From: Peter Collingbourne <pcc@google.com>
To: Dave Martin <Dave.Martin@arm.com>
Cc: Andrey Konovalov <andreyknvl@google.com>,
Kevin Brodsky <kevin.brodsky@arm.com>,
Kostya Serebryany <kcc@google.com>,
Evgenii Stepanov <eugenis@google.com>,
Catalin Marinas <catalin.marinas@arm.com>,
Vincenzo Frascino <vincenzo.frascino@arm.com>,
Will Deacon <will@kernel.org>,
Linux ARM <linux-arm-kernel@lists.infradead.org>,
Richard Henderson <rth@twiddle.net>
Subject: Re: [PATCH v3] arm64: Expose original FAR_EL1 value in sigcontext
Date: Wed, 13 May 2020 11:00:08 -0700 [thread overview]
Message-ID: <CAMn1gO5GvTjUwFZh=JqWw6Xfx+TkaT7zWNb=ykSzjrVApYcuqg@mail.gmail.com> (raw)
In-Reply-To: <20200513172745.GX21779@arm.com>
On Wed, May 13, 2020 at 10:27 AM Dave Martin <Dave.Martin@arm.com> wrote:
>
> On Thu, May 07, 2020 at 10:55:02AM -0700, Peter Collingbourne wrote:
> > On Mon, May 4, 2020 at 3:19 AM Dave Martin <Dave.Martin@arm.com> wrote:
> > >
> > > On Fri, Mar 27, 2020 at 12:19:15PM -0700, Peter Collingbourne wrote:
> > > > The kernel currently clears the tag bits (i.e. bits 56-63) in the fault
> > > > address exposed via siginfo.si_addr and sigcontext.fault_address. However,
> > > > the tag bits may be needed by tools in order to accurately diagnose
> > > > memory errors, such as HWASan [1] or future tools based on the Memory
> > > > Tagging Extension (MTE).
> > > >
> > > > We should not stop clearing these bits in the existing fault address
> > > > fields, because there may be existing userspace applications that are
> > > > expecting the tag bits to be cleared. Instead, create a far_context in
> > > > sigcontext (similar to the existing esr_context), and store the original
> > > > value of FAR_EL1 (including the tag bits) there.
> > > >
> > > > [1] http://clang.llvm.org/docs/HardwareAssistedAddressSanitizerDesign.html
> > > >
> > > > Signed-off-by: Peter Collingbourne <pcc@google.com>
> > > > ---
> > > > v3:
> > > > - add documentation to tagged-pointers.rst
> > > > - update comments in sigcontext.h
> > > >
> > > > v2:
> > > > - revert changes to hw_breakpoint.c
> > > > - rename set_thread_esr to set_thread_far_esr
> > > >
> > > > Documentation/arm64/tagged-pointers.rst | 17 +++++----
> > > > arch/arm64/include/asm/exception.h | 2 +-
> > > > arch/arm64/include/asm/processor.h | 2 +-
> > > > arch/arm64/include/uapi/asm/sigcontext.h | 21 +++++++----
> > > > arch/arm64/kernel/entry-common.c | 2 --
> > > > arch/arm64/kernel/signal.c | 20 ++++++++++-
> > > > arch/arm64/mm/fault.c | 45 ++++++++++++++----------
> > > > 7 files changed, 74 insertions(+), 35 deletions(-)
> > >
> > > [...]
