From: Dave Martin <Dave.Martin@arm.com>
To: Mark Rutland <mark.rutland@arm.com>
Cc: "Paul Elliott" <paul.elliott@arm.com>,
"Peter Zijlstra" <peterz@infradead.org>,
"Catalin Marinas" <catalin.marinas@arm.com>,
"Will Deacon" <will.deacon@arm.com>,
"Yu-cheng Yu" <yu-cheng.yu@intel.com>,
"Amit Kachhap" <amit.kachhap@arm.com>,
"Vincenzo Frascino" <vincenzo.frascino@arm.com>,
linux-arch@vger.kernel.org,
"Eugene Syromiatnikov" <esyr@redhat.com>,
"Szabolcs Nagy" <szabolcs.nagy@arm.com>,
"H.J. Lu" <hjl.tools@gmail.com>,
"Andrew Jones" <drjones@redhat.com>,
"Kees Cook" <keescook@chromium.org>,
"Arnd Bergmann" <arnd@arndb.de>, "Jann Horn" <jannh@google.com>,
"Richard Henderson" <richard.henderson@linaro.org>,
"Kristina Martšenko" <kristina.martsenko@arm.com>,
"Mark Brown" <broonie@kernel.org>,
"Thomas Gleixner" <tglx@linutronix.de>,
linux-arm-kernel@lists.infradead.org,
"Florian Weimer" <fweimer@redhat.com>,
linux-kernel@vger.kernel.org, "Sudakshina Das" <sudi.das@arm.com>
Subject: Re: [PATCH v2 05/12] arm64: Basic Branch Target Identification support
Date: Fri, 11 Oct 2019 17:01:13 +0100 [thread overview]
Message-ID: <20191011160113.GO27757@arm.com> (raw)
In-Reply-To: <20191011154444.GN27757@arm.com>
On Fri, Oct 11, 2019 at 04:44:45PM +0100, Dave Martin wrote:
> On Fri, Oct 11, 2019 at 04:40:43PM +0100, Mark Rutland wrote:
> > On Fri, Oct 11, 2019 at 04:32:26PM +0100, Dave Martin wrote:
> > > On Fri, Oct 11, 2019 at 11:25:33AM -0400, Richard Henderson wrote:
> > > > On 10/11/19 11:10 AM, Mark Rutland wrote:
> > > > > On Thu, Oct 10, 2019 at 07:44:33PM +0100, Dave Martin wrote:
> > > > >> @@ -730,6 +730,11 @@ static void setup_return
> > > > >> regs->regs[29] = (unsigned long)&user->next_frame->fp;
> > > > >> regs->pc = (unsigned long)ka->sa.sa_handler;
> > > > >>
> > > > >> + if (system_supports_bti()) {
> > > > >> + regs->pstate &= ~PSR_BTYPE_MASK;
> > > > >> + regs->pstate |= PSR_BTYPE_CALL;
> > > > >> + }
> > > > >> +
> > > > >
> > > > > I think we might need a comment as to what we're trying to ensure here.
> > > > >
> > > > > I was under the (perhaps mistaken) impression that we'd generate a
> > > > > pristine pstate for a signal handler, and it's not clear to me that we
> > > > > must ensure the first instruction is a target instruction.
> > > >
> > > > I think it makes sense to treat entry into a signal handler as a call. Code
> > > > that has been compiled for BTI, and whose page has been marked with PROT_BTI,
> > > > will already have the pauth/bti markup at the beginning of the signal handler
> > > > function; we might as well verify that.
> > > >
> > > > Otherwise sigaction becomes a hole by which an attacker can force execution to
> > > > start at any arbitrary address.
> > >
> > > Ack, that's the intended rationale -- I also outlined this in the commit
> > > message.
> >
> > Ah, sorry. I evidently did not read that thoroughly enough.
> >
> > > Does this sound reasonable?
> > >
> > >
> > > Either way, I feel we should do this: any function in a PROT_BTI page
> > > should have a suitable landing pad. There's no reason I can see why
> > > a protection given to any other callback function should be omitted
> > > for a signal handler.
> > >
> > > Note, if the signal handler isn't in a PROT_BTI page then overriding
> > > BTYPE here will not trigger a Branch Target exception.
> > >
> > > I'm happy to drop a brief comment into the code also, once we're
> > > agreed on what the code should be doing.
> >
> > So long as there's a comment as to why, I have no strong feelings here.
