From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wr0-f198.google.com (mail-wr0-f198.google.com [209.85.128.198]) by kanga.kvack.org (Postfix) with ESMTP id 4D51D6B0279 for ; Fri, 23 Jun 2017 09:59:39 -0400 (EDT) Received: by mail-wr0-f198.google.com with SMTP id l34so12954748wrc.12 for ; Fri, 23 Jun 2017 06:59:39 -0700 (PDT) Received: from mx1.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id g24si4078762wrb.24.2017.06.23.06.59.36 for (version=TLS1 cipher=AES128-SHA bits=128/128); Fri, 23 Jun 2017 06:59:37 -0700 (PDT) Date: Fri, 23 Jun 2017 15:59:24 +0200 From: Michal Hocko Subject: Re: [PATCH] exec: Account for argv/envp pointers Message-ID: <20170623135924.GC5314@dhcp22.suse.cz> References: <20170622001720.GA32173@beast> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170622001720.GA32173@beast> Sender: owner-linux-mm@kvack.org List-ID: To: Kees Cook Cc: Andrew Morton , Alexander Viro , Qualys Security Advisory , linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-hardening@lists.openwall.com On Wed 21-06-17 17:17:20, Kees Cook wrote: > When limiting the argv/envp strings during exec to 1/4 of the stack limit, > the storage of the pointers to the strings was not included. This means > that an exec with huge numbers of tiny strings could eat 1/4 of the > stack limit in strings and then additional space would be later used > by the pointers to the strings. For example, on 32-bit with a 8MB stack > rlimit, an exec with 1677721 single-byte strings would consume less than > 2MB of stack, the max (8MB / 4) amount allowed, but the pointers to the > strings would consume the remaining additional stack space (1677721 * > 4 == 6710884). The result (1677721 + 6710884 == 8388605) would exhaust > stack space entirely. Controlling this stack exhaustion could result in > pathological behavior in setuid binaries (CVE-2017-1000365). > > Fixes: b6a2fea39318 ("mm: variable length argument support") > Cc: stable@vger.kernel.org > Signed-off-by: Kees Cook > --- > fs/exec.c | 20 ++++++++++++++++---- > 1 file changed, 16 insertions(+), 4 deletions(-) > > diff --git a/fs/exec.c b/fs/exec.c > index 72934df68471..8079ca70cfda 100644 > --- a/fs/exec.c > +++ b/fs/exec.c > @@ -220,8 +220,18 @@ static struct page *get_arg_page(struct linux_binprm *bprm, unsigned long pos, > > if (write) { > unsigned long size = bprm->vma->vm_end - bprm->vma->vm_start; > + unsigned long ptr_size; > struct rlimit *rlim; > > + /* > + * Since the stack will hold pointers to the strings, we > + * must account for them as well. > + */ > + ptr_size = (bprm->argc + bprm->envc) * sizeof(void *); > + if (ptr_size > ULONG_MAX - size) > + goto fail; > + size += ptr_size; > + > acct_arg_size(bprm, size / PAGE_SIZE); Doesn't this over account? I mean this gets called for partial arguments as they fit into a page so a single argument can get into this function multiple times AFAIU. I also do not understand why would you want to account bprm->argc + bprm->envc pointers for each argument. > > /* > @@ -239,13 +249,15 @@ static struct page *get_arg_page(struct linux_binprm *bprm, unsigned long pos, > * to work from. > */ > rlim = current->signal->rlim; > - if (size > ACCESS_ONCE(rlim[RLIMIT_STACK].rlim_cur) / 4) { > - put_page(page); > - return NULL; > - } > + if (size > READ_ONCE(rlim[RLIMIT_STACK].rlim_cur) / 4) > + goto fail; > } > > return page; > + > +fail: > + put_page(page); > + return NULL; > } > > static void put_arg_page(struct page *page) > -- > 2.7.4 > > > -- > Kees Cook > Pixel Security > > -- > To unsubscribe, send a message with 'unsubscribe linux-mm' in > the body to majordomo@kvack.org. For more info on Linux MM, > see: http://www.linux-mm.org/ . > Don't email: email@kvack.org -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org