Re: [PATCH v20 22/28] x86/traps: Attempt to fixup exceptions in vDSO before signaling
From: Xing, Cedric
Date: Thu Jul 11 2019 - 18:12:44 EST
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...