Re: [PATCH v17 03/15] arm64: Introduce prctl() options to control the tagged user addresses ABI

From: Catalin Marinas <catalin.marinas@arm.com>
To: Dave Martin <Dave.Martin@arm.com>
Cc: Andrey Konovalov <andreyknvl@google.com>,
	linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org, amd-gfx@lists.freedesktop.org,
	dri-devel@lists.freedesktop.org, linux-rdma@vger.kernel.org,
	linux-media@vger.kernel.org, kvm@vger.kernel.org,
	linux-kselftest@vger.kernel.org,
	Mark Rutland <mark.rutland@arm.com>,
	Szabolcs Nagy <Szabolcs.Nagy@arm.com>,
	Will Deacon <will.deacon@arm.com>,
	Kostya Serebryany <kcc@google.com>,
	Khalid Aziz <khalid.aziz@oracle.com>,
	Felix Kuehling <Felix.Kuehling@amd.com>,
	Vincenzo Frascino <vincenzo.frascino@arm.com>,
	Jacob Bramley <Jacob.Bramley@arm.com>,
	Leon Romanovsky <leon@kernel.org>,
	Christoph Hellwig <hch@infradead.org>,
	Jason Gunthorpe <jgg@ziepe.ca>,
	Evgeniy Stepanov <eugenis@google.com>
Subject: Re: [PATCH v17 03/15] arm64: Introduce prctl() options to control the tagged user addresses ABI
Date: Thu, 13 Jun 2019 16:35:07 +0100	[thread overview]
Message-ID: <20190613153505.GU28951@C02TF0J2HF1T.local> (raw)
In-Reply-To: <20190613111659.GX28398@e103592.cambridge.arm.com>

On Thu, Jun 13, 2019 at 12:16:59PM +0100, Dave P Martin wrote:
> On Wed, Jun 12, 2019 at 01:43:20PM +0200, Andrey Konovalov wrote:
> > From: Catalin Marinas <catalin.marinas@arm.com>
> > 
> > It is not desirable to relax the ABI to allow tagged user addresses into
> > the kernel indiscriminately. This patch introduces a prctl() interface
> > for enabling or disabling the tagged ABI with a global sysctl control
> > for preventing applications from enabling the relaxed ABI (meant for
> > testing user-space prctl() return error checking without reconfiguring
> > the kernel). The ABI properties are inherited by threads of the same
> > application and fork()'ed children but cleared on execve().
> > 
> > The PR_SET_TAGGED_ADDR_CTRL will be expanded in the future to handle
> > MTE-specific settings like imprecise vs precise exceptions.
> > 
> > Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
> > ---
> >  arch/arm64/include/asm/processor.h   |  6 +++
> >  arch/arm64/include/asm/thread_info.h |  1 +
> >  arch/arm64/include/asm/uaccess.h     |  3 +-
> >  arch/arm64/kernel/process.c          | 67 ++++++++++++++++++++++++++++
> >  include/uapi/linux/prctl.h           |  5 +++
> >  kernel/sys.c                         | 16 +++++++
> >  6 files changed, 97 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/arm64/include/asm/processor.h b/arch/arm64/include/asm/processor.h
> > index fcd0e691b1ea..fee457456aa8 100644
> > --- a/arch/arm64/include/asm/processor.h
> > +++ b/arch/arm64/include/asm/processor.h
> > @@ -307,6 +307,12 @@ extern void __init minsigstksz_setup(void);
> >  /* PR_PAC_RESET_KEYS prctl */
> >  #define PAC_RESET_KEYS(tsk, arg)	ptrauth_prctl_reset_keys(tsk, arg)
> >  
> > +/* PR_TAGGED_ADDR prctl */
> 
> (A couple of comments I missed in my last reply:)
> 
> Name mismatch?

Yeah, it went through several names but it seems that I didn't update
all places.

> > +long set_tagged_addr_ctrl(unsigned long arg);
> > +long get_tagged_addr_ctrl(void);
> > +#define SET_TAGGED_ADDR_CTRL(arg)	set_tagged_addr_ctrl(arg)
> > +#define GET_TAGGED_ADDR_CTRL()		get_tagged_addr_ctrl()
> > +
> 
> [...]
> 
> > diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
> > index 3767fb21a5b8..69d0be1fc708 100644
> > --- a/arch/arm64/kernel/process.c
> > +++ b/arch/arm64/kernel/process.c
> > @@ -30,6 +30,7 @@
> >  #include <linux/kernel.h>
> >  #include <linux/mm.h>
> >  #include <linux/stddef.h>
> > +#include <linux/sysctl.h>
> >  #include <linux/unistd.h>
> >  #include <linux/user.h>
> >  #include <linux/delay.h>
> > @@ -323,6 +324,7 @@ void flush_thread(void)
> >  	fpsimd_flush_thread();
> >  	tls_thread_flush();
> >  	flush_ptrace_hw_breakpoint(current);
> > +	clear_thread_flag(TIF_TAGGED_ADDR);
> >  }
> >  
> >  void release_thread(struct task_struct *dead_task)
> > @@ -552,3 +554,68 @@ void arch_setup_new_exec(void)
> >  
> >  	ptrauth_thread_init_user(current);
> >  }
> > +
> > +/*
> > + * Control the relaxed ABI allowing tagged user addresses into the kernel.
> > + */
> > +static unsigned int tagged_addr_prctl_allowed = 1;
> > +
> > +long set_tagged_addr_ctrl(unsigned long arg)
> > +{
> > +	if (!tagged_addr_prctl_allowed)
> > +		return -EINVAL;
> 
> So, tagging can actually be locked on by having a process enable it and
> then some possibly unrelated process clearing tagged_addr_prctl_allowed.
> That feels a bit weird.

The problem is that if you disable the ABI globally, lots of
applications would crash. This sysctl is meant as a way to disable the
opt-in to the TBI ABI. Another option would be a kernel command line
option (I'm not keen on a Kconfig option).

> Do we want to allow a process that has tagging on to be able to turn
> it off at all?  Possibly things like CRIU might want to do that.

I left it in for symmetry but I don't expect it to be used. A potential
use-case is doing it per subsequent threads in an application.

> > +	if (is_compat_task())
> > +		return -EINVAL;
> > +	if (arg & ~PR_TAGGED_ADDR_ENABLE)
> > +		return -EINVAL;
> 
> How do we expect this argument to be extended in the future?

Yes, for MTE. That's why I wouldn't allow random bits here.

> I'm wondering whether this is really a bitmask or an enum, or a mixture
> of the two.  Maybe it doesn't matter.

User may want to set PR_TAGGED_ADDR_ENABLE | PR_MTE_PRECISE in a single
call.

> > +	if (arg & PR_TAGGED_ADDR_ENABLE)
> > +		set_thread_flag(TIF_TAGGED_ADDR);
> > +	else
> > +		clear_thread_flag(TIF_TAGGED_ADDR);
> 
> I think update_thread_flag() could be used here.

Yes. I forgot you added this.

-- 
Catalin