linux-kernel - Re: [PATCH] mm/migrate: fix do_pages

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Message-Id: <f8d621c8-f5eb-4ba6-ab25-d3f782562abd@app.fastmail.com>
Date:   Tue, 03 Oct 2023 19:01:45 +0200
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Andrew Morton" <akpm@...ux-foundation.org>,
        "Gregory Price" <gourry.memverge@...il.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        "Gregory Price" <gregory.price@...verge.com>
Subject: Re: [PATCH] mm/migrate: fix do_pages_move for compat pointers

On Tue, Oct 3, 2023, at 18:57, Arnd Bergmann wrote:
> On Tue, Oct 3, 2023, at 18:49, Andrew Morton wrote:
>> On Tue,  3 Oct 2023 10:48:56 -0400 Gregory Price 
>> <gourry.memverge@...il.com> wrote:
>>
>>> do_pages_move does not handle compat pointers for the page list.
>>> correctly.  Add in_compat_syscall check and appropriate get_user
>>> fetch when iterating the page list.
>>
>> What are the userspace visible effects of this change?
>
> It makes the syscall in compat mode (32-bit userspace, 64-bit kernel)
> work the same way as the native 32-bit syscall again, restoring the
> behavior before my broken commit 5b1b561ba73c ("mm: simplify
> compat_sys_move_pages").

More specifically, my patch moved the parsing of the 'pages'
array from the main entry point into do_pages_stat(), which left
the syscall working correctly for the 'stat' operation (nodes = NULL),
while the 'move' operation (nodes != NULL) is now missing
the conversion and interprets 'pages' as an array of 64-bit
pointers instead of the intended 32-bit userspace pointers.

     Arnd