[PATCH 1/2] fs/exec: Explicitly unshare fs_struct on exec

[Eric W. Biederman]

David Laight <David.Laight at ACULAB.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