Lutomirski wrote:
>> On Mon, Aug 11, 2014 at 12:00 AM, Denys Vlasenko <dvlasenk@xxxxxxxxxx> wrote:
>>> On 08/09/2014 12:59 AM, Andy Lutomirski wrote:
>>>>> + * When returning through fast path, userspace sees rcx = return address,
>>>>> + * r11 = rflags. When returning through iret (e.g. if audit is active),
>>>>> + * these registers may contain garbage.
>>>>> + * For ptrace we manage to avoid that: when we hit slow path on entry,
>>>>> + * we do save rcx and r11 in pt_regs, so ptrace on exit also sees them.
>>>>> + * If slow path is entered only on exit, there will be garbage.
>>>>
>>>> I don't like this.  At least the current code puts something
>>>> deterministic in there (-1) for the slow path, even though it's wrong
>>>> and makes the slow path behave visibly differently from the fast path.
>>>>
>>>> Leaking uninitialized data here is extra bad, though.  Keep in mind
>>>> that, when a syscall entry is interrupted before fully setting up
>>>> pt_regs, the interrupt frame overlaps task_pt_regs, so it's possible,
>>>> depending on the stack slot ordering, for a kernel secret
>>>> (kernel_stack?) to end up somewhere in task_pt_regs.
>>>
>>> It's easy to fix. We jump off fast path to slow path here:
>>>
>>>         movl TI_flags+THREAD_INFO(%rsp,SIZEOF_PTREGS),%edx
>>>         andl %edi,%edx
>>>         jnz  sysret_careful
>>>
>>> This is the only use of "sysret_careful" label.
>>> Therefore, there we don't need to think about any other scenarios
>>> besides "we are returning from syscall here".
>>
>> I may be missing something here (on vacation, not really testing
>> things, no big monitor, etc), but how is this compatible with things
>> like rt_sigreturn?  rt_sigreturn is call from the fastpath, via a
>> stub, and it returns through int_ret_from_syscall.  The C part needs
>> to modify all the regs, and those regs need to stick, so fixing up rcx
>> and r11 after rt_sigreturn can't work.
>
> Code at "sysret_careful" label is only reachable
> on fast path return.
> We don't go down this code path after rt_sigreturn.
> stub_rt_sigreturn manually steers into iret code path instead:
>
> ENTRY(stub_rt_sigreturn)
>         CFI_STARTPROC
>         addq $8, %rsp
>         DEFAULT_FRAME 0
>         SAVE_EXTRA_REGS
>         STORE_IRET_FRAME_CS_SS
>         call sys_rt_sigreturn
>         movq %rax,RAX(%rsp)
>         RESTORE_EXTRA_REGS
>         jmp int_ret_from_sys_call   <==== NOTE THIS
>
> So, we don't do any rcx/r11 fixups after sys_rt_sigreturn.

Oh, right.  rt_sigreturn overwrites all regs, so it doesn't need a
fixup in advance.

That still leaves fork and everything that calls ptrace_event, though.

--Andy
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