From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from canpmsgout05.his.huawei.com (canpmsgout05.his.huawei.com [113.46.200.220]) (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 1965724501D for ; Tue, 7 Jul 2026 12:47:06 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=113.46.200.220 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783428430; cv=none; b=amH34Ow0R81gN4C3vohfQR9Na1eNKzIKWhl+UY9z90yF52GLvZq7iVIa6RCOJC0lHKnGzK+w5YHWI6Yfqz1BVqgaOp8iKiL9/N964Qd1zT+SUBQwd58ElPAjsu67TABfUyxiPmHkGTYlxfE39wwyVnHE1gR5cEKabPz9MAj4mHE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783428430; c=relaxed/simple; bh=9Q5ZMooVWFd+hNxGYyTI4Ile9siJGan2z6Jtejhu6QM=; h=Message-ID:Date:MIME-Version:Subject:To:CC:References:From: In-Reply-To:Content-Type; b=dsf37fGpCo+XApg9cKim1VzFsZ+x/Xlp+cuWj2hfUuNV2dPOP16i9n0bPUmCNg3zWRew7/1XKdQSQv2VSNI1Pc5Vq3TsWg6wmAGm9zk9dwtbJLbvitHTi/DsqJHP27xBwsr1Df9NamNucxsPZ/+6s9xJcMT9wiFwfm0laJXTIsM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=huawei.com; spf=pass smtp.mailfrom=huawei.com; dkim=pass (1024-bit key) header.d=huawei.com header.i=@huawei.com header.b=TALjfW1B; arc=none smtp.client-ip=113.46.200.220 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=huawei.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=huawei.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=huawei.com header.i=@huawei.com header.b="TALjfW1B" dkim-signature: v=1; a=rsa-sha256; d=huawei.com; s=dkim; c=relaxed/relaxed; q=dns/txt; h=From; bh=GQRdIed9FxvGoM8zGNvZRyfwCPAFp/XYSbDPrPh4wGc=; b=TALjfW1BXCDsVYIOhNk3xDerpRvOXDdd+5Jf3kCiH8Uud7hxIQjKLAouoCMgWOFAF91eJ47Ui FyYb89uMNUwkVanzjvQIlVXVF0N+AAggbf5hAK1QLB0zclikUafxeTFm16aMADo6qN3kOlRNEA9 gcyU6AJ7Dx7CgTwpSGADeuE= Received: from mail.maildlp.com (unknown [172.19.162.197]) by canpmsgout05.his.huawei.com (SkyGuard) with ESMTPS id 4gvgkQ55rmz12LDB; Tue, 7 Jul 2026 20:38:22 +0800 (CST) Received: from kwepemo200010.china.huawei.com (unknown [7.202.195.178]) by mail.maildlp.com (Postfix) with ESMTPS id 6223340579; Tue, 7 Jul 2026 20:47:02 +0800 (CST) Received: from [10.174.178.56] (10.174.178.56) by kwepemo200010.china.huawei.com (7.202.195.178) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.2.1544.36; Tue, 7 Jul 2026 20:47:01 +0800 Message-ID: <5d00c0d4-89ab-40eb-ba50-6eafe4957f50@huawei.com> Date: Tue, 7 Jul 2026 20:47:00 +0800 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 v3 1/2] ARM: mm: fix use-after-free in __do_user_fault() under CONFIG_DEBUG_USER To: Russell King CC: Xie Yuanbin , , , , , , , , , , , , , , , References: <20260626073048.3595106-2-xiqi2@huawei.com> <20260706133247.145485-1-xieyuanbin1@huawei.com> Content-Language: en-GB From: Qi Xi In-Reply-To: Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 8bit X-ClientProxiedBy: kwepems100002.china.huawei.com (7.221.188.206) To kwepemo200010.china.huawei.com (7.202.195.178) On 07/07/2026 19:57, Russell King wrote: > On Tue, Jul 07, 2026 at 07:48:12PM +0800, Qi Xi wrote: >> >> On 06/07/2026 21:32, Xie Yuanbin wrote: >>> On Fri, 26 Jun 2026 15:30:47 +0800, Qi Xi wrote: >>>> @@ -181,7 +181,9 @@ __do_user_fault(unsigned long addr, unsigned int fsr, unsigned int sig, >>>> pr_err("8<--- cut here ---\n"); >>>> pr_err("%s: unhandled page fault (%d) at 0x%08lx, code 0x%03x\n", >>>> tsk->comm, sig, addr, fsr); >>>> + mmap_read_lock(tsk->mm); >>>> show_pte(KERN_ERR, tsk->mm, addr); >>>> + mmap_read_unlock(tsk->mm); >>>> show_regs(regs); >>>> } >>>> #endif >>> I found that this fix does not completely solve the problem. For a user >>> fault, the addr could also be a kernel address. For arm32/x86, the kernel >>> address space and user address space share the same pgd page table, >>> but the kernel address space's page table is not protected by >>> current->mm->mmap_lock. >>> >>> I have written a use case to construct and verify this point. When A user >>> program accesses a kernel address and triggers __do_user_fault(), >>> show_pte() will directly print the kernel page table. >>> >>> So, I suggest that: >>> ```c >>> if (user_mode(regs)) { >>> struct mm_struct *const pt_mm = addr >= TASK_SIZE ? >>> &init_mm : current->mm; >>> >>> mmap_read_lock(pt_mm); >>> show_pte(KERN_ALERT, pt_mm, addr); >>> mmap_read_unlock(pt_mm); >>> } else { >>> // .. keep nothing change >>> show_pte(KERN_ALERT, current->mm, addr); >>> } >>> ``` >>> >>> I have read this article: >>> Link: https://docs.kernel.org/mm/process_addrs.html >>> `mmap_read_lock(&init_mm)` should be able to ensure that the kernel >>> address's page tables can be traversed. But I'm not quite sure if >>> `mmap_read_lock(¤t->mm)` provides protection for user-space non-VMA >>> addresses? >> You're right. And I think the fix is to simply skip show_pte() for kernel >> addresses. > No. This information is useful debug for kernel oops. > For addr >= TASK_SIZE, concurrent munmap cannot free the kernel page tables. So there is no uaf risk, and show_pte() is still called without the lock as before. Does the following change look acceptable? if (user_mode(regs) && addr < TASK_SIZE) { mmap_read_lock(current->mm); show_pte(KERN_ALERT, current->mm, addr); mmap_read_unlock(current->mm); } else { // .. keep nothing change show_pte(KERN_ALERT, current->mm, addr); }