Re: [PATCH V5 08/11] riscv: Support HAVE_IRQ_EXIT_ON_IRQ_STACK

Guo Ren
2022-09-20T06:09:31Z

On Mon, Sep 19, 2022 at 9:45 PM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Sun, Sep 18, 2022 at 11:52:43AM -0400, guoren@kernel.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 found:
https://lore.kernel.org/all/20210204204903.350275743@linutronix.de/

- The fact that the stack switching code ended up being an easy to find
exploit gadget.

What's the exploit gadget? Do you have a ref link? Thx.

>
> I'm not much of an exploit writer and I've no idea how effective our
> inline stategy is, perhaps other can comment.
It seems that I should move to an inline flavor. a0cfc74d0b00
("x86/irq: Provide macro for inlining irq stack switching")

--
Best Regards
Guo Ren