linux-kernel - Re: [PATCHv5 bpf-next 6/8] x86/shstk: Add return uprobe support

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Date: Tue, 14 May 2024 18:44:33 -0700
From: Deepak Gupta <debug@...osinc.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
Cc: "olsajiri@...il.com" <olsajiri@...il.com>,
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Subject: Re: [PATCHv5 bpf-next 6/8] x86/shstk: Add return uprobe support

On Wed, May 15, 2024 at 01:10:03AM +0000, Edgecombe, Rick P wrote:
>On Mon, 2024-05-13 at 15:23 -0600, Jiri Olsa wrote:
>> so at the moment the patch 6 changes shadow stack for
>>
>> 1) current uretprobe which are not working at the moment and we change
>>    the top value of shadow stack with shstk_push_frame
>> 2) optimized uretprobe which needs to push new frame on shadow stack
>>    with shstk_update_last_frame
>>
>> I think we should do 1) and have current uretprobe working with shadow
>> stack, which is broken at the moment
>>
>> I'm ok with not using optimized uretprobe when shadow stack is detected
>> as enabled and we go with current uretprobe in that case
>>
>> would this work for you?
>
>Sorry for the delay. It seems reasonable to me due to 1 being at a fixed address
>where 2 was arbitrary address. But Peterz might have felt the opposite earlier.
>Not sure.
>
>I'd also love to get some second opinions from broonie (arm shadow stack) and
>Deepak (riscv shadow stack).
>
>Deepak, even if riscv has a special instruction that pushes to the shadow stack,
>will it be ok if there is a callable operation that does the same thing? Like,
>aren't you relying on endbranches or the compiler or something such that
>arbitrary data can't be pushed via that instruction?

Instruction is `sspush x1/ra`. It pushes contents of register return address (ra 
also called x1) onto shadow stack. `ra` is like arm's equivalent of link register.
Prologue of function is supposed to have `sspush x1` to save it away.
ISA doesn't allow encodings with register in risc-v GPRs (except register x5
because some embedded riscv space toolchains have used x5 as ra too).

On question of callable operation, I think still need to fully understand who manages
the probe and forward progress.

Question,

Is it kernel who is maintaining all return probes, meaning original return addresses
are saved in kernel data structures on per task basis. Once uretprobe did its job then
its kernel who is ensuring return to original return address ?

>
>BTW Jiri, thanks for considering shadow stack in your work.