From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ig0-f179.google.com (mail-ig0-f179.google.com [209.85.213.179]) by kanga.kvack.org (Postfix) with ESMTP id 19C03828EB for ; Mon, 11 Jan 2016 17:45:59 -0500 (EST) Received: by mail-ig0-f179.google.com with SMTP id h5so80271178igh.0 for ; Mon, 11 Jan 2016 14:45:59 -0800 (PST) Received: from mail-io0-x235.google.com (mail-io0-x235.google.com. [2607:f8b0:4001:c06::235]) by mx.google.com with ESMTPS id j6si28085274igj.2.2016.01.11.14.45.58 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 11 Jan 2016 14:45:58 -0800 (PST) Received: by mail-io0-x235.google.com with SMTP id g73so168924681ioe.3 for ; Mon, 11 Jan 2016 14:45:58 -0800 (PST) MIME-Version: 1.0 In-Reply-To: References: <20160108232727.GA23490@www.outflux.net> Date: Mon, 11 Jan 2016 14:45:57 -0800 Message-ID: Subject: Re: [PATCH v6] fs: clear file privilege bits when mmap writing From: Kees Cook Content-Type: text/plain; charset=UTF-8 Sender: owner-linux-mm@kvack.org List-ID: To: Konstantin Khlebnikov Cc: Alexander Viro , Andy Lutomirski , Jan Kara , yalin wang , Willy Tarreau , Andrew Morton , linux-fsdevel , linux-arch , Linux API , "linux-mm@kvack.org" On Mon, Jan 11, 2016 at 2:39 PM, Konstantin Khlebnikov wrote: > On Mon, Jan 11, 2016 at 10:38 PM, Kees Cook wrote: >> On Sun, Jan 10, 2016 at 7:48 AM, Konstantin Khlebnikov wrote: >>> On Sat, Jan 9, 2016 at 2:27 AM, 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). We >>>> could do this during vm_mmap_pgoff, but that would need coverage in >>>> mprotect as well, but to check for MAP_SHARED, we'd need to hold mmap_sem >>>> again. We could clear at open() time, but it's possible things are >>>> accidentally opening with O_RDWR and only reading. Better to clear on >>>> close and error failures (i.e. an improvement over now, which is not >>>> clearing at all). >>> >>> I think this should be done in mmap/mprotect. Code in sys_mmap is trivial. >>> >>> In sys_mprotect you can check file_needs_remove_privs() and VM_SHARED >>> under mmap_sem, then if needed grab reference to struct file from vma and >>> clear suid after unlocking mmap_sem. >>> >>> I haven't seen previous iterations, probably this approach has known flaws. >> >> mmap_sem is still needed in mprotect (to find and hold the vma), so >> it's not possible. I'd love to be proven wrong, but I didn't see a >> way. > > something like this > > @@ -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 +418,21 @@ SYSCALL_DEFINE3(mprotect, unsigned long, start, > size_t, len, > goto out; > } > > + if ((newflags & VM_WRITE) && !(vma->vm_flags & VM_WRITE) && > + vma->vm_file && file_needs_remove_privs(vma->vm_file)) { > + struct file *file = get_file(vma->vm_file); > + > + start = vma->vm_start; > + up_write(¤t->mm->mmap_sem); > + mutex_lock(&file_inode(file)->i_mutex); > + error = file_remove_privs(file); > + mutex_unlock(&file_inode(file)->i_mutex); > + fput(file); > + if (error) > + return error; > + goto restart; > + } > + Is this safe against the things Al mentioned? I still don't like the mmap/mprotect approach because it makes the change before anything was actually written... -Kees > > >> >> -Kees >> >> -- >> Kees Cook >> Chrome OS & Brillo Security -- 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