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Message-ID: <202504290957.1D6835B89@keescook>
Date: Tue, 29 Apr 2025 09:57:24 -0700
From: Kees Cook <kees@...nel.org>
To: Oleg Nesterov <oleg@...hat.com>
Cc: brauner@...nel.org, viro@...iv.linux.org.uk, jack@...e.cz,
linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-mm@...ck.org, syzkaller-bugs@...glegroups.com,
mjguzik@...il.com, Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH] exec: fix the racy usage of fs_struct->in_exec
On Tue, Apr 29, 2025 at 05:49:44PM +0200, Oleg Nesterov wrote:
> Damn, I am stupid.
>
> On 03/24, Oleg Nesterov wrote:
> >
> > check_unsafe_exec() sets fs->in_exec under cred_guard_mutex, then execve()
> > paths clear fs->in_exec lockless. This is fine if exec succeeds, but if it
> > fails we have the following race:
> >
> > T1 sets fs->in_exec = 1, fails, drops cred_guard_mutex
> >
> > T2 sets fs->in_exec = 1
> >
> > T1 clears fs->in_exec
>
> When I look at this code again, I think this race was not possible and thus
> this patch (applied as af7bb0d2ca45) was not needed.
>
> Yes, begin_new_exec() can drop cred_guard_mutex on failure, but only after
> de_thread() succeeds, when we can't race with another sub-thread.
>
> I hope this patch didn't make the things worse so we don't need to revert it.
> Plus I think it makes this (confusing) logic a bit more clear. Just, unless
> I am confused again, it wasn't really needed.
>
> -----------------------------------------------------------------------------
> But. I didn't read the original report from syzbot,
> https://lore.kernel.org/all/67dc67f0.050a0220.25ae54.001f.GAE@google.com/#t
> because I wasn't CC'ed. and then - sorry Kees!!! - I didn't bother to read
> your first reply carefully.
>
> So yes, with or without this patch the "if (fs->in_exec)" check in copy_fs()
> can obviously hit the 1 -> 0 transition.
>
> This is harmless, but should be probably fixed just to avoid another report
> from KCSAN.
>
> I do not want to add another spin_lock(fs->lock). We can change copy_fs() to
> use data_race(), but I'd prefer the patch below. Yes, it needs the additional
> comment(s) to explain READ_ONCE().
>
> What do you think? Did I miss somthing again??? Quite possibly...
>
> Mateusz, I hope you will cleanup this horror sooner or later ;)
>
> Oleg.
> ---
>
> diff --git a/fs/exec.c b/fs/exec.c
> index 5d1c0d2dc403..42a7f9b43911 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1495,7 +1495,7 @@ static void free_bprm(struct linux_binprm *bprm)
> free_arg_pages(bprm);
> if (bprm->cred) {
> /* in case exec fails before de_thread() succeeds */
> - current->fs->in_exec = 0;
> + WRITE_ONCE(current->fs->in_exec, 0);
> mutex_unlock(¤t->signal->cred_guard_mutex);
> abort_creds(bprm->cred);
> }
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 4c2df3816728..381af8c8ece8 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -1802,7 +1802,7 @@ static int copy_fs(unsigned long clone_flags, struct task_struct *tsk)
> /* tsk->fs is already what we want */
> spin_lock(&fs->lock);
> /* "users" and "in_exec" locked for check_unsafe_exec() */
> - if (fs->in_exec) {
> + if (READ_ONCE(fs->in_exec)) {
> spin_unlock(&fs->lock);
> return -EAGAIN;
> }
>
Yeah, this seems reasonable.
--
Kees Cook
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