linux-kernel - Re: [PATCH] mm/mremap: Fix address wraparound in move_page

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Message-ID: <9d1a9d6c-5c3d-401f-8646-828bb9c282cd@bytedance.com>
Date: Tue, 12 Nov 2024 11:00:14 +0800
From: Qi Zheng <zhengqi.arch@...edance.com>
To: Jann Horn <jannh@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 "Joel Fernandes (Google)" <joel@...lfernandes.org>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>, Vlastimil Babka
 <vbabka@...e.cz>, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 stable@...r.kernel.org
Subject: Re: [PATCH] mm/mremap: Fix address wraparound in move_page_tables()



On 2024/11/12 03:34, Jann Horn wrote:
> On 32-bit platforms, it is possible for the expression
> `len + old_addr < old_end` to be false-positive if `len + old_addr` wraps
> around. `old_addr` is the cursor in the old range up to which page table
> entries have been moved; so if the operation succeeded, `old_addr` is the
> *end* of the old region, and adding `len` to it can wrap.
> 
> The overflow causes mremap() to mistakenly believe that PTEs have been
> copied; the consequence is that mremap() bails out, but doesn't move the
> PTEs back before the new VMA is unmapped, causing anonymous pages in the
> region to be lost. So basically if userspace tries to mremap() a
> private-anon region and hits this bug, mremap() will return an error and
> the private-anon region's contents appear to have been zeroed.
> 
> The idea of this check is that `old_end - len` is the original start
> address, and writing the check that way also makes it easier to read; so
> fix the check by rearranging the comparison accordingly.
> 
> (An alternate fix would be to refactor this function by introducing an
> "orig_old_start" variable or such.)
> 
> Cc: stable@...r.kernel.org
> Fixes: af8ca1c14906 ("mm/mremap: optimize the start addresses in move_page_tables()")
> Signed-off-by: Jann Horn <jannh@...gle.com>

Acked-by: Qi Zheng <zhengqi.arch@...edance.com>

Thanks!