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

[Sean Christopherson]

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.

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...