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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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  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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