From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id F198746D09A for ; Mon, 5 Oct 2026 10:27:56 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791196078; cv=none; b=NQjz/1iGye/UsEuLGSJcy/k2aZVW0D3au0lPM57H4ZYKVv+Arhu6QKWsXpYdLEs3tPzr7zKgLMP/sh5Ci2LYytvAXvXnI25fTHJT/xjVkE7yO5I4crDaW3HyFgEml+ogZOvwRhsG64hua8PSaMigLyfWsm000vqSw24Pi/dGikg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791196078; c=relaxed/simple; bh=C9Zvbv33nUSf8gI5EoxZZ1MUetN0ESAqqDRSnBMiEng=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=DlJAs+X4ckTqm1MHcnZi7K4yMQtEgnR+bAEPqqLehIFGD8hoPo+U3ff4qLR+zofLJ9r96azzKlecUjAhVEhqCKKWbk+KS1y1gGMm+G82H008eRUAG5+edC59HMAcesTz11Og+S50MMQHt43FOeqP1Mn2LCG1qr+v8y64LMKGZPo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=oKGrAF3Y; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="oKGrAF3Y" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 7A3291F000FF; Mon, 5 Oct 2026 10:27:53 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1791196076; bh=YV0ahyh+qROmkFhWhWDlwwYmMNujGqoOPIW1A6VxBOk=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=oKGrAF3Y6UveLrA2UOY5wx0zRSLKy04JzXsiOxj3ysMaZPU993WGae1/63tiWDXuP StuarUNmsSOtX8r+bhrwT7f76+/Ej4S0PuDqW48GA2hdZiKejogy7nZLg4/rIAkM1V 1mde+oIHX9VOk5yzlXz8e7Rjqv8uD5VER42XZoR5D5bJ6r+RvcrM1ZS01NXWIE75Ox x4xyDy5KgbHYLxmsY6c3XTK1IFNOCxux1W7wISTv+yyuwofen4r+m/Cl84l102VPW5 RxvtZNU8JJR/pQEXXTPzJAZ85feEKmEQokKl3zjvM+7ePNjY+AbnDav77u3sKEo8AL UcRX5TS64TU5w== Message-ID: Date: Mon, 5 Oct 2026 12:27:50 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/gup: fix NULL pointer dereference in fixup_user_fault() To: Lance Yang , akpm@linux-foundation.org Cc: neganhat@gmail.com, jgg@ziepe.ca, jhubbard@nvidia.com, peterx@redhat.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20261004125601.c06953a6f0660df9859b0c2a@linux-foundation.org> <20261005021957.5254-1-lance.yang@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 10/5/26 04:19, Lance Yang wrote: > > On Sun, Oct 04, 2026 at 12:56:01PM -0700, Andrew Morton wrote: >> On Sun, 4 Oct 2026 02:48:46 +0700 Nguyen Duy Nhat Anh wrote: >> >>> In fixup_user_fault(), the 'unlocked' parameter is checked for NULL >>> early on, allowing callers to pass NULL if they do not track whether the >>> mmap lock was dropped. >>> >>> However, if handle_mm_fault() returns VM_FAULT_COMPLETED, line 1597 >>> dereferences 'unlocked' directly (*unlocked = true) without checking >>> if it is NULL. Callers like s390's pci_mmio.c pass NULL for 'unlocked', >>> leading to a kernel NULL pointer dereference when VM_FAULT_COMPLETED >>> occurs. >>> >>> Fix this by checking if 'unlocked' is non-NULL before assigning to it. >> >> This code is too subtle so you aren't the first to attempt to "fix" it. >> >> The key hint is in the kerneldoc: >> >> * @unlocked: did we unlock the mmap_lock while retrying, maybe NULL if caller >> * does not allow retry. If NULL, the caller must guarantee >> * that fault_flags does not contain FAULT_FLAG_ALLOW_RETRY. >> >> Trace through the >> FAULT_FLAG_ALLOW_RETRY/VM_FAULT_COMPLETED/VM_FAULT_RETRY logic >> to confirm that the null deref is a cant-happen. > > IIUC, that's indeed a can't-happen :) https://lore.kernel.org/linux-mm/c44e5924-091f-43ed-8cff-34e87c3d9763@kernel.org/ People should stop trusting tool output and use their brain ;) If you did mmap_read_lock() and wouldn't have a way to indicate that to the user something would be seriously messed up. -- Cheers, David