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Date: Mon, 13 May 2024 11:31:23 -0700
From: Deepak Gupta <debug@rivosinc.com>
To: Charlie Jenkins <charlie@rivosinc.com>
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Subject: Re: [PATCH v3 17/29] prctl: arch-agnostic prctl for indirect branch
 tracking
Message-ID: <ZkJce/hix5kRhubS@debug.ba.rivosinc.com>
References: <20240403234054.2020347-1-debug@rivosinc.com>
 <20240403234054.2020347-18-debug@rivosinc.com>
 <Zj6tzxWFj4H+250p@ghost>
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In-Reply-To: <Zj6tzxWFj4H+250p@ghost>

On Fri, May 10, 2024 at 04:29:19PM -0700, Charlie Jenkins wrote:
>On Wed, Apr 03, 2024 at 04:35:05PM -0700, Deepak Gupta wrote:
>> Three architectures (x86, aarch64, riscv) have support for indirect branch
>> tracking feature in a very similar fashion. On a very high level, indirect
>> branch tracking is a CPU feature where CPU tracks branches which uses
>> memory operand to perform control transfer in program. As part of this
>> tracking on indirect branches, CPU goes in a state where it expects a
>> landing pad instr on target and if not found then CPU raises some fault
>> (architecture dependent)
>>
>> x86 landing pad instr - `ENDBRANCH`
>> aarch64 landing pad instr - `BTI`
>> riscv landing instr - `lpad`
>>
>> Given that three major arches have support for indirect branch tracking,
>> This patch makes `prctl` for indirect branch tracking arch agnostic.
>>
>> To allow userspace to enable this feature for itself, following prtcls are
>> defined:
>>  - PR_GET_INDIR_BR_LP_STATUS: Gets current configured status for indirect
>>    branch tracking.
>>  - PR_SET_INDIR_BR_LP_STATUS: Sets a configuration for indirect branch
>>    tracking.
>>    Following status options are allowed
>>        - PR_INDIR_BR_LP_ENABLE: Enables indirect branch tracking on user
>>          thread.
>>        - PR_INDIR_BR_LP_DISABLE; Disables indirect branch tracking on user
>>          thread.
>>  - PR_LOCK_INDIR_BR_LP_STATUS: Locks configured status for indirect branch
>>    tracking for user thread.
>>
>> Signed-off-by: Deepak Gupta <debug@rivosinc.com>
>> ---
>>  include/uapi/linux/prctl.h | 27 +++++++++++++++++++++++++++
>>  kernel/sys.c               | 30 ++++++++++++++++++++++++++++++
>>  2 files changed, 57 insertions(+)
>>
>> diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
>> index 3c66ed8f46d8..b7a8212a068e 100644
>> --- a/include/uapi/linux/prctl.h
>> +++ b/include/uapi/linux/prctl.h
>> @@ -328,4 +328,31 @@ struct prctl_mm_map {
>>   */
>>  #define PR_LOCK_SHADOW_STACK_STATUS      73
>>
>> +/*
>> + * Get the current indirect branch tracking configuration for the current
>> + * thread, this will be the value configured via PR_SET_INDIR_BR_LP_STATUS.
>> + */
>> +#define PR_GET_INDIR_BR_LP_STATUS      74
>> +
>> +/*
>> + * Set the indirect branch tracking configuration. PR_INDIR_BR_LP_ENABLE will
>> + * enable cpu feature for user thread, to track all indirect branches and ensure
>> + * they land on arch defined landing pad instruction.
>> + * x86 - If enabled, an indirect branch must land on `ENDBRANCH` instruction.
>> + * arch64 - If enabled, an indirect branch must land on `BTI` instruction.
>> + * riscv - If enabled, an indirect branch must land on `lpad` instruction.
>> + * PR_INDIR_BR_LP_DISABLE will disable feature for user thread and indirect
>> + * branches will no more be tracked by cpu to land on arch defined landing pad
>> + * instruction.
>> + */
>> +#define PR_SET_INDIR_BR_LP_STATUS      75
>> +# define PR_INDIR_BR_LP_ENABLE		   (1UL << 0)
>> +
>> +/*
>> + * Prevent further changes to the specified indirect branch tracking
>> + * configuration.  All bits may be locked via this call, including
>> + * undefined bits.
>> + */
>> +#define PR_LOCK_INDIR_BR_LP_STATUS      76
>> +
>>  #endif /* _LINUX_PRCTL_H */
>> diff --git a/kernel/sys.c b/kernel/sys.c
>> index 242e9f147791..c770060c3f06 100644
>> --- a/kernel/sys.c
>> +++ b/kernel/sys.c
>> @@ -2330,6 +2330,21 @@ int __weak arch_lock_shadow_stack_status(struct task_struct *t, unsigned long st
>>  	return -EINVAL;
>>  }
>>
>> +int __weak arch_get_indir_br_lp_status(struct task_struct *t, unsigned long __user *status)
>> +{
>> +	return -EINVAL;
>> +}
>> +
>> +int __weak arch_set_indir_br_lp_status(struct task_struct *t, unsigned long __user *status)
>> +{
>> +	return -EINVAL;
>> +}
>> +
>> +int __weak arch_lock_indir_br_lp_status(struct task_struct *t, unsigned long __user *status)
>> +{
>> +	return -EINVAL;
>> +}
>> +
>
>These weak references each cause a warning:
>
>kernel/sys.c:2333:12: warning: no previous prototype for 'arch_get_indir_br_lp_status' [-Wmissing-prototypes]
> 2333 | int __weak arch_get_indir_br_lp_status(struct task_struct *t, unsigned long __user *status)
>      |            ^~~~~~~~~~~~~~~~~~~~~~~~~~~
>kernel/sys.c:2338:12: warning: no previous prototype for 'arch_set_indir_br_lp_status' [-Wmissing-prototypes]
> 2338 | int __weak arch_set_indir_br_lp_status(struct task_struct *t, unsigned long __user *status)
>      |            ^~~~~~~~~~~~~~~~~~~~~~~~~~~
>kernel/sys.c:2343:12: warning: no previous prototype for 'arch_lock_indir_br_lp_status' [-Wmissing-prototypes]
> 2343 | int __weak arch_lock_indir_br_lp_status(struct task_struct *t, unsigned long __user *status)
>
>Can the definitions be added to include/linux/mm.h alongside the
>*_shadow_stack_status() definitions?

