On Fri, Aug 20, 2021 at 11:03:00AM +0200, Peter Zijlstra wrote:
> On Fri, Aug 20, 2021 at 09:24:58AM +0100, Will Deacon wrote:
>
> > > gcc-10.2.1, x86_64-defconfig
> > >
> > > kernel/event/core.o-inline-ud1: 96454
> > > kernel/event/core.o-outofline-ud1: 96604
> > > kernel/event/core.o-outofline-call: 97072
>
> kernel/event/core.o-outofline-saturate-ud2: 96954
> kernel/event/core.o: 97248
>
> > Is that with the saturation moved to the UD handler as well?
>
> Yep, that's the full function call replaced with an exception.
>
> > I think it would be good to keep that as close to the point at which
> > we detect the problem as we can, so perhaps we can inline that part
> > and leave the diagnostics to the exception handler?
>
> That's simpler execption code too, we can abuse the existing WARN/UD2
> stuff.
>
> ---
> arch/x86/include/asm/refcount.h | 31 +++++++++++++++++++++++++++++++
> include/asm-generic/bug.h | 4 ++++
> include/linux/refcount.h | 15 +++++++++++----
> lib/bug.c | 13 ++++++++++++-
> lib/refcount.c | 7 ++-----
> 5 files changed, 60 insertions(+), 10 deletions(-)
>
> diff --git a/arch/x86/include/asm/refcount.h b/arch/x86/include/asm/refcount.h
> new file mode 100644
> index 000000000000..bed52b95d24c
> --- /dev/null
> +++ b/arch/x86/include/asm/refcount.h
> @@ -0,0 +1,31 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _ASM_X86_REFCOUNT_H
> +#define _ASM_X86_REFCOUNT_H
> +
> +#define refcount_warn_saturate refcount_warn_saturate
> +static __always_inline void refcount_warn_saturate(refcount_t *r, const enum refcount_saturation_type t)
> +{
> + refcount_set(r, REFCOUNT_SATURATED);
> + __WARN_FLAGS(BUGFLAG_ONCE|BUGFLAG_REFCOUNT|BUGFLAG_REFCOUNT_TYPE(t));
> +}
Instead of using up warn flags, what was done in
the past was to use an explicit EXTABLE in a cold text section:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/include/asm/asm.h?h=v4.15#n80
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/mm/extable.c?h=v4.15#n45
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/include/asm/refcount.h?h=v4.15
> +
> +#define refcount_dec_and_test refcount_dec_and_test
> +static inline bool refcount_dec_and_test(refcount_t *r)
> +{
> + asm_volatile_goto (LOCK_PREFIX "decl %[var]\n\t"
> + "jz %l[cc_zero]\n\t"
> + "jl %l[cc_error]"
> + : : [var] "m" (r->refs.counter)
> + : "memory" : cc_zero, cc_error);
> +
> + return false;
> +
> +cc_zero:
> + return true;
> +
> +cc_error:
> + refcount_warn_saturate(r, REFCOUNT_SUB_UAF);
> + return false;
> +}
This looks basically the same as what we had before (i.e. the earlier
REFCOUNT_CHECK_LE_ZERO within GEN_UNARY_SUFFIXED_RMWcc).
--
Kees Cook