> > >
> > > > diff --git a/arch/arm64/include/uapi/asm/sigcontext.h b/arch/arm64/include/uapi/asm/sigcontext.h
> > > > index 8b0ebce92427..6782394633cb 100644
> > > > --- a/arch/arm64/include/uapi/asm/sigcontext.h
> > > > +++ b/arch/arm64/include/uapi/asm/sigcontext.h
> > > > @@ -44,11 +44,12 @@ struct sigcontext {
> > > > *
> > > > * 0x210 fpsimd_context
> > > > * 0x10 esr_context
> > > > + * 0x10 far_context
> > > > * 0x8a0 sve_context (vl <= 64) (optional)
> > > > * 0x20 extra_context (optional)
> > > > * 0x10 terminator (null _aarch64_ctx)
> > > > *
> > > > - * 0x510 (reserved for future allocation)
> > > > + * 0x500 (reserved for future allocation)
> > > > *
> > > > * New records that can exceed this space need to be opt-in for userspace, so
> > > > * that an expanded signal frame is not generated unexpectedly. The mechanism
> > > > @@ -94,17 +95,25 @@ struct esr_context {
> > > > __u64 esr;
> > > > };
> > > >
> > > > +/* FAR_EL1 context */
> > > > +#define FAR_MAGIC 0x46415201
> > > > +
> > > > +struct far_context {
> > > > + struct _aarch64_ctx head;
> > > > + __u64 far;
> > > > +};
> > > > +
> > > > /*
> > > > * extra_context: describes extra space in the signal frame for
> > > > * additional structures that don't fit in sigcontext.__reserved[].
> > > > *
> > > > * Note:
> > > > *
> > > > - * 1) fpsimd_context, esr_context and extra_context must be placed in
> > > > - * sigcontext.__reserved[] if present. They cannot be placed in the
> > > > - * extra space. Any other record can be placed either in the extra
> > > > - * space or in sigcontext.__reserved[], unless otherwise specified in
> > > > - * this file.
> > > > + * 1) fpsimd_context, esr_context, far_context and extra_context must be
> > > > + * placed in sigcontext.__reserved[] if present. They cannot be placed
> > > > + * in the extra space. Any other record can be placed either in the
> > > > + * extra space or in sigcontext.__reserved[], unless otherwise specified
> > > > + * in this file.
> > >
> > > This is for backwards compatibility only. We don't need this constraint
> > > for any new field, so you can probably leave the paragraph as-is.
> > >
> > > Removing this would mean constraint would mean that userspace must be
> > > prepared to traverse extra_context when looking for far_context. But
> > > really we want modern userspace to do this anyway, since it reduces
> > > backwards compatibilty worries when adding more new records in the
> > > future.
> >
> > My original reason for updating this comment was that I figured that
> > this record was small enough that we could just always include it in
> > __reserved.
> >
> > But thinking about this a bit more, it doesn't seem that just wanting
> > userspace to read extra_context will guarantee that it will do so. In
> > practice, it would be easy to write userspace code that works right
> > now but doesn't read extra_context correctly (either because
> > extra_context wasn't considered at all, or because the code purporting
> > to read the record from extra_context contains a latent bug because it
> > wasn't exercised). Since we may be practically constrained from moving
> > the record anyway, we might as well document it and allow the
> > userspace code to be a little simpler.
> >
> > I guess one alternative is that we always place this record in
> > extra_context, which would force userspace to read it correctly. That
> > has something of the opposite problem (userspace code could be written
> > to only expect the record in extra_context), but at least we're less
> > constrained there, and it's more likely that the code would be parsing
> > __reserved correctly since it would need to do so in order to find
> > extra_context.
> >
> > Anyway, I've reverted the comment change for now in v4, but let me
> > know what you think.
>
> Apologies for the delay in responding -- I think it does make sense to
> reserve space in __reserved[] for the new record, the the location you
> suggested for it is sensible.
>
> __reserved[] is a scarce resource, and should only be burned on "small"
> records, but far_context is small.
>
>
> here's another reason too, which is that we don't want to needlessly
> block new software from using this field without allocating larger
> stacks -- not least because they just won't, and the problem won't
> bite them until much later.
>
>
> Hope that helps clarify things.
Thanks, that makes sense. I will send a v6 with the comment brought back.