> > :)
>
> OK, I think it's worth a brief comment in the code either way, so I'll
> add something.
Hmm, come to think of it we do need special logic for a particular case
here:
If we are delivering a SIGILL here and the SIGILL handler was registered
with SA_NODEFER then we will get into a spin, repeatedly delivering
the BTI-triggered SIGILL to the same (bad) entry point.
Without SA_NODEFER, the SIGILL becomes fatal, which is the desired
behaviour, but we'll need to catch this recursion explicitly.
It's similar to the special force_sigsegv() case in
linux/kernel/signal.c...
Thoughts?
Cheers
---Dave
next prev parent reply other threads:[~2019-10-11 16:01 UTC|newest]
Thread overview: 48+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-10 18:44 [PATCH v2 00/12] arm64: ARMv8.5-A: Branch Target Identification support Dave Martin
2019-10-10 18:44 ` [PATCH v2 01/12] ELF: UAPI and Kconfig additions for ELF program properties Dave Martin
2019-10-10 18:44 ` [PATCH v2 02/12] ELF: Add ELF program property parsing support Dave Martin
2019-10-10 18:44 ` [PATCH v2 03/12] mm: Reserve asm-generic prot flag 0x10 for arch use Dave Martin
2019-10-10 18:44 ` [PATCH v2 04/12] arm64: docs: cpu-feature-registers: Document ID_AA64PFR1_EL1 Dave Martin
2019-10-11 13:19 ` Alex Bennée
2019-10-11 14:51 ` Dave Martin
2019-10-21 19:18 ` Mark Brown
2019-10-22 10:32 ` Will Deacon
2019-10-10 18:44 ` [PATCH v2 05/12] arm64: Basic Branch Target Identification support Dave Martin
2019-10-11 15:06 ` [FIXUP 0/2] Fixups to patch 5 Dave Martin
2019-10-11 15:06 ` [FIXUP 1/2] squash! arm64: Basic Branch Target Identification support Dave Martin
2019-10-11 15:06 ` [FIXUP 2/2] " Dave Martin
2019-10-11 15:10 ` [PATCH v2 05/12] " Mark Rutland
2019-10-11 15:25 ` Richard Henderson
2019-10-11 15:32 ` Dave Martin
2019-10-11 15:40 ` Mark Rutland
2019-10-11 15:44 ` Dave Martin
2019-10-11 16:01 ` Dave Martin [this message]
2019-10-11 16:42 ` Dave Martin
2019-10-18 11:05 ` Mark Rutland
2019-10-18 13:36 ` Dave Martin
2019-10-11 17:20 ` Dave Martin
2019-10-18 11:10 ` Mark Rutland
2019-10-18 13:37 ` Dave Martin
2019-10-18 11:16 ` Mark Rutland
2019-10-18 13:40 ` Dave Martin
2019-10-10 18:44 ` [PATCH v2 06/12] elf: Allow arch to tweak initial mmap prot flags Dave Martin
2019-10-10 18:44 ` [PATCH v2 07/12] arm64: elf: Enable BTI at exec based on ELF program properties Dave Martin
2019-10-10 18:44 ` [PATCH v2 08/12] arm64: BTI: Decode BYTPE bits when printing PSTATE Dave Martin
2019-10-11 15:31 ` Richard Henderson
2019-10-11 15:33 ` Dave Martin
2019-10-10 18:44 ` [PATCH v2 09/12] arm64: traps: Fix inconsistent faulting instruction skipping Dave Martin
2019-10-11 15:24 ` Mark Rutland
2019-10-15 15:21 ` Dave Martin
2019-10-15 16:42 ` Mark Rutland
2019-10-15 16:49 ` Dave Martin
2019-10-18 16:40 ` Dave Martin
2019-10-22 11:09 ` Robin Murphy
2019-10-10 18:44 ` [PATCH v2 10/12] arm64: traps: Shuffle code to eliminate forward declarations Dave Martin
2019-10-10 18:44 ` [PATCH v2 11/12] arm64: BTI: Reset BTYPE when skipping emulated instructions Dave Martin
2019-10-11 14:21 ` Mark Rutland
2019-10-11 14:47 ` Dave Martin
2019-10-18 11:04 ` Mark Rutland
2019-10-18 14:49 ` Dave Martin
2019-10-10 18:44 ` [PATCH v2 12/12] KVM: " Dave Martin
2019-10-11 14:24 ` Mark Rutland
2019-10-11 14:44 ` Dave Martin
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