Re: [PATCH v16 02/16] arm64: untag user pointers in access_ok and __uaccess_mask_ptr

[Catalin Marinas <catalin.marinas@arm.com>]

Hi Vincenzo,

On Tue, Jun 11, 2019 at 06:09:10PM +0100, Vincenzo Frascino wrote:
> > 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);
> 
> Nit: in line we the other functions in thread_flush we could have something like
> "tagged_addr_thread_flush", maybe inlined.

The other functions do a lot more than clearing a TIF flag, so they
deserved their own place. We could do this when adding MTE support. I
think we also need to check what other TIF flags we may inadvertently
pass on execve(), maybe have a mask clearing.

> > diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
> > index 094bb03b9cc2..2e927b3e9d6c 100644
> > --- a/include/uapi/linux/prctl.h
> > +++ b/include/uapi/linux/prctl.h
> > @@ -229,4 +229,9 @@ struct prctl_mm_map {
> >  # define PR_PAC_APDBKEY			(1UL << 3)
> >  # define PR_PAC_APGAKEY			(1UL << 4)
> >  
> > +/* Tagged user address controls for arm64 */
> > +#define PR_SET_TAGGED_ADDR_CTRL		55
> > +#define PR_GET_TAGGED_ADDR_CTRL		56
> > +# define PR_TAGGED_ADDR_ENABLE		(1UL << 0)
> > +
> >  #endif /* _LINUX_PRCTL_H */
> > diff --git a/kernel/sys.c b/kernel/sys.c
> > index 2969304c29fe..ec48396b4943 100644
> > --- a/kernel/sys.c
> > +++ b/kernel/sys.c
> > @@ -124,6 +124,12 @@
> >  #ifndef PAC_RESET_KEYS
> >  # define PAC_RESET_KEYS(a, b)	(-EINVAL)
> >  #endif
> > +#ifndef SET_TAGGED_ADDR_CTRL
> > +# define SET_TAGGED_ADDR_CTRL(a)	(-EINVAL)
> > +#endif
> > +#ifndef GET_TAGGED_ADDR_CTRL
> > +# define GET_TAGGED_ADDR_CTRL()		(-EINVAL)
> > +#endif
> >  
> >  /*
> >   * this is where the system-wide overflow UID and GID are defined, for
> > @@ -2492,6 +2498,16 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
> >  			return -EINVAL;
> >  		error = PAC_RESET_KEYS(me, arg2);
> >  		break;
> > +	case PR_SET_TAGGED_ADDR_CTRL:
> > +		if (arg3 || arg4 || arg5)
> > +			return -EINVAL;
> > +		error = SET_TAGGED_ADDR_CTRL(arg2);
> > +		break;
> > +	case PR_GET_TAGGED_ADDR_CTRL:
> > +		if (arg2 || arg3 || arg4 || arg5)
> > +			return -EINVAL;
> > +		error = GET_TAGGED_ADDR_CTRL();
> > +		break;
> 
> Why do we need two prctl here? We could have only one and use arg2 as set/get
> and arg3 as a parameter. What do you think?

This follows the other PR_* options, e.g. PR_SET_VL/GET_VL,
PR_*_FP_MODE. We will use other bits in arg2, for example to set the
precise vs imprecise MTE trapping.

-- 
Catalin