From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-15.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id ED23AC433E6 for ; Tue, 16 Mar 2021 08:32:14 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id C7A8265006 for ; Tue, 16 Mar 2021 08:32:14 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234855AbhCPIbr (ORCPT ); Tue, 16 Mar 2021 04:31:47 -0400 Received: from mx2.suse.de ([195.135.220.15]:52350 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234850AbhCPIb0 (ORCPT ); Tue, 16 Mar 2021 04:31:26 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1615883484; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=Pf+nc+lIl8kMGUiohYuj/WARbXesnkwspb9bTfbVJ5k=; b=fYCNUhM9G+jM/lC2L5iP8K5xwVZgD0e7S00eWqsagRV8vjV/eNF7/EreaylgS+j3v58FLk ZFoKRHFopLT2AoQ5/3F5eK4+CXvpp40b+R3w5snFN1cDy2vn2nLgUKo1TAIN4UMTZN328n ACb+akvRNm0HlEhhfSWrB6oHJo9cOxQ= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id BB7D9AE5C; Tue, 16 Mar 2021 08:31:24 +0000 (UTC) Date: Tue, 16 Mar 2021 09:31:23 +0100 From: Michal Hocko To: Kees Cook Cc: Andrew Morton , Greg Kroah-Hartman , Alexey Dobriyan , Lee Duncan , Chris Leech , Adam Nichols , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-hardening@vger.kernel.org Subject: Re: [PATCH v2] seq_file: Unconditionally use vmalloc for buffer Message-ID: References: <20210315174851.622228-1-keescook@chromium.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210315174851.622228-1-keescook@chromium.org> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon 15-03-21 10:48:51, Kees Cook wrote: > The sysfs interface to seq_file continues to be rather fragile, as seen > with some recent exploits[1]. Move the seq_file buffer to the vmap area > (while retaining the accounting flag), since it has guard pages that > will catch and stop linear overflows. This seems justified given that > seq_file already uses kvmalloc(), is almost always using a PAGE_SIZE or > larger allocation, has allocations are normally short lived, and is not > normally on a performance critical path. I have already objected without having my concerns really addressed. Your observation that most of buffers are PAGE_SIZE in the vast majority cases matches my experience and kmalloc should perform better than vmalloc. You should check the most common /proc readers at least. Also this cannot really be done for configurations with a very limited vmalloc space (32b for example). Those systems are more and more rare but you shouldn't really allow userspace to deplete the vmalloc space. I would be also curious to see how vmalloc scales with huge number of single page allocations which would be easy to trigger with this patch. > [1] https://blog.grimm-co.com/2021/03/new-old-bugs-in-linux-kernel.html > > Signed-off-by: Kees Cook > --- > fs/seq_file.c | 15 ++++++++++----- > 1 file changed, 10 insertions(+), 5 deletions(-) > > diff --git a/fs/seq_file.c b/fs/seq_file.c > index cb11a34fb871..16fb4a4e61e3 100644 > --- a/fs/seq_file.c > +++ b/fs/seq_file.c > @@ -32,7 +32,12 @@ static void seq_set_overflow(struct seq_file *m) > > static void *seq_buf_alloc(unsigned long size) > { > - return kvmalloc(size, GFP_KERNEL_ACCOUNT); > + /* > + * To be proactively defensive against buggy seq_get_buf() callers > + * (i.e. sysfs handlers), use the vmap area to gain the trailing > + * guard page which will protect against linear buffer overflows. > + */ > + return __vmalloc(size, GFP_KERNEL_ACCOUNT); > } > > /** > @@ -130,7 +135,7 @@ static int traverse(struct seq_file *m, loff_t offset) > > Eoverflow: > m->op->stop(m, p); > - kvfree(m->buf); > + vfree(m->buf); > m->count = 0; > m->buf = seq_buf_alloc(m->size <<= 1); > return !m->buf ? -ENOMEM : -EAGAIN; > @@ -237,7 +242,7 @@ ssize_t seq_read_iter(struct kiocb *iocb, struct iov_iter *iter) > goto Fill; > // need a bigger buffer > m->op->stop(m, p); > - kvfree(m->buf); > + vfree(m->buf); > m->count = 0; > m->buf = seq_buf_alloc(m->size <<= 1); > if (!m->buf) > @@ -349,7 +354,7 @@ EXPORT_SYMBOL(seq_lseek); > int seq_release(struct inode *inode, struct file *file) > { > struct seq_file *m = file->private_data; > - kvfree(m->buf); > + vfree(m->buf); > kmem_cache_free(seq_file_cache, m); > return 0; > } > @@ -585,7 +590,7 @@ int single_open_size(struct file *file, int (*show)(struct seq_file *, void *), > return -ENOMEM; > ret = single_open(file, show, data); > if (ret) { > - kvfree(buf); > + vfree(buf); > return ret; > } > ((struct seq_file *)file->private_data)->buf = buf; > -- > 2.25.1 -- Michal Hocko SUSE Labs