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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-488fb7c0eacsm148798195e9.35.2026.04.22.13.45.08 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 22 Apr 2026 13:45:08 -0700 (PDT) Date: Wed, 22 Apr 2026 21:45:07 +0100 From: David Laight To: "Shengzhuo Wei" Cc: "Richard Weinberger" , "Anton Ivanov" , "Johannes Berg" , "Dan Carpenter" , "Andrew Morton" , "Jeff Dike" , , Subject: Re: [PATCH] um: proc/exitcode: fix simple_strtol() out-of-bounds read Message-ID: <20260422214507.0078c8ba@pumpkin> In-Reply-To: <20260423-fix_exitcode-v1-1-7e4508913d68@cherr.cc> References: <20260423-fix_exitcode-v1-1-7e4508913d68@cherr.cc> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260422_134511_786182_6BC0618A X-CRM114-Status: GOOD ( 22.39 ) X-BeenThere: linux-um@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-um" Errors-To: linux-um-bounces+linux-um=archiver.kernel.org@lists.infradead.org On Thu, 23 Apr 2026 01:39:25 +0800 "Shengzhuo Wei" wrote: > The stack buffer 'buf' is declared as char[sizeof("nnnnn\0")] (7 bytes) > and the copy size is min(count, sizeof(buf)). When a user writes 7 or > more bytes, copy_from_user fills all 7 bytes without a NUL terminator. > The subsequent call to simple_strtol() expects a NUL-terminated string > and will read past the end of buf on the stack. You should probably also mention that write(, "123", 3) will lead to buf[3] being read - which is uninitialised stack. David > > write(2) should report the number of bytes consumed. Returning the original > count would claim success even when the input was truncated, so userspace > cannot detect it. > > Clamp the copy length to sizeof(buf)-1, add a terminator, and return the > consumed length. > > Fixes: 201f99f170df ("uml: check length in exitcode_proc_write()") > Fixes: e16f5350d4cf ("uml: get declaration of simple_strtoul") > Signed-off-by: Shengzhuo Wei > --- > arch/um/kernel/exitcode.c | 5 +++-- > 1 file changed, 3 insertions(+), 2 deletions(-) > > diff --git a/arch/um/kernel/exitcode.c b/arch/um/kernel/exitcode.c > index 43edc2aa57e4fbd4a3d24f96878c76f9f8fd4eaa..8de404ff21a213918c5351bc20a6e047bf1b93f5 100644 > --- a/arch/um/kernel/exitcode.c > +++ b/arch/um/kernel/exitcode.c > @@ -43,16 +43,17 @@ static ssize_t exitcode_proc_write(struct file *file, > size_t size; > int tmp; > > - size = min(count, sizeof(buf)); > + size = min(count, sizeof(buf) - 1); > if (copy_from_user(buf, buffer, size)) > return -EFAULT; > + buf[size] = '\0'; > > tmp = simple_strtol(buf, &end, 0); > if ((*end != '\0') && !isspace(*end)) > return -EINVAL; > > uml_exitcode = tmp; > - return count; > + return size; > } > > static const struct proc_ops exitcode_proc_ops = { > > --- > base-commit: 6596a02b207886e9e00bb0161c7fd59fea53c081 > change-id: 20260423-fix_exitcode-908061ece624 > > Best regards,