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smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=CEIDyS2d; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 8EAC3606EE93; Thu, 5 Nov 2026 10:07:49 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=CEIDyS2d Received: by smtp.kernel.org (Postfix) with ESMTPSA id D6F1D1F00A17; Thu, 5 Nov 2026 10:07:46 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793873268; bh=GzdAvB2/oHNqGKeRfI1OIOpKJXPB5hmLnx90lxaP2zA=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=CEIDyS2djknf3O5c/eBT+dmxNXdN/G/oUW1+HL7y7blsCddi9eA80g/V5XSEFEC5O GWzeYpQW/88FuFjd3od24xYlzw1Y8NFhRT9CRAKfOxPxHoEVcWcuhbBFbnYJe5uJ3x 7znzB02zi5HOAp15fi78ez0zS7Xj27yJ7kQw7ICRyVUz6rAYf/2YbkbPXYFvZUG8WV LWvF2aht/cNIhoVT90q3Y2joyaxE/0lsIxm3UEWmJFmwQuHxu7g3uFWJfcB8187xQc XUdN+wa8na43x1Uw/a/neuqh2m6zrpJVGHLRYmer9FcDzhG671aBEzU5eMqcXSG2cb uBUJ5qlFBvWig== Message-ID: Date: Thu, 5 Nov 2026 11:07:44 +0100 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] mm/filemap: stop fault-around when the walk steps back To: "Jose Fernandez (Anthropic)" , "Matthew Wilcox (Oracle)" , Jan Kara , Andrew Morton Cc: linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Krystian Kaniewski , stable@vger.kernel.org References: <20261031-jf-mm-filemap-map-pages-lower-bound-v2-1-21029744e589@linux.dev> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 10/31/26 05:33, Jose Fernandez (Anthropic) wrote: > filemap_map_pages() can dereference a PTE pointer outside the page table > it locked. KASAN has reported this as a use-after-free read [1]. The > same path can also write out of bounds. When the stray entry reads as > none, the fault-around loop maps a page there with set_pte_range(). > > Its fault-around loop walks the page cache and moves vmf->pte by the > change in index. A lower index moves it back. xas_find() can step back > to a lower index when a multi-index entry is split during the walk. > > [1] fixes xas_find(), but the loop still relies on the walk never > stepping back. Stop the fault-around when the walk steps back. ->fault() > maps the faulting page if the loop did not. > > Link: https://lore.kernel.org/20260904121301.200049-1-krystianmkaniewski@gmail.com [1] > Fixes: 6b24ca4a1a8d ("mm: Use multi-index entries in the page cache") > Cc: stable@vger.kernel.org > Signed-off-by: Jose Fernandez (Anthropic) > --- > Changes in v2: > - Add Fixes: and Cc: stable tags, and say in the message that the loop > can also write out of bounds (David Hildenbrand). > https://patch.msgid.link/0a1ad7d2-ff0e-4248-8f03-2a64a9134aae@kernel.org > - Link to v1: https://patch.msgid.link/20261028-jf-mm-filemap-map-pages-lower-bound-v1-1-0bf126e876d4@linux.dev > --- > mm/filemap.c | 7 +++++++ > 1 file changed, 7 insertions(+) > > diff --git a/mm/filemap.c b/mm/filemap.c > index b74bc1e5015c..1df776907140 100644 > --- a/mm/filemap.c > +++ b/mm/filemap.c > @@ -3968,6 +3968,13 @@ vm_fault_t filemap_map_pages(struct vm_fault *vmf, > unsigned long end; > vm_fault_t map_ret; > > + /* A lower index could move vmf->pte out of its page table. */ > + if (unlikely(xas.xa_index < last_pgoff)) { > + folio_unlock(folio); > + folio_put(folio); > + break; > + } > + > addr += (xas.xa_index - last_pgoff) << PAGE_SHIFT; > vmf->pte += xas.xa_index - last_pgoff; > last_pgoff = xas.xa_index; > > --- > base-commit: 65ba8f34ec886ec72b2c07ddc8cf3d815c32869d > change-id: 20261028-jf-mm-filemap-map-pages-lower-bound-a623ed8a81c9 > > Best regards, > -- > Jose Fernandez (Anthropic) > I'm trusting on Andrew to pick this up :) -- Cheers, David