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SubjectRe: [PATCH 17/17] module: Prevent module removal racing with text_poke()
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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.

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.

(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.)

-hpa

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