Re: [PATCH v20 22/28] x86/traps: Attempt to fixup exceptions in vDSO before signaling

["Xing, Cedric" <cedric.xing@intel.com>]

On 7/11/2019 8:54 AM, Sean Christopherson wrote:
> On Thu, Jun 27, 2019 at 01:32:58PM -0700, Xing, Cedric wrote:
>> Just a reminder that #DB/#BP shall be treated differently because they are
>> used by debuggers. So instead of branching to the fixup address, the kernel
>> shall just signal the process.
> 
> More importantly, doing fixup on #DB and #BP simply doesn't work.

What's really needed is a signal, as if the fixup entry didn't exist.

You don't have to care whether a debugger is attached or not.

> On Tue, Apr 23, 2019 at 11:59:37AM -0700, Sean Christopherson wrote:
>> On Mon, Apr 22, 2019 at 06:29:06PM -0700, Andy Lutomirski wrote:
>>> What's not tested here is running this code with EFLAGS.TF set and
>>> making sure that it unwinds correctly.  Also, Jarkko, unless I missed
>>> something, the vDSO extable code likely has a bug.  If you run the
>>> instruction right before ENCLU with EFLAGS.TF set, then do_debug()
>>> will eat the SIGTRAP and skip to the exception handler.  Similarly, if
>>> you put an instruction breakpoint on ENCLU, it'll get skipped.  Or is
>>> the code actually correct and am I just remembering wrong?
>>
>> The code is indeed broken, and I don't see a sane way to make it not
>> broken other than to never do vDSO fixup on #DB or #BP.  But that's
>> probably the right thing to do anyways since an attached debugger is
>> likely the intended recipient the 99.9999999% of the time.
>>
>> The crux of the matter is that it's impossible to identify whether or
>> not a #DB/#BP originated from within an enclave, e.g. an INT3 in an
>> enclave will look identical to an INT3 at the AEP.  Even if hardware
>> provided a magic flag, #DB still has scenarios where the intended
>> recipient is ambiguous, e.g. data breakpoint encountered in the enclave
>> but on an address outside of the enclave, breakpoint encountered in the
>> enclave and a code breakpoint on the AEP, etc...