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Message-ID: <87sfi3rmuk.fsf@email.froward.int.ebiederm.org>
Date: Mon, 28 Nov 2022 11:49:07 -0600
From: "Eric W. Biederman" <ebiederm@...ssion.com>
To: David Laight <David.Laight@...LAB.COM>
Cc: 'Andy Lutomirski' <luto@...nel.org>, Jann Horn <jannh@...gle.com>,
Christian Brauner <brauner@...nel.org>,
Kees Cook <keescook@...omium.org>,
Jorge Merlino <jorge.merlino@...onical.com>,
Al Viro <viro@...iv.linux.org.uk>,
Thomas Gleixner <tglx@...utronix.de>,
"Sebastian Andrzej Siewior" <bigeasy@...utronix.de>,
Andrew Morton <akpm@...ux-foundation.org>,
"linux-mm@...ck.org" <linux-mm@...ck.org>,
"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
"John Johansen" <john.johansen@...onical.com>,
Paul Moore <paul@...l-moore.com>,
James Morris <jmorris@...ei.org>,
"Serge E. Hallyn" <serge@...lyn.com>,
Stephen Smalley <stephen.smalley.work@...il.com>,
Eric Paris <eparis@...isplace.org>,
Richard Haines <richard_c_haines@...nternet.com>,
Casey Schaufler <casey@...aufler-ca.com>,
Xin Long <lucien.xin@...il.com>,
"David S. Miller" <davem@...emloft.net>,
Todd Kjos <tkjos@...gle.com>,
"Ondrej Mosnacek" <omosnace@...hat.com>,
Prashanth Prahlad <pprahlad@...hat.com>,
Micah Morton <mortonm@...omium.org>,
Fenghua Yu <fenghua.yu@...el.com>,
Andrei Vagin <avagin@...il.com>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
"apparmor@...ts.ubuntu.com" <apparmor@...ts.ubuntu.com>,
"linux-security-module@...r.kernel.org"
<linux-security-module@...r.kernel.org>,
"selinux@...r.kernel.org" <selinux@...r.kernel.org>,
"linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>
Subject: Re: [PATCH 1/2] fs/exec: Explicitly unshare fs_struct on exec
David Laight <David.Laight@...LAB.COM> writes:
> From: Andy Lutomirski
>> Sent: 14 October 2022 04:18
> ...
>> But seriously, this makes no sense at all. It should not be possible to exec a program and then,
>> without ptrace, change its cwd out from under it. Do we really need to preserve this behavior?
>
> it maybe ok if the exec'ed program also 'bought-in' to the
> fact that its cwd and open files might get changed.
> But imagine someone doing it to a login shell!
I am slowly catching up on my email and I saw this conversation.
When I initially saw this thread I was confused and thought this
might run into an issue with fs/locks.c. I was close but wrong.
fs/locks.c uses current->files as a sort of process identifier
and so is very sensitive to when it is unshared. Making
unsharing current->files unconditionally a bug. Not relevant to
this conversation.
There are several clone options that were only relevant for the old
LinuxThreads implementation including CLONE_FS and CLONE_SIGHAND.
The LinuxThreads implementation has not been needed since
the introduction of CLONE_THREAD in linux-2.6.0 in 17 Dec 2003.
Almost 20 years ago.
I suggest we introduce CONFIG_CLONE_FS and CONFIG_SIGHAND to allow
disabling support of these clone options. No known user space will
care. The are both getting in the way of kernel maintenance so there
is a reason to start pushing them out.
Further simply not worrying about UNSHARE_FS during exec fixes the
race so it essentially a bug fix by code removal.
I believe something like the patch below should get the job done.
diff --git a/fs/exec.c b/fs/exec.c
index a0b1f0337a62..7ff13c77ad04 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1186,7 +1186,8 @@ static int unshare_sighand(struct task_struct *me)
{
struct sighand_struct *oldsighand = me->sighand;
- if (refcount_read(&oldsighand->count) != 1) {
+ if (IS_ENABLED(CONFIG_CLONE_SIGHAND) &&
+ refcount_read(&oldsighand->count) != 1) {
struct sighand_struct *newsighand;
/*
* This ->sighand is shared with the CLONE_SIGHAND
@@ -1568,6 +1569,9 @@ static void check_unsafe_exec(struct linux_binprm *bprm)
if (task_no_new_privs(current))
bprm->unsafe |= LSM_UNSAFE_NO_NEW_PRIVS;
+ if (!IS_ENABLED(CONFIG_CLONE_FS))
+ return;
+
t = p;
n_fs = 1;
spin_lock(&p->fs->lock);
diff --git a/init/Kconfig b/init/Kconfig
index 94125d3b6893..8660a6bcc1cf 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1764,6 +1764,23 @@ config KALLSYMS_BASE_RELATIVE
time constants, and no relocation pass is required at runtime to fix
up the entries based on the runtime load address of the kernel.
+config CLONE_FS
+ bool
+ default y
+ help
+ Support CLONE_FS being passed to clone. The only known user
+ is the old LinuxThreads package so it should be safe to disable
+ this option.
+
+config CLONE_SIGHAND
+ bool
+ default y
+ help
+ Support CLONE_SIGHAND being passed to clone. The only known user
+ is the old LinuxThreads package so it should be safe to disable
+ this option.
+
+
# end of the "standard kernel features (expert users)" menu
# syscall, maps, verifier
diff --git a/kernel/fork.c b/kernel/fork.c
index 08969f5aa38d..da9017b51da4 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2023,6 +2023,16 @@ static __latent_entropy struct task_struct *copy_process(
if ((clone_flags & CLONE_SIGHAND) && !(clone_flags & CLONE_VM))
return ERR_PTR(-EINVAL);
+ /* Don't allow CLONE_FS if not enabled */
+ if (!IS_ENABLED(CONFIG_CLONE_FS) &&
+ ((clone_flags & (CLONE_THREAD | CLONE_FS)) == CLONE_FS))
+ return ERR_PTR(-EINVAL);
+
+ /* Don't allow CLONE_SIGHAND if not enabled */
+ if (!IS_ENABLED(CONFIG_CLONE_SIGHAND) &&
+ ((clone_flags & (CLONE_THREAD | CLONE_SIGHAND)) == CLONE_SIGHAND))
+ return ERR_PTR(-EINVAL);
+
/*
* Siblings of global init remain as zombies on exit since they are
* not reaped by their parent (swapper). To solve this and to avoid
@@ -3101,6 +3111,9 @@ static int unshare_fs(unsigned long unshare_flags, struct fs_struct **new_fsp)
if (fs->users == 1)
return 0;
+ if (!IS_ENABLED(CONFIG_CLONE_FS))
+ return -EINVAL;
+
*new_fsp = copy_fs_struct(fs);
if (!*new_fsp)
return -ENOMEM;
Eric
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