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

[Vincenzo Frascino]

On 13/06/2019 16:35, Catalin Marinas wrote:
> 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@xxxxxxx>
>>>
>>> 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@xxxxxxx>
>>> ---
>>>  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).
>

Why you are 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.
> 

-- 
Regards,
Vincenzo