Peter
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prev parent reply other threads:[~2020-05-13 18:00 UTC|newest]
Thread overview: 63+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-12 17:17 [PATCH] arm64: Expose original FAR_EL1 value in sigcontext Peter Collingbourne
2020-03-25 13:10 ` Catalin Marinas
2020-03-25 17:41 ` Peter Collingbourne
2020-03-25 17:40 ` [PATCH v2] " Peter Collingbourne
2020-03-26 16:45 ` Catalin Marinas
2020-03-27 7:56 ` Will Deacon
2020-03-27 11:39 ` Catalin Marinas
2020-03-27 19:26 ` Peter Collingbourne
2020-03-27 19:19 ` [PATCH v3] " Peter Collingbourne
2020-04-22 14:25 ` Catalin Marinas
2020-04-29 21:08 ` Will Deacon
2020-04-29 21:42 ` Peter Collingbourne
2020-05-04 17:03 ` Will Deacon
2020-05-07 17:57 ` [PATCH v4] arm64: Expose FAR_EL1 tag bits " Peter Collingbourne
2020-05-08 2:01 ` [PATCH v5] " Peter Collingbourne
2020-05-12 16:25 ` Catalin Marinas
2020-05-13 18:09 ` [PATCH v6] " Peter Collingbourne
2020-05-13 20:28 ` Dave Martin
2020-05-15 0:58 ` Peter Collingbourne
2020-05-18 9:53 ` Dave Martin
2020-05-19 22:00 ` Peter Collingbourne
2020-05-20 8:55 ` Will Deacon
2020-05-20 9:26 ` Dave Martin
2020-05-21 2:28 ` Peter Collingbourne
2020-05-21 2:29 ` [PATCH v6 0/3] " Peter Collingbourne
2020-05-21 2:29 ` [PATCH v6 1/3] signal: Allow architectures to store arch-specific data in kernel_siginfo Peter Collingbourne
2020-05-21 2:29 ` [PATCH v6 2/3] arm64: Move fault address and fault code into kernel_siginfo Peter Collingbourne
2020-05-21 13:34 ` kbuild test robot
2020-05-21 2:29 ` [PATCH v6 3/3] arm64: Expose FAR_EL1 tag bits in sigcontext Peter Collingbourne
2020-05-21 12:35 ` [PATCH v6] " Eric W. Biederman
2020-05-21 18:03 ` Peter Collingbourne
2020-05-21 19:24 ` Eric W. Biederman
2020-05-21 20:48 ` Peter Collingbourne
2020-06-08 18:12 ` Peter Collingbourne
2020-06-08 18:14 ` [PATCH v7] arm64: Expose FAR_EL1 tag bits in siginfo Peter Collingbourne
[not found] ` <20200623020134.16655-1-pcc@google.com>
[not found] ` <87sgemrlgc.fsf@x220.int.ebiederm.org>
2020-06-23 14:38 ` [PATCH v8] " Dave Martin
2020-06-23 17:47 ` Eric W. Biederman
2020-06-24 0:40 ` Peter Collingbourne
2020-06-24 9:28 ` Dave Martin
2020-06-24 16:51 ` Peter Collingbourne
2020-06-24 17:12 ` Dave Martin
2020-06-24 19:51 ` Peter Collingbourne
2020-07-06 16:41 ` Dave Martin
2020-07-06 19:20 ` Peter Collingbourne
2020-07-07 14:19 ` Dave Martin
2020-07-07 19:07 ` Peter Collingbourne
2020-07-08 11:00 ` Dave Martin
2020-07-08 13:58 ` Dave Martin
2020-07-08 22:21 ` Peter Collingbourne
2020-07-13 13:24 ` Dave Martin
2020-07-13 20:50 ` Peter Collingbourne
2020-07-14 17:36 ` Dave Martin
2020-08-18 3:16 ` Peter Collingbourne
2020-08-18 13:50 ` Dave Martin
2020-06-23 14:57 ` Dave Martin
2020-05-26 13:03 ` [PATCH v6] arm64: Expose FAR_EL1 tag bits in sigcontext Dave Martin
2020-04-30 9:50 ` [PATCH v3] arm64: Expose original FAR_EL1 value " Catalin Marinas
2020-04-30 9:59 ` Will Deacon
2020-04-30 13:34 ` Catalin Marinas
2020-05-04 10:19 ` Dave Martin
2020-05-07 17:55 ` Peter Collingbourne
2020-05-13 17:27 ` Dave Martin
2020-05-13 18:00 ` Peter Collingbourne [this message]
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