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[188.32.236.56]) by smtp.gmail.com with ESMTPSA id m12sm682023lji.110.2021.01.21.12.15.23 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 21 Jan 2021 12:15:23 -0800 (PST) Reply-To: efremov@linux.com To: Steven Rostedt Cc: Gaurav Kohli , linux-kernel@vger.kernel.org, linux-arm-msm@vger.kernel.org, stable@vger.kernel.org, Julia Lawall References: <1601976833-24377-1-git-send-email-gkohli@codeaurora.org> <20210121140951.2a554a5e@gandalf.local.home> From: Denis Efremov Subject: Re: [PATCH v1] trace: Fix race in trace_open and buffer resize call Message-ID: <021b1b38-47ce-bc8b-3867-99160cc85523@linux.com> Date: Thu, 21 Jan 2021 23:15:22 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.6.0 MIME-Version: 1.0 In-Reply-To: <20210121140951.2a554a5e@gandalf.local.home> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-arm-msm@vger.kernel.org On 1/21/21 10:09 PM, Steven Rostedt wrote: > On Thu, 21 Jan 2021 17:30:40 +0300 > Denis Efremov wrote: > >> Hi, >> >> This patch (CVE-2020-27825) was tagged with >> Fixes: b23d7a5f4a07a ("ring-buffer: speed up buffer resets by avoiding synchronize_rcu for each CPU") >> >> I'm not an expert here but it seems like b23d7a5f4a07a only refactored >> ring_buffer_reset_cpu() by introducing reset_disabled_cpu_buffer() without >> significant changes. Hence, mutex_lock(&buffer->mutex)/mutex_unlock(&buffer->mutex) >> can be backported further than b23d7a5f4a07a~ and to all LTS kernels. Is >> b23d7a5f4a07a the actual cause of the bug? >> > > Ug, that looks to be a mistake. Looking back at the thread about this: > > https://lore.kernel.org/linux-arm-msm/20200915141304.41fa7c30@gandalf.local.home/ I see from the link that it was planned to backport the patch to LTS kernels: > Actually we are seeing issue in older kernel like 4.19/4.14/5.4 and there below patch was not > present in stable branches: > Commit b23d7a5f4a07 ("ring-buffer: speed up buffer resets by avoiding synchronize_rcu for each CPU") The point is that it's not backported yet. Maybe because of Fixes tag. I've discovered this while trying to formalize CVE-2020-27825 bug in cvehound https://github.com/evdenis/cvehound/blob/master/cvehound/cve/CVE-2020-27825.cocci I think that the backport to the 4.4+ should be something like: diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c index 547a3a5ac57b..2171b377bbc1 100644 --- a/kernel/trace/ring_buffer.c +++ b/kernel/trace/ring_buffer.c @@ -4295,6 +4295,8 @@ void ring_buffer_reset_cpu(struct ring_buffer *buffer, int cpu) if (!cpumask_test_cpu(cpu, buffer->cpumask)) return; + mutex_lock(&buffer->mutex); + atomic_inc(&buffer->resize_disabled); atomic_inc(&cpu_buffer->record_disabled); @@ -4317,6 +4319,8 @@ void ring_buffer_reset_cpu(struct ring_buffer *buffer, int cpu) atomic_dec(&cpu_buffer->record_disabled); atomic_dec(&buffer->resize_disabled); + + mutex_unlock(&buffer->mutex); } EXPORT_SYMBOL_GPL(ring_buffer_reset_cpu); Thanks, Denis