From mboxrd@z Thu Jan 1 00:00:00 1970 From: Catalin Marinas 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 Message-ID: <20190613153505.GU28951@C02TF0J2HF1T.local> References: <20190613111659.GX28398@e103592.cambridge.arm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20190613111659.GX28398@e103592.cambridge.arm.com> Sender: linux-kernel-owner@vger.kernel.org To: Dave Martin Cc: Andrey Konovalov , 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 , Szabolcs Nagy , Will Deacon , Kostya Serebryany , Khalid Aziz , Felix Kuehling , Vincenzo Frascino , Jacob Bramley , Leon Romanovsky , Christoph Hellwig , Jason Gunthorpe , Evgeniy Stepanov List-Id: dri-devel@lists.freedesktop.org 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 > > > > 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 > > --- > > 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 > > #include > > #include > > +#include > > #include > > #include > > #include > > @@ -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