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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 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-Originating-IP: [10.174.178.56] X-ClientProxiedBy: kwepems100002.china.huawei.com (7.221.188.206) To kwepemo200010.china.huawei.com (7.202.195.178) X-Rspam-User: X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 591F7180006 X-Stat-Signature: qsrm4rmangz9z83yksqq9xzykqqfxpfz X-HE-Tag: 1783428426-107476 X-HE-Meta: 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 uw5oNT52 rKifpQuuZ2LOZVeKd9guPDuMQJ6OS6PQEVaZLccvu3+pmyaGKvYlrKqOwx7Nw0W6kYpMeR70wA3GtkmWsdrNQXB4ahtkQvbDcbKy8TydVhMh9OGfAy4fPHI3XyBVW5o8PnF4BUAsj/djm03wU2/LFOGJzTkeYlPC38MmtQgtdlEYccHRBhRSlWiob5Lbi3l/inQ4AHmun4bHK/kfrVPE/yGs9GMt3DmnYu42QjDcIf+kBIm+tSteqwuIhz6LL/V6tV3ktlLCpSQNkLzAF0wQahVZUdz94pBz+IdXpTTOmzz4qjDC//4bT7NIzk9pYOwt7n+FXWrnma4GvVsKQAmjNsLjZft94B2UISTYolGVvR1eSqsITzZUOilUXzIm47pawxWOsI2MDknPLpTmZ9/9MpCzDiPntC/wXNsqv6Wnagei4or5nUslMR8ULJQ== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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); }