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SubjectRe: [PATCH 17/17] module: Prevent module removal racing with text_poke()
On Thu, 17 Jan 2019 17:15:27 -0800
Nadav Amit <nadav.amit@gmail.com> wrote:

> > On Jan 17, 2019, at 3:58 PM, H. Peter Anvin <hpa@zytor.com> wrote:
> >
> > On 1/16/19 11:54 PM, Masami Hiramatsu wrote:
> >> On Wed, 16 Jan 2019 16:32:59 -0800
> >> Rick Edgecombe <rick.p.edgecombe@intel.com> wrote:
> >>
> >>> From: Nadav Amit <namit@vmware.com>
> >>>
> >>> It seems dangerous to allow code modifications to take place
> >>> concurrently with module unloading. So take the text_mutex while the
> >>> memory of the module is freed.
> >>
> >> At that point, since the module itself is removed from module list,
> >> it seems no actual harm. Or would you have any concern?
> >
> > The issue isn't the module list, but rather when it is safe to free the
> > contents, so we don't clobber anything. We absolutely need to enforce
> > that we can't text_poke() something that might have already been freed.
> >
> > That being said, we *also* really would prefer to enforce that we can't
> > text_poke() memory that doesn't actually contain code; as far as I can
> > tell we don't currently do that check.
>
> Yes, that what the mutex was supposed to achieve. It’s not supposed just
> to check whether it is a code page, but also that it is the same code
> page that you wanted to patch.
>
> > This, again, is a good use for a separate mm context. We can enforce
> > that that context will only ever contain valid page mappings for actual
> > code pages.
>
> This will not tell you that you have the *right* code-page. The module
> notifiers help to do so, since they synchronize the text poking with
> the module removal.
>
> > (Note: in my proposed algorithm, with a separate mm, replace INVLPG with
> > switching CR3 if we have to do a rollback or roll forward in the
> > breakpoint handler.)
>
> I really need to read your patches more carefully to see what you mean.
>
> Anyhow, so what do you prefer? I’m ok with either one:
> 1. Keep this patch
> 2. Remove this patch and change into a comment on text_poke()
> 3. Just drop the patch

I would prefer 2. so at least we should add a comment to text_poke().

Thank you,


--
Masami Hiramatsu <mhiramat@kernel.org>

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