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

Borislav Petkov
2020-06-29T19:39:17Z

On Thu, Jun 18, 2020 at 01:08:37AM +0300, Jarkko Sakkinen wrote:
...
> intended benefit of massaging GCC's inlining algorithm is unlikely to
> realized in the vDSO any time soon, if ever.

That is a very good explanation and I would prefer if it would be in a
sgx-specific README or so instead of it getting lost in git...

> +bool fixup_vdso_exception(struct pt_regs *regs, int trapnr,
> + unsigned long error_code, unsigned long fault_addr)
> +{
> + const struct vdso_image *image = current->mm->context.vdso_image;
> + const struct vdso_exception_table_entry *extable;
> + unsigned int nr_entries, i;
> + unsigned long base;
> +
> + /*
> + * Do not attempt to fixup #DB or #BP. It's impossible to identify
> + * whether or not a #DB/#BP originated from within an SGX enclave and
> + * SGX enclaves are currently the only use case for vDSO fixup.
> + */

So this is all fine and dandy but nowhere do I see the code doing:

if (am_I_an_sgx_enclave(tsk))
fixup_vdso_exception()

because that vDSO exception fixup, albeit it looking kinda generic, is
SGX-only for now. So it should be designed to run only for SGX enclaves
for now.

Also, is there any particular reason for fixup_vdso_exception() to be in
arch/x86/entry/vdso/extable.c instead of in arch/x86/mm/extable.c?

I mean, it gets called by traps.c so it looks like normal kernel code to
me or am I missing some vdso magic?

And built only when CONFIG_INTEL_SGX is enabled.

And so on...

...

> diff --git a/arch/x86/entry/vdso/extable.h b/arch/x86/entry/vdso/extable.h
> new file mode 100644
> index 000000000000..aafdac396948
> --- /dev/null
> +++ b/arch/x86/entry/vdso/extable.h
> @@ -0,0 +1,29 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef __VDSO_EXTABLE_H
> +#define __VDSO_EXTABLE_H
> +
> +/*
> + * Inject exception fixup for vDSO code. Unlike normal exception fixup,
> + * vDSO uses a dedicated handler the addresses are relative to the overall
> + * exception table, not each individual entry.
> + */
> +#ifdef __ASSEMBLY__
> +#define _ASM_VDSO_EXTABLE_HANDLE(from, to) \
> + ASM_VDSO_EXTABLE_HANDLE from to
> +
> +.macro ASM_VDSO_EXTABLE_HANDLE from:req to:req
> + .pushsection __ex_table, "a"
> + .long (\from) - __ex_table
> + .long (\to) - __ex_table
> + .popsection
> +.endm
> +#else
> +#define _ASM_VDSO_EXTABLE_HANDLE(from, to) \
> + ".pushsection __ex_table, \"a\"\n" \
> + ".long (" #from ") - __ex_table\n" \
> + ".long (" #to ") - __ex_table\n" \
> + ".popsection\n"
> +#endif
> +
> +#endif /* __VDSO_EXTABLE_H */
> +

.git/rebase-apply/patch:122: new blank line at EOF.
+

Thx.

--
Regards/Gruss,
Boris.