From: Dave Martin <Dave.Martin@arm.com>
To: Mark Brown <broonie@kernel.org>
Cc: Tamas Zsoldos <tamas.zsoldos@arm.com>,
Will Deacon <will@kernel.org>,
kernel-team@android.com, linux-arm-kernel@lists.infradead.org,
Daniel Kiss <daniel.kiss@arm.com>
Subject: Re: [PATCH 1/3] arm64: vdso: Don't prefix sigreturn trampoline with a BTI C instruction
Date: Wed, 20 May 2020 12:08:33 +0100 [thread overview]
Message-ID: <20200520110833.GP5031@arm.com> (raw)
In-Reply-To: <20200520104653.GA4823@sirena.org.uk>
On Wed, May 20, 2020 at 11:46:53AM +0100, Mark Brown wrote:
> On Wed, May 20, 2020 at 10:48:45AM +0100, Dave Martin wrote:
> > On Tue, May 19, 2020 at 04:42:47PM +0100, Mark Brown wrote:
> > > On Tue, May 19, 2020 at 03:55:15PM +0100, Dave Martin wrote:
>
> > > > > > If BTI {nothing} allows this while disallowing all BR/BLR then we could
> > > > > > use that (I can't remember what BTI {nothing} is useful for, if anything).
>
> > > > > > Otherwise, it's less clear what we should have here.
>
> > > > > I can't remember anything that distinguishes it from an explicit NOP.
>
> > > > I think it rejects everything other then fallthrough execution
> > > > (BTYPE==0, which includes RET). I might have misunderstood something
>
> > > Right, but since BTI only generates an exception when BTYPE != 0 I'm
> > > having trouble differentiating this from a NOP in practical terms.
>
> > The idea would be that if an attacker could fudge some function pointer
> > to point at __kernel_rt_sigreturn, attempting to do a call via that
> > pointer would still trigger a BTI trap.
>
> We'll get a BTI exception no matter what instruction is here so long as
> it's not an appropriate BTI landing pad so unless we want to prevent one
> being generated there's no need to change the instruction sequence. Or
> perhaps I'm not quite getting the scenario you're thinking of?
Duh, yes. I guess we're good, then.
Cheers
---Dave
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next prev parent reply other threads:[~2020-05-20 11:08 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-19 12:18 [PATCH 0/3] arm64 sigreturn unwinding fixes Will Deacon
2020-05-19 12:18 ` [PATCH 1/3] arm64: vdso: Don't prefix sigreturn trampoline with a BTI C instruction Will Deacon
2020-05-19 12:38 ` Mark Brown
2020-05-19 13:25 ` Dave Martin
2020-05-19 14:35 ` Mark Brown
2020-05-19 14:55 ` Dave Martin
2020-05-19 15:42 ` Mark Brown
2020-05-20 9:48 ` Dave Martin
2020-05-20 10:46 ` Mark Brown
2020-05-20 11:08 ` Dave Martin [this message]
2020-05-19 15:26 ` Will Deacon
2020-05-19 13:21 ` Dave Martin
2020-05-19 13:29 ` Will Deacon
2020-05-19 12:18 ` [PATCH 2/3] arm64: vdso: Add a comment to justify the mysterious NOP in sigreturn Will Deacon
2020-05-19 13:26 ` Dave Martin
2020-05-19 12:18 ` [PATCH 3/3] arm64: vdso: Fix CFI directives in sigreturn trampoline Will Deacon
2020-05-19 13:09 ` Dave P Martin
2020-05-19 13:39 ` Will Deacon
2020-05-19 13:55 ` Dave Martin
2020-05-19 15:24 ` Will Deacon
2020-05-19 15:30 ` Daniel Kiss
2020-05-19 15:55 ` Will Deacon
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