Re: [PATCH v33 15/21] x86/vdso: Add support for exception fixup in vDSO functions

Andy Lutomirski
2020-06-30T16:48:38Z

On Tue, Jun 30, 2020 at 1:41 AM Borislav Petkov <bp@alien8.de> wrote:
>
> On Mon, Jun 29, 2020 at 11:00:55PM -0700, Sean Christopherson wrote:
> > E.g. the vDSO function should get the fixup even if userspace screws
> > up mmap() and invokes __vdso_sgx_enter_enclave() without being tagged
> > an SGX task.
>
> I sincerely hope you don't mean this seriously.
>
> Please add a member to task_struct which denotes that a task is an
> sgx task, test that member where needed and forget real quickly about
> running *any* *fixup* for unrelated tasks.

I don't see the problem. If you call this magic vDSO function and get
a fault, it gets handled. What's the failure mode?

>
> > No hard dependency, it's normal kernel code. My reasoning for dropping it
> > in .../vdso was largely to co-locate it with vdso/extable.h due to the
> > dependency on the format of 'struct vdso_exception_table_entry'.
>
> A struct which you defined instead of simply using struct
> exception_table_entry even if it has a handler member which would remain
> unused?

Don't forget the cross-arch issue. We need that structure to have
constant layout so that the -m32 build from the vDSO has the same
layout as in the kernel.

So my only actual objection to the patch is that there should probably
be a comment above the structure reminding people that it needs to
have the same layout on 32-bit, x32, and x86_64. So maybe the entries
should be s32 instead of int, although this doesn't really matter.