[PATCH] riscv: Invalid instruction cache after copy the xol area

[Po-Kai Chi]

Hello Palmer,

Replacing flush_icache_range() with flush_icache_all() looks good to me.
And I think this is the necessary evil because we don't know whether
hardware is performing speculative access to the XOL area or not.


On Fri, Jun 17, 2022 at 6:04 AM Palmer Dabbelt <palmer at dabbelt.com> wrote:
>
> On Wed, 18 May 2022 01:17:53 PDT (-0700), po-kai.chi at sifive.com wrote:
> > We need to invalid the relevant instruction cache after
> > copying the xol area, to ensure the changes takes effect.
> >
> > Signed-off-by: Po-Kai Chi <po-kai.chi at sifive.com>
> > ---
> >  arch/riscv/kernel/probes/uprobes.c | 8 ++++----
> >  1 file changed, 4 insertions(+), 4 deletions(-)
> >
> > diff --git a/arch/riscv/kernel/probes/uprobes.c b/arch/riscv/kernel/probes/uprobes.c
> > index 7a057b5f0adc..9d52beeac73c 100644
> > --- a/arch/riscv/kernel/probes/uprobes.c
> > +++ b/arch/riscv/kernel/probes/uprobes.c
> > @@ -165,6 +165,7 @@ void arch_uprobe_copy_ixol(struct page *page, unsigned long vaddr,
> >       /* Initialize the slot */
> >       void *kaddr = kmap_atomic(page);
> >       void *dst = kaddr + (vaddr & ~PAGE_MASK);
> > +     unsigned long addr = (unsigned long)dst;
> >
> >       memcpy(dst, src, len);
> >
> > @@ -177,10 +178,9 @@ void arch_uprobe_copy_ixol(struct page *page, unsigned long vaddr,
> >       kunmap_atomic(kaddr);
> >
> >       /*
> > -      * We probably need flush_icache_user_page() but it needs vma.
> > -      * This should work on most of architectures by default. If
> > -      * architecture needs to do something different it can define
> > -      * its own version of the function.
> > +      * Flush both I/D cache to ensure instruction modification
> > +      * takes effect.
> >        */
> >       flush_dcache_page(page);
> > +     flush_icache_range(addr, addr + len);
> >  }
>
> This brings up a handful of issues:
>
> * This always inserts a 32-bit breakpoint, but that's not quite correct.
>   This should really be checking the _next_ instruction as well to
>   insert a 16-bit breakpoint if it's a 16-bit instruction as otherwise
>   userspace might jump into the middle of the breakpoint.
> * These instructions can be concurrently executing, which relies on some
>   instruction fetch ordering that's very lightly specified.  We don't
>   rely on that elsewhere (see stop_machine() in kprobes), but we
>   probably should.  It's probably worth adding something to probe the HW
>   to make sure this is supported.
> * Adding the icache flush defeats a uprobes advantage in that we'll now
>   be triggering remote execution (to do the remote fence.i).  One option
>   could be to defer the fence and wait on it, but not sure if that's
>   sane and it'd likely require a lot of
>
> This also leaves a bit undefined WRT icache aliasing, at least in terms
> of the API used.  IMO it'd be
>
>     diff --git a/arch/riscv/kernel/probes/uprobes.c b/arch/riscv/kernel/probes/uprobes.c
>     index 9d52beeac73c..c857346864fc 100644
>     --- a/arch/riscv/kernel/probes/uprobes.c
>     +++ b/arch/riscv/kernel/probes/uprobes.c
>     @@ -165,7 +165,6 @@ void arch_uprobe_copy_ixol(struct page *page, unsigned long vaddr,
>         /* Initialize the slot */
>         void *kaddr = kmap_atomic(page);
>         void *dst = kaddr + (vaddr & ~PAGE_MASK);
>     -   unsigned long addr = (unsigned long)dst;
>
>         memcpy(dst, src, len);
>
>     @@ -179,8 +178,10 @@ void arch_uprobe_copy_ixol(struct page *page, unsigned long vaddr,
>
>         /*
>          * Flush both I/D cache to ensure instruction modification
>     -    * takes effect.
>     +    * takes effect.  We don't need to flush the whole icache, but that's
>     +    * all RISC-V defines so rather than worry about aliasing this just
>     +    * flushes everything.
>          */
>         flush_dcache_page(page);
>     -   flush_icache_range(addr, addr + len);
>     +   flush_icache_all();
>      }
>
> which will end up doing the same thing but avoids the ambiguity.  I went
> ahead and put this at palmer/riscv-uprobe_fencei with that and some
> other minor things fixed up, LMK if that looks OK and I'll take it on
> fixes.
>
> Thanks!



-- 
BR,
Po-Kai Chi