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

[Vincenzo Frascino <vincenzo.frascino@arm.com>]

Hi Catalin,

On 12/06/2019 10:32, Catalin Marinas wrote:
> 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.
> 

Agreed. All the comments I provided are meant to simplify the addition of MTE
support.

>>> 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.
> 

Indeed. I was not questioning the pre-existing interface definition, but trying
more to reduce the changes to the ABI to the minimum since:
 - prctl does not mandate how to use the arg[2-5]
 - prctl interface is flexible enough for the problem to be solved with only one
   PR_ command.

I agree on reusing the interface for MTE for the purposes you specified.

-- 
Regards,
Vincenzo