From: Chen Gang Subject: Re: [Qemu-trivial] [PATCH] target-ppc: kvm: Fix memory overflow issue about strncat() Date: Fri, 24 Oct 2014 16:19:43 +0800 Message-ID: <544A0B9F.9040500@gmail.com> References: <543BE352.3080609@gmail.com> <543BE616.1000707@suse.de> <544A046D.4090204@msgid.tls.msk.ru> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Cc: qemu-trivial@nongnu.org, qemu-ppc@nongnu.org, qemu-devel , kvm@vger.kernel.org To: Michael Tokarev , Alexander Graf , pbonzini@redhat.com Return-path: Received: from mail-pd0-f176.google.com ([209.85.192.176]:42897 "EHLO mail-pd0-f176.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752608AbaJXINv (ORCPT ); Fri, 24 Oct 2014 04:13:51 -0400 Received: by mail-pd0-f176.google.com with SMTP id fp1so1039830pdb.35 for ; Fri, 24 Oct 2014 01:13:51 -0700 (PDT) In-Reply-To: <544A046D.4090204@msgid.tls.msk.ru> Sender: kvm-owner@vger.kernel.org List-ID: On 10/24/14 15:49, Michael Tokarev wrote: > On 10/13/2014 06:47 PM, Alexander Graf wrote: >> On 13.10.14 16:36, Chen Gang wrote: >>> strncat() will append additional '\0' to destination buffer, so need >>> additional 1 byte for it, or may cause memory overflow, just like other >>> area within QEMU have done. >>> >>> Signed-off-by: Chen Gang >> >> I agree with this patch. However, the code is pretty ugly - I'm sure it >> must've been me who wrote it :). >> >> Could you please instead rewrite it to use g_strdup_printf() rather than >> strncat()s? That way we resolve all string pitfalls automatically - and >> this code is not the fast path, so doing an extra memory allocation is ok. > > I'd just use snprintf() like this: > > diff --git a/target-ppc/kvm.c b/target-ppc/kvm.c > index 9c23c6b..5eaa36c 100644 > --- a/target-ppc/kvm.c > +++ b/target-ppc/kvm.c > @@ -1794,8 +1794,7 @@ static uint64_t kvmppc_read_int_cpu_dt(const char *propname) > return -1; > } > > - strncat(buf, "/", sizeof(buf) - strlen(buf)); > - strncat(buf, propname, sizeof(buf) - strlen(buf)); > + snprintf(buf + strlen(buf), sizeof(buf) - strlen(buf), "/%s", propname); > > f = fopen(buf, "rb"); > if (!f) { > > the buffer is of size PATH_MAX, and we're looking at /proc filesystem where > names should be rather short so we're extremly unlikely to hit this prob in > practice, there's no need to dynamically allocate a buffer for this stuff. > > (Or alternatively there's asprintf(), but still I think it is overkill). > > I can apply the above if everyone agrees. > For me, what you said is reasonable, although I am not sure whether the patch v2 for g_strdup_printf() was already applied or not. Thanks. -- Chen Gang Open, share, and attitude like air, water, and life which God blessed