linux-kernel - Re: [syzbot] [ntfs?] WARNING in do_open

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Date:   Fri, 18 Aug 2023 10:33:26 -0700
From:   Kees Cook <keescook@...omium.org>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     syzbot <syzbot+6ec38f7a8db3b3fb1002@...kaller.appspotmail.com>,
        anton@...era.com, brauner@...nel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-ntfs-dev@...ts.sourceforge.net,
        syzkaller-bugs@...glegroups.com, viro@...iv.linux.org.uk
Subject: Re: [syzbot] [ntfs?] WARNING in do_open_execat

On Fri, Aug 18, 2023 at 11:26:51AM -0500, Eric W. Biederman wrote:
> syzbot <syzbot+6ec38f7a8db3b3fb1002@...kaller.appspotmail.com> writes:
> 
> > Hello,
> >
> > syzbot found the following issue on:
> 
> Not an issue.
> Nothing to do with ntfs.
> 
> The code is working as designed and intended.
> 
> syzbot generated a malformed exec and the kernel made it
> well formed and warned about it.
> 
> Human beings who run syzbot please mark this as not an issue in your
> system.  The directions don't have a way to say that the code is working
> as expected and designed.

WARN and BUG should not be reachable from userspace, so if this can be
tripped we should take a closer look and likely fix it...

> > HEAD commit:    16931859a650 Merge tag 'nfsd-6.5-4' of git://git.kernel.or..
> > git tree:       upstream
> > console+strace: https://syzkaller.appspot.com/x/log.txt?x=13e2673da80000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=aa796b6080b04102
> > dashboard link: https://syzkaller.appspot.com/bug?extid=6ec38f7a8db3b3fb1002
> > compiler:       gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
> > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=17cdbc65a80000
> > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1262d8cfa80000
> >
> > Downloadable assets:
> > disk image: https://storage.googleapis.com/syzbot-assets/eecc010800b4/disk-16931859.raw.xz
> > vmlinux: https://storage.googleapis.com/syzbot-assets/f45ae06377a7/vmlinux-16931859.xz
> > kernel image: https://storage.googleapis.com/syzbot-assets/68891896edba/bzImage-16931859.xz
> > mounted in repro: https://storage.googleapis.com/syzbot-assets/4b6ab78b223a/mount_0.gz
> >
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+6ec38f7a8db3b3fb1002@...kaller.appspotmail.com
> >
> > ntfs: volume version 3.1.
> > process 'syz-executor300' launched './file1' with NULL argv: empty string added
> > ------------[ cut here ]------------
> > WARNING: CPU: 0 PID: 5020 at fs/exec.c:933 do_open_execat+0x18f/0x3f0 fs/exec.c:933

This is a double-check I left in place, since it shouldn't have been reachable:

        /*
         * 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.
         */
        err = -EACCES;
        if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode) ||
                         path_noexec(&file->f_path)))
                goto exit;

So yes, let's figure this out...

-- 
Kees Cook