Noted. Will work on a fix for this.

>
>- Charlie
>
>>  #define PR_IO_FLUSHER (PF_MEMALLOC_NOIO | PF_LOCAL_THROTTLE)
>>
>>  #ifdef CONFIG_ANON_VMA_NAME
>> @@ -2787,6 +2802,21 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
>>  			return -EINVAL;
>>  		error = arch_lock_shadow_stack_status(me, arg2);
>>  		break;
>> +	case PR_GET_INDIR_BR_LP_STATUS:
>> +		if (arg3 || arg4 || arg5)
>> +			return -EINVAL;
>> +		error = arch_get_indir_br_lp_status(me, (unsigned long __user *) arg2);
>> +		break;
>> +	case PR_SET_INDIR_BR_LP_STATUS:
>> +		if (arg3 || arg4 || arg5)
>> +			return -EINVAL;
>> +		error = arch_set_indir_br_lp_status(me, (unsigned long __user *) arg2);
>> +		break;
>> +	case PR_LOCK_INDIR_BR_LP_STATUS:
>> +		if (arg3 || arg4 || arg5)
>> +			return -EINVAL;
>> +		error = arch_lock_indir_br_lp_status(me, (unsigned long __user *) arg2);
>> +		break;
>>  	default:
>>  		error = -EINVAL;
>>  		break;
>> --
>> 2.43.2
>>