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

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Message-Id: <b977b720-f9e7-482f-af09-f9c94c0491ec@app.fastmail.com>
Date:   Tue, 03 Oct 2023 19:36:27 +0200
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Andrew Morton" <akpm@...ux-foundation.org>
Cc:     "Gregory Price" <gourry.memverge@...il.com>, 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 19:17, Andrew Morton wrote:
> On Tue, 03 Oct 2023 19:01:45 +0200 "Arnd Bergmann" <arnd@...db.de> wrote:
>> 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.
>> 
>
> Thanks.  So is a cc:stable warranted?

Yes, absolutely. It is possible that nobody noticed this
bug because the few applications that actually call move_pages are
unlikely to run in compat mode because of their large memory
requirements, but this clearly fixes a user-visible regression
and should have been caught by ltp.

      Arnd