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Date: Thu, 22 Jun 2017 21:27:10 -0700
From: Kees Cook <keescook@...omium.org>
To: Solar Designer <solar@...nwall.com>
Cc: "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>
Subject: Re: symlink/hardlink/FIFO restrictions
On Wed, Jun 7, 2017 at 5:06 AM, Solar Designer <solar@...nwall.com> wrote:
> On Tue, Jun 06, 2017 at 09:10:43PM -0700, Kees Cook wrote:
>> On Tue, Jun 6, 2017 at 2:55 PM, Solar Designer <solar@...nwall.com> wrote:
>> > The hardlink restrictions differ between -ow vs. grsecurity and upstream.
>> > In -ow, I had the check in vfs_link(). In grsecurity (oldest I checked
>> > now is grsecurity-2.1.11-2.4.35-200708071800), it's in sys_link() and
>> > later in sys_linkat(), which is also called by sys_link(). Upstream
>> > also has it right in the linkat() syscall function. Maybe there were
>> > good reasons not to do it in vfs_link(), but I am unaware of those.
>> > We need to know what currently happens and what we'd like to happen for
>> > other callers to vfs_link(), such as kernel nfsd and CRIU - do we want
>> > the restrictions to apply there? Maybe for some of those other callers,
>> > but not for all? Do the same checks work correctly when called from
>> > those other contexts or do we need revised checks there?
>>
>> Hm, yeah, I don't remember sys_linkat() vs vfs_link() coming up during
>> review, but it's been a while. The nfsd thing is interesting... would
>> it be right to refuse a client's linkat based on the server's link
>> restriction sysctl? Hmm.
>
> Probably it would, because otherwise we have a bypass in case the
> server's filesystem is or will be later also mounted elsewhere.
>
>> > As to the FIFO restrictions, those don't appear to have been merged
>> > upstream yet.
>>
>> Do the FIFO restrictions exist in -ow too?
>
> Yes, and they pre-date grsecurity too.
>
>> I wonder what enabling that protection might break...
>
> I was adding notes on what broke to the -ow patch FAQ, but it lists
> nothing for the FIFOs. So apparently nothing of note broke, maybe
> because the restrictions were limited to what was required (e.g., didn't
> apply to FIFOs opened without O_CREAT).
>
> +Restricted FIFOs in /tmp
> +CONFIG_HARDEN_FIFO
> + In addition to restricting links, you might also want to restrict
> + writes into untrusted FIFOs (named pipes), to make data spoofing
> + attacks harder. Enabling this option disallows writing into FIFOs
> + not owned by the user in +t directories, unless the owner is the
> + same as that of the directory or the FIFO is opened without the
> + O_CREAT flag.
>
> @@ -1084,6 +1126,32 @@ do_last:
> /*
> * It already exists.
> */
> +
> +#ifdef CONFIG_HARDEN_FIFO
> + /*
> + * Don't write to FIFOs that we don't own in +t directories,
> + * unless the FIFO is owned by the owner of the directory.
> + *
> + * Do this check early while we hold the directory.
> + */
> + inode = dentry->d_inode;
> + if (S_ISFIFO(inode->i_mode) && !(flag & O_EXCL) &&
> + (dir->d_inode->i_mode & S_ISVTX) &&
> + inode->i_uid != dir->d_inode->i_uid &&
> + current->fsuid != inode->i_uid) {
> + up(&dir->d_inode->i_sem);
> + if (!permission(inode, acc_mode))
> + security_alert("denied writing FIFO of %d.%d "
> + "by UID %d, EUID %d, process %s:%d",
> + "writes into a FIFO denied",
> + inode->i_uid, inode->i_gid,
> + current->uid, current->euid,
> + current->comm, current->pid);
> + error = -EACCES;
> + goto exit_dput;
> + }
> +#endif
>
> The intent was to detect unintentional writes to FIFOs, where the
> program expected to create a regular file with this very same call.
>
> An argument against restricting this is that similar attacks are also
> possible via attacker-created regular files, especially now that we have
> inotify, although FIFOs made the attacks particularly simple and
> reliable (no need to win a race).
[trying to get back to older emails...]
Looks reasonable. Do you want to put together a patch for this? It
seems like it'd be a useful addition to complement the link
restrictions.
-Kees
--
Kees Cook
Pixel Security
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