From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-lf0-f53.google.com (mail-lf0-f53.google.com [209.85.215.53]) by kanga.kvack.org (Postfix) with ESMTP id C9C8D828DF for ; Wed, 13 Jan 2016 15:23:14 -0500 (EST) Received: by mail-lf0-f53.google.com with SMTP id h129so67947272lfh.3 for ; Wed, 13 Jan 2016 12:23:14 -0800 (PST) Received: from mail-lf0-x243.google.com (mail-lf0-x243.google.com. [2a00:1450:4010:c07::243]) by mx.google.com with ESMTPS id i132si1602291lfb.191.2016.01.13.12.23.13 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 13 Jan 2016 12:23:13 -0800 (PST) Received: by mail-lf0-x243.google.com with SMTP id z62so7403357lfd.0 for ; Wed, 13 Jan 2016 12:23:13 -0800 (PST) MIME-Version: 1.0 In-Reply-To: References: <20160112190903.GA9421@www.outflux.net> <20160113090330.GA14630@quack.suse.cz> Date: Wed, 13 Jan 2016 23:23:13 +0300 Message-ID: Subject: Re: [PATCH v8] fs: clear file privilege bits when mmap writing From: Konstantin Khlebnikov Content-Type: text/plain; charset=UTF-8 Sender: owner-linux-mm@kvack.org List-ID: To: Kees Cook Cc: Jan Kara , Alexander Viro , Andy Lutomirski , yalin wang , Willy Tarreau , Andrew Morton , Linux-MM , LKML On Wed, Jan 13, 2016 at 7:09 PM, Kees Cook wrote: > On Wed, Jan 13, 2016 at 1:03 AM, Jan Kara wrote: >> On Tue 12-01-16 11:09:04, Kees Cook wrote: >>> Normally, when a user can modify a file that has setuid or setgid bits, >>> those bits are cleared when they are not the file owner or a member >>> of the group. This is enforced when using write and truncate but not >>> when writing to a shared mmap on the file. This could allow the file >>> writer to gain privileges by changing a binary without losing the >>> setuid/setgid/caps bits. >>> >>> Changing the bits requires holding inode->i_mutex, so it cannot be done >>> during the page fault (due to mmap_sem being held during the fault). >>> Instead, clear the bits if PROT_WRITE is being used at mmap open time, >>> or added at mprotect time. >>> >>> Since we can't do the check in the right place inside mmap (due to >>> holding mmap_sem), we have to do it before holding mmap_sem, which >>> means duplicating some checks, which have to be available to the non-MMU >>> builds too. >>> >>> When walking VMAs during mprotect, we need to drop mmap_sem (while >>> holding a file reference) and restart the walk after clearing privileges. >> >> ... >> >>> @@ -375,6 +376,7 @@ SYSCALL_DEFINE3(mprotect, unsigned long, start, size_t, len, >>> >>> vm_flags = calc_vm_prot_bits(prot); >>> >>> +restart: >>> down_write(¤t->mm->mmap_sem); >>> >>> vma = find_vma(current->mm, start); >>> @@ -416,6 +418,28 @@ SYSCALL_DEFINE3(mprotect, unsigned long, start, size_t, len, >>> goto out; >>> } >>> >>> + /* >>> + * If we're adding write permissions to a shared file, >>> + * we must clear privileges (like done at mmap time), >>> + * but we have to juggle the locks to avoid holding >>> + * mmap_sem while holding i_mutex. >>> + */ >>> + if ((vma->vm_flags & VM_SHARED) && vma->vm_file && >>> + (newflags & VM_WRITE) && !(vma->vm_flags & VM_WRITE) && >>> + !IS_NOSEC(file_inode(vma->vm_file))) { >> >> This code assumes that IS_NOSEC gets set for inode once file_remove_privs() >> is called. However that is not true for two reasons: >> >> 1) When you are root, SUID bit doesn't get cleared and thus you cannot set >> IS_NOSEC. >> >> 2) Some filesystems do not have MS_NOSEC set and for those IS_NOSEC is >> never true. >> >> So in these cases you'll loop forever. > > UUuugh. > >> >> You can check SUID bits without i_mutex so that could be done without >> dropping mmap_sem but you cannot easily call security_inode_need_killpriv() >> without i_mutex as that checks extended attributes (IMA) and that needs >> i_mutex to be held to avoid races with someone else changing the attributes >> under you. > > Yeah, that's why I changed this from Konstantin's original suggestion. > >> Honestly, I don't see a way of implementing this in mprotect() which would >> be reasonably elegant. > > Konstantin, any thoughts here? Getxattr works fine without i_mutex: sys_getxattr/vfs_getxattr doesn't lock it. If somebody changes xattrs under us we'll end up in race anyway. But this still safe: setxattrs are sychronized. > > -Kees > > -- > Kees Cook > Chrome OS & Brillo 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