Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933486AbbELPGi (ORCPT ); Tue, 12 May 2015 11:06:38 -0400 Received: from mail-ie0-f171.google.com ([209.85.223.171]:34162 "EHLO mail-ie0-f171.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932537AbbELPGd (ORCPT ); Tue, 12 May 2015 11:06:33 -0400 MIME-Version: 1.0 In-Reply-To: <20150512094305.24768.51807.stgit@buzz> References: <20150512090156.24768.2521.stgit@buzz> <20150512094305.24768.51807.stgit@buzz> Date: Tue, 12 May 2015 08:06:32 -0700 X-Google-Sender-Auth: apS8rV24pEE_yRkRidooqnwEtz0 Message-ID: Subject: Re: [PATCH v2 2/3] pagemap: hide physical addresses from non-privileged users From: Linus Torvalds To: Konstantin Khlebnikov Cc: linux-mm , Naoya Horiguchi , Linux Kernel Mailing List , Andrew Morton , Mark Williamson , Pavel Emelyanov , Linux API , Andy Lutomirski , Vlastimil Babka , Pavel Machek , Mark Seaborn , "Kirill A. Shutemov" , Daniel James , Finn Grimwood Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2510 Lines: 68 On Tue, May 12, 2015 at 2:43 AM, Konstantin Khlebnikov wrote: > @@ -1260,6 +1269,8 @@ static ssize_t pagemap_read(struct file *file, char __user *buf, > if (!count) > goto out_task; > > + /* do not disclose physical addresses: attack vector */ > + pm.show_pfn = capable(CAP_SYS_ADMIN); > pm.v2 = soft_dirty_cleared; > pm.len = (PAGEMAP_WALK_SIZE >> PAGE_SHIFT); > pm.buffer = kmalloc(pm.len * PM_ENTRY_BYTES, GFP_TEMPORARY); NO! Dammit, no, no, no! How many times must people do this major security faux-pas before we learn? WE DO NOT CHECK CURRENT CAPABILITIES AT READ/WRITE TIME! It's a bug. It's a security issue. It's not how Unix capabilities work! Capabilities are checked at open time.: > @@ -1335,9 +1346,6 @@ out: > > static int pagemap_open(struct inode *inode, struct file *file) > { > - /* do not disclose physical addresses: attack vector */ > - if (!capable(CAP_SYS_ADMIN)) > - return -EPERM; THIS is where you are supposed to check for capabilities. The place where you removed it! The reason we check capabilities at open time, and open time ONLY is because that is really very integral to the whole Unix security model. Otherwise, you get into this situation: - unprivileged process opens file - unprivileged process tricks suid process to do the actual access for it where the traditional model is to just force a "write()" by opening the file as stderr, and then executing a suid process (traditionally "sudo") that writes an error message to it. So *don't* do permission checks using read/write time credentials. They are wrong. Now, if there is some reason that you really can't do it when opening the file, and you actually need to use capability information at read/write time, you use the "file->f_cred" field, which is the open-time capabilities. So you _can_ do permission checks at read/write time, but you have to use the credentials of the opener, not "current". So in this case, I guess you could use pm.show_pfn = file_ns_capable(file, &init_user_ns, CAP_SYS_ADMIN); if you really need to do this at read time, and cannot fill in that "show_pfn" at open-time. Linus -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/