Message-ID: <20230818173625.by6bud4u7uz2k4be@f>
Date:   Fri, 18 Aug 2023 19:36:25 +0200
From:   Mateusz Guzik <mjguzik@...il.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     syzbot <syzbot+6ec38f7a8db3b3fb1002@...kaller.appspotmail.com>,
        anton@...era.com, brauner@...nel.org, keescook@...omium.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.
> 

There is definitely an issue here.

The warn on comes from:
        /*
         * 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;

Where path_noexec is:
        return (path->mnt->mnt_flags & MNT_NOEXEC) ||
               (path->mnt->mnt_sb->s_iflags & SB_I_NOEXEC);

So as is I think this can race with mount update *adding* noexec after
may_open() returned but before the code gets to recheck.

I suspect i_mode can also change in the time window.

However, the reproducer does not try to race anything so this is not why
the warn.

>From my reading the kernel always lands in may_open() (as in, not
fs-specific) so as is I'm puzzled as to what happened. Maybe I'll try to
repro later.

> > 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
> > Modules linked in:
> > CPU: 0 PID: 5020 Comm: syz-executor300 Not tainted 6.5.0-rc6-syzkaller-00038-g16931859a650 #0
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/26/2023
> > RIP: 0010:do_open_execat+0x18f/0x3f0 fs/exec.c:933
> > Code: 8e 46 02 00 00 41 0f b7 1e bf 00 80 ff ff 66 81 e3 00 f0 89 de e8 b1 67 9b ff 66 81 fb 00 80 0f 84 8b 00 00 00 e8 51 6c 9b ff <0f> 0b 48 c7 c3 f3 ff ff ff e8 43 6c 9b ff 4c 89 e7 e8 4b c9 fe ff
> > RSP: 0018:ffffc90003b0fd10 EFLAGS: 00010293
> > RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
> > RDX: ffff888028401dc0 RSI: ffffffff81ea9c4f RDI: 0000000000000003
> > RBP: 1ffff92000761fa2 R08: 0000000000000003 R09: 0000000000008000
> > R10: 0000000000000000 R11: 0000000000000000 R12: ffff88802bf18780
> > R13: ffff888075d70000 R14: ffff8880742776a0 R15: 0000000000000001
> > FS:  000055555706b380(0000) GS:ffff8880b9800000(0000) knlGS:0000000000000000
> > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > CR2: 00007ffe0f1d3ff8 CR3: 0000000015f97000 CR4: 0000000000350ef0
> > Call Trace:
> >  <TASK>
> >  bprm_execve fs/exec.c:1830 [inline]
> >  bprm_execve+0x49d/0x1a50 fs/exec.c:1811
> >  do_execveat_common.isra.0+0x5d3/0x740 fs/exec.c:1963
> >  do_execve fs/exec.c:2037 [inline]
> >  __do_sys_execve fs/exec.c:2113 [inline]
> >  __se_sys_execve fs/exec.c:2108 [inline]
> >  __x64_sys_execve+0x8c/0xb0 fs/exec.c:2108
> >  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
> >  do_syscall_64+0x38/0xb0 arch/x86/entry/common.c:80
> >  entry_SYSCALL_64_after_hwframe+0x63/0xcd
> > RIP: 0033:0x7fee7ec27b39
> > Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 61 17 00 00 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b8 ff ff ff f7 d8 64 89 01 48
> > RSP: 002b:00007ffe6c369d28 EFLAGS: 00000246 ORIG_RAX: 000000000000003b
> > RAX: ffffffffffffffda RBX: 0031656c69662f2e RCX: 00007fee7ec27b39
> > RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000020000480
> > RBP: 00007fee7ec7004b R08: 000000000001ee3b R09: 0000000000000000
> > R10: 00007ffe6c369bf0 R11: 0000000000000246 R12: 00007fee7ec70055
> > R13: 00007ffe6c369f08 R14: 0000000000000001 R15: 0000000000000001
> >  </TASK>
> >
> >
> > ---
> > This report is generated by a bot. It may contain errors.
> > See https://goo.gl/tpsmEJ for more information about syzbot.
> > syzbot engineers can be reached at syzkaller@...glegroups.com.
> >
> > syzbot will keep track of this issue. See:
> > https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
> >
> > If the bug is already fixed, let syzbot know by replying with:
> > #syz fix: exact-commit-title
> >
> > If you want syzbot to run the reproducer, reply with:
> > #syz test: git://repo/address.git branch-or-commit-hash
> > If you attach or paste a git patch, syzbot will apply it before testing.
> >
> > If you want to overwrite bug's subsystems, reply with:
> > #syz set subsystems: new-subsystem
> > (See the list of subsystem names on the web dashboard)
> >
> > If the bug is a duplicate of another bug, reply with:
> > #syz dup: exact-subject-of-another-report
> >
> > If you want to undo deduplication, reply with:
> > #syz undup

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