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Message-ID: <202205181215.D448675BEA@keescook>
Date: Wed, 18 May 2022 12:17:45 -0700
From: Kees Cook <keescook@...omium.org>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Zhihao Cheng <chengzhihao1@...wei.com>, viro@...iv.linux.org.uk,
linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
linux-kernel@...r.kernel.org, yukuai3@...wei.com
Subject: Re: [PATCH -next] exec: Remove redundant check in
do_open_execat/uselib
On Wed, May 18, 2022 at 10:46:01AM -0700, Andrew Morton wrote:
> On Wed, 18 May 2022 16:12:27 +0800 Zhihao Cheng <chengzhihao1@...wei.com> wrote:
>
> > There is a false positive WARNON happening in execve(2)/uselib(2)
> > syscalls with concurrent noexec-remount.
> >
> > execveat remount
> > do_open_execat(path/bin)
> > do_filp_open
> > path_openat
> > do_open
> > may_open
> > path_noexec() // PASS
> > remount(path->mnt, MS_NOEXEC)
> > WARNON(path_noexec(&file->f_path)) // path_noexec() checks fail
Did you encounter this in the real world?
>
> You're saying this is a race condition? A concurrent remount causes
> this warning?
It seems not an unreasonable thing to warn about. Perhaps since it's
technically reachable from userspace, it could be downgraded to
pr_warn(), but I certainly don't want to remove the checks.
>
> > Since may_open() has already checked the same conditions, fix it by
> > removing 'S_ISREG' and 'path_noexec' check in do_open_execat()/uselib(2).
> >
> > ...
> >
> > --- a/fs/exec.c
> > +++ b/fs/exec.c
> > @@ -141,16 +141,6 @@ SYSCALL_DEFINE1(uselib, const char __user *, library)
> > if (IS_ERR(file))
> > goto out;
> >
> > - /*
> > - * may_open() has already checked for this, so it should be
> > - * impossible to trip now. But we need to be extra cautious
> > - * and check again at the very end too.
> > - */
> > - error = -EACCES;
> > - if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode) ||
> > - path_noexec(&file->f_path)))
> > - goto exit;
> > -
>
> Maybe we should retain the `goto exit'. The remount has now occurred,
> so the execution attempt should be denied. If so, the comment should
> be updated to better explain what's happening.
>
> I guess we'd still be racy against `mount -o exec', but accidentally
> denying something seems less serious than accidentally permitting it.
I'd like to leave this as-is, since we _do_ want to find the cases where
we're about to allow an exec and a very important security check was NOT
handled.
--
Kees Cook
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