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

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