linux-kernel - Re: [PATCH v12 12/28] riscv: Implements arch agnostic shadow stack prctls

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Message-ID: <aAu6toR4VkcPMTlH@debug.ba.rivosinc.com>
Date: Fri, 25 Apr 2025 09:39:18 -0700
From: Deepak Gupta <debug@...osinc.com>
To: Radim Krčmář <rkrcmar@...tanamicro.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
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	linux-riscv <linux-riscv-bounces@...ts.infradead.org>
Subject: Re: [PATCH v12 12/28] riscv: Implements arch agnostic shadow stack
 prctls

On Fri, Apr 25, 2025 at 01:42:44PM +0200, Radim Krčmář wrote:
>2025-04-24T11:16:19-07:00, Deepak Gupta <debug@...osinc.com>:
>> On Thu, Apr 24, 2025 at 03:36:54PM +0200, Radim Krčmář wrote:
>>>2025-04-23T21:44:09-07:00, Deepak Gupta <debug@...osinc.com>:
>>>> On Thu, Apr 10, 2025 at 11:45:58AM +0200, Radim Krčmář wrote:
>>>>>2025-03-14T14:39:31-07:00, Deepak Gupta <debug@...osinc.com>:
>>>>>> diff --git a/arch/riscv/include/asm/usercfi.h b/arch/riscv/include/asm/usercfi.h
>>>>>> @@ -14,7 +15,8 @@ struct kernel_clone_args;
>>>>>>  struct cfi_status {
>>>>>>  	unsigned long ubcfi_en : 1; /* Enable for backward cfi. */
>>>>>> -	unsigned long rsvd : ((sizeof(unsigned long) * 8) - 1);
>>>>>> +	unsigned long ubcfi_locked : 1;
>>>>>> +	unsigned long rsvd : ((sizeof(unsigned long) * 8) - 2);
>>>>>
>>>>>The rsvd field shouldn't be necessary as the container for the bitfield
>>>>>is 'unsigned long' sized.
>>>>>
>>>>>Why don't we use bools here, though?
>>>>>It might produce a better binary and we're not hurting for struct size.
>>>>
>>>> If you remember one of the previous patch discussion, this goes into
>>>> `thread_info` Don't want to bloat it. Even if we end shoving into task_struct,
>>>> don't want to bloat that either. I can just convert it into bitmask if
>>>> bitfields are an eyesore here.
>>>
>>>  "unsigned long rsvd : ((sizeof(unsigned long) * 8) - 2);"
>>>
>>>is an eyesore that defines exactly the same as the two lines alone
>>>
>>>  unsigned long ubcfi_en : 1;
>>>  unsigned long ubcfi_locked : 1;
>>>
>>>That one should be removed.
>>>
>>>If we have only 4 bits in 4/8 bytes, then bitfields do generate worse
>>>code than 4 bools and a 0/4 byte hole.  The struct size stays the same.
>>>
>>>I don't care much about the switch to bools, though, because this code
>>>is not called often.
>>
>> I'll remove the bitfields, have single `unsigned long cfi_control_state`
>> And do `#define RISCV_UBCFI_EN 1` and so on.
>
>I might have seemed too much against the bitfieds, sorry.  I am against
>the rsvd fields, because it is a pointless cognitive overhead and even
>this series already had a bug in them.

Aah got it.

>
>#defines should generate the same code as bitfields (worse than bools),
>so the source code is really a matter of personal preference.
>(I do prefer bitfields.)
>
>>>>>> @@ -262,3 +292,83 @@ void shstk_release(struct task_struct *tsk)
>>>>>> +int arch_lock_shadow_stack_status(struct task_struct *task,
>>>>>> +				  unsigned long arg)
>>>>>> +{
>>>>>> +	/* If shtstk not supported or not enabled on task, nothing to lock here */
>>>>>> +	if (!cpu_supports_shadow_stack() ||
>>>>>> +	    !is_shstk_enabled(task) || arg != 0)
>>>>>> +		return -EINVAL;
>>>>>
>>>>>The task might want to prevent shadow stack from being enabled?
>>>>
>>>> But Why would it want to do that? Task can simply not issue the prctl. There
>>>> are glibc tunables as well using which it can be disabled.
>>>
>>>The task might do it as some last resort to prevent a buggy code from
>>>enabling shadow stacks that would just crash.  Or whatever complicated
>>>reason userspace can think of.
>>>
>>>It's more the other way around.  I wonder why we're removing this option
>>>when we don't really care what userspace does to itself.
>>>I think it's complicating the kernel without an obvious gain.
>>
>> It just feels wierd. There isn't anything like this for other features lit-up
>> via envcfg. Does hwprobe allow this on per-task basis? I'll look into it.
>
>I think PMM doesn't allow to lock and the rest don't seem configurable
>from userspace.
>
>It's not that important and we hopefully won't be breaking any userspace
>if we decided to allow it later, so I'm fine with this version.