linux-kernel - Re: [PATCHv4 bpf-next 2/7] uprobe: Add uretprobe syscall to speed up return probe

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Message-ID: <ZjVs4bxi7CpWhtEQ@debug.ba.rivosinc.com>
Date: Fri, 3 May 2024 16:01:53 -0700
From: Deepak Gupta <debug@...osinc.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
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Subject: Re: [PATCHv4 bpf-next 2/7] uprobe: Add uretprobe syscall to speed up
 return probe

On Fri, May 03, 2024 at 07:38:18PM +0000, Edgecombe, Rick P wrote:
>+Some more shadow stack folks from other archs. We are discussing how uretprobes
>work with shadow stack.
>
>Context:
>https://lore.kernel.org/lkml/ZjU4ganRF1Cbiug6@krava/

Thanks Rick.

Yeah I didn't give enough attention to uprobes either.
Although now that I think for RISC-V shadow stack, it shouldn't be an issue.
On RISC-V return addresses don't get pushed as part of call instruction.
There is a distinct instruction "shadow stack push of return address" in prolog.
Similarly in epilog there is distinct instruction "shadow stack pop and check with
link register".

On RISC-V, uretprobe would install a uprobe on function start and when it's hit.
It'll replace pt_regs->ra = trampoline_handler. As function will resume, trampoline
addr will get pushed and popped. Although trampoline_handler would have to be enlightened
to eventually return to original return site.

>
>On Fri, 2024-05-03 at 21:18 +0200, Jiri Olsa wrote:
>>
>> hack below seems to fix it for the current uprobe setup,
>> we need similar fix for the uretprobe syscall trampoline setup
>
>It seems like a reasonable direction.
>
>Security-wise, applications cannot do this on themselves, or it is an otherwise
>privileged thing right?
>
>