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

Guo Ren
2022-09-21T06:16:35Z

On Tue, Sep 20, 2022 at 3:34 PM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Tue, Sep 20, 2022 at 09:27:51AM +0200, Peter Zijlstra wrote:
> > On Tue, Sep 20, 2022 at 02:08:55PM +0800, Guo Ren wrote:
> > > 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.
> >
> > Sadly no, I do not. Kees might. But basically it boils down to this
> > function taking both a stack pointer and a function pointer as
> > arguments (@a1 and @a2 resp. if I'm not reading this wrong).
> >
> > If an attacker can call this with arguments of its choice then it gains
> > full control of subsequent execution.
>
> If you inline it the hope is that the function pointers go away or at
> least the encompassing function doesn't have quite such a 'convenient'
> signature to hijack control flow.
Thanks for mentioning it. I would change to an inline style.

--
Best Regards
Guo Ren