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[24.205.208.113]) by smtp.gmail.com with ESMTPSA id q4sm13092321otk.39.2022.02.14.18.16.45 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 14 Feb 2022 18:16:46 -0800 (PST) Subject: Re: [PATCH] mctp: fix use after free To: Jeremy Kerr , matt@codeconstruct.com.au, davem@davemloft.net, kuba@kernel.org, nathan@kernel.org, ndesaulniers@google.com Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org, llvm@lists.linux.dev References: <20220214175138.2902947-1-trix@redhat.com> From: Tom Rix Message-ID: <6590666e-524d-51c3-0859-f8bf0c43c5ca@redhat.com> Date: Mon, 14 Feb 2022 18:16:44 -0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.10.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2/14/22 4:44 PM, Jeremy Kerr wrote: > Hi Tom, > >> Clang static analysis reports this problem >> route.c:425:4: warning: Use of memory after it is freed >>   trace_mctp_key_acquire(key); >>   ^~~~~~~~~~~~~~~~~~~~~~~~~~~ >> When mctp_key_add() fails, key is freed but then is later >> used in trace_mctp_key_acquire().  Add an else statement >> to use the key only when mctp_key_add() is successful. > Looks good to me, thanks for the fix. > > However, the Fixes tag will need an update; at the point of > 4a992bbd3650 ("mctp: Implement message fragmentation"), there was no > use of 'key' after the kfree() there. > > Instead, this is the hunk that introduced the trace event: > > @@ -365,12 +368,16 @@ > if (rc) > kfree(key); > > + trace_mctp_key_acquire(key); > + > /* we don't need to release key->lock on exit */ > key = NULL; > > - which is from 4f9e1ba6de45. The unref() comes in later, but the > initial uaf is caused by this change. > > So, I'd suggest this instead: > > Fixes: 4f9e1ba6de45 ("mctp: Add tracepoints for tag/key handling") ok - see v2 > > (this just means we need the fix for 5.16+, rather than 5.15+). > > Also, can you share how you're doing the clang static analysis there? > I'll get that included in my checks too. build clang, then use it scan-build \     --use-cc=clang \     make CC=clang There are a couple of configs that aren't happy with clang, these you can sed away with sed -e 's/CONFIG_FRAME_WARN=2048/CONFIG_FRAME_WARN=0/; s/CONFIG_RETPOLINE=y/CONFIG_RETPOLINE=n/; s/CONFIG_READABLE_ASM=y/CONFIG_READABLE_ASM=n/; s/CONFIG_FORTIFY_SOURCE=y/CONFIG_FORTIFY_SOURCE=n/' I am using clang 14 Tom > > Cheers, > > > Jeremy >