linux-kernel - Re: [PATCH V5 08/11] riscv: Support HAVE_IRQ_EXIT_ON_IRQ

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Message-ID: <Yyhyap+Xi3UtV+T0@hirez.programming.kicks-ass.net>
Date:   Mon, 19 Sep 2022 15:45:14 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     guoren@...nel.org
Cc:     arnd@...db.de, palmer@...osinc.com, tglx@...utronix.de,
        luto@...nel.org, conor.dooley@...rochip.com, heiko@...ech.de,
        jszhang@...nel.org, lazyparser@...il.com, falcon@...ylab.org,
        chenhuacai@...nel.org, apatel@...tanamicro.com,
        atishp@...shpatra.org, palmer@...belt.com,
        paul.walmsley@...ive.com, mark.rutland@....com,
        zouyipeng@...wei.com, bigeasy@...utronix.de,
        David.Laight@...lab.com, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-riscv@...ts.infradead.org,
        Guo Ren <guoren@...ux.alibaba.com>, keescook@...omium.org
Subject: Re: [PATCH V5 08/11] riscv: Support HAVE_IRQ_EXIT_ON_IRQ_STACK

On Sun, Sep 18, 2022 at 11:52:43AM -0400, guoren@...nel.org wrote:

> +ENTRY(call_on_stack)
> +	/* Create a frame record to save our ra and fp */
> +	addi	sp, sp, -RISCV_SZPTR
> +	REG_S	ra, (sp)
> +	addi	sp, sp, -RISCV_SZPTR
> +	REG_S	fp, (sp)
> +
> +	/* Save sp in fp */
> +	move	fp, sp
> +
> +	/* Move to the new stack and call the function there */
> +	li	a3, IRQ_STACK_SIZE
> +	add	sp, a1, a3
> +	jalr	a2
> +
> +	/*
> +	 * Restore sp from prev fp, and fp, ra from the frame
> +	 */
> +	move	sp, fp
> +	REG_L	fp, (sp)
> +	addi	sp, sp, RISCV_SZPTR
> +	REG_L	ra, (sp)
> +	addi	sp, sp, RISCV_SZPTR
> +	ret
> +ENDPROC(call_on_stack)

IIRC x86_64 moved away from a stack-switch function like this because it
presents a convenient exploit gadget.

I'm not much of an exploit writer and I've no idea how effective our
inline stategy is, perhaps other can comment.