Re: [syzbot] general protection fault in legacy_parse_param

[Christian Brauner <christian.brauner@ubuntu.com>]

On Fri, Aug 27, 2021 at 08:40:15AM -0700, Casey Schaufler wrote:
> On 8/27/2021 8:30 AM, Christian Brauner wrote:
> > On Tue, Jul 06, 2021 at 08:50:44AM -0400, Paul Moore wrote:
> >> On Mon, Jul 5, 2021 at 1:52 AM Dmitry Vyukov <dvyukov@google.com> wrote:
> >>> On Sun, Jul 4, 2021 at 4:14 PM Paul Moore <paul@paul-moore.com> wrote:
> >>>> On Sat, Jul 3, 2021 at 6:16 PM Casey Schaufler <casey@schaufler-ca.com> wrote:
> >>>>> On 7/2/2021 10:51 PM, Dmitry Vyukov wrote:
> >>>>>> On Sat, Jul 3, 2021 at 7:41 AM syzbot
> >>>>>> <syzbot+d1e3b1d92d25abf97943@syzkaller.appspotmail.com> wrote:
> >>>>>>> Hello,
> >>>>>>>
> >>>>>>> syzbot found the following issue on:
> >>>>>>>
> >>>>>>> HEAD commit:    62fb9874 Linux 5.13
> >>>>>>> git tree:       upstream
> >>>>>>> console output: https://syzkaller.appspot.com/x/log.txt?x=12ffa118300000
> >>>>>>> kernel config:  https://syzkaller.appspot.com/x/.config?x=19404adbea015a58
> >>>>>>> dashboard link: https://syzkaller.appspot.com/bug?extid=d1e3b1d92d25abf97943
> >>>>>>> compiler:       Debian clang version 11.0.1-2
> >>>>>>>
> >>>>>>> Unfortunately, I don't have any reproducer for this issue yet.
> >>>>>>>
> >>>>>>> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> >>>>>>> Reported-by: syzbot+d1e3b1d92d25abf97943@syzkaller.appspotmail.com
> >>>>>> +Casey for what looks like a smackfs issue
> >>>>> This is from the new mount infrastructure introduced by
> >>>>> David Howells in November 2018. It makes sense that there
> >>>>> may be a problem in SELinux as well, as the code was introduced
> >>>>> by the same developer at the same time for the same purpose.
> >>>>>
> >>>>>> The crash was triggered by this test case:
> >>>>>>
> >>>>>> 21:55:33 executing program 1:
> >>>>>> r0 = fsopen(&(0x7f0000000040)='ext3\x00', 0x1)
> >>>>>> fsconfig$FSCONFIG_SET_STRING(r0, 0x1, &(0x7f00000002c0)='smackfsroot',
> >>>>>> &(0x7f0000000300)='default_permissions', 0x0)
> >>>>>>
> >>>>>> And I think the issue is in smack_fs_context_parse_param():
> >>>>>> https://elixir.bootlin.com/linux/latest/source/security/smack/smack_lsm.c#L691
> >>>>>>
> >>>>>> But it seems that selinux_fs_context_parse_param() contains the same issue:
> >>>>>> https://elixir.bootlin.com/linux/latest/source/security/selinux/hooks.c#L2919
> >>>>>> +So selinux maintainers as well.
> >>>>>>
> >>>>>>> general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] PREEMPT SMP KASAN
> >>>>>>> KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
> >>>>>>> CPU: 0 PID: 20300 Comm: syz-executor.1 Not tainted 5.13.0-syzkaller #0
> >>>>>>> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> >>>>>>> RIP: 0010:memchr+0x2f/0x70 lib/string.c:1054
> >>>>>>> Code: 41 54 53 48 89 d3 41 89 f7 45 31 f6 49 bc 00 00 00 00 00 fc ff df 0f 1f 44 00 00 48 85 db 74 3b 48 89 fd 48 89 f8 48 c1 e8 03 <42> 0f b6 04 20 84 c0 75 0f 48 ff cb 48 8d 7d 01 44 38 7d 00 75 db
> >>>>>>> RSP: 0018:ffffc90001dafd00 EFLAGS: 00010246
> >>>>>>> RAX: 0000000000000000 RBX: 0000000000000013 RCX: dffffc0000000000
> >>>>>>> RDX: 0000000000000013 RSI: 000000000000002c RDI: 0000000000000000
> >>>>>>> RBP: 0000000000000000 R08: ffffffff81e171bf R09: ffffffff81e16f95
> >>>>>>> R10: 0000000000000002 R11: ffff88807e96b880 R12: dffffc0000000000
> >>>>>>> R13: ffff888020894000 R14: 0000000000000000 R15: 000000000000002c
> >>>>>>> FS:  00007fe01ae27700(0000) GS:ffff8880b9a00000(0000) knlGS:0000000000000000
> >>>>>>> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> >>>>>>> CR2: 00000000005645a8 CR3: 0000000018afc000 CR4: 00000000001506f0
> >>>>>>> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> >>>>>>> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> >>>>>>> Call Trace:
> >>>>>>>  legacy_parse_param+0x461/0x7e0 fs/fs_context.c:537
> >>>>>>>  vfs_parse_fs_param+0x1e5/0x460 fs/fs_context.c:117
> >>>> It's Sunday morning and perhaps my mind is not yet in a "hey, let's
> >>>> look at VFS kernel code!" mindset, but I'm not convinced the problem
> >>>> is the 'param->string = NULL' assignment in the LSM hooks.  In both
> >>>> the case of SELinux and Smack that code ends up returning either a 0
> >>>> (Smack) or a 1 (SELinux) - that's a little odd in it's own way, but I
> >>>> don't believe it is relevant here - either way these return values are
> >>>> not equal to -ENOPARAM so we should end up returning early from
> >>>> vfs_parse_fs_param before it calls down into legacy_parse_param():
> >>>>
> >>>> Taken from https://elixir.bootlin.com/linux/latest/source/fs/fs_context.c#L109 :
> >>>>
> >>>>   ret = security_fs_context_parse_param(fc, param);
> >>>>   if (ret != -ENOPARAM)
> >>>>     /* Param belongs to the LSM or is disallowed by the LSM; so
> >>>>      * don't pass to the FS.
> >>>>      */
> >>>>     return ret;
> >>>>
> >>>>   if (fc->ops->parse_param) {
> >>>>     ret = fc->ops->parse_param(fc, param);
> >>>>     if (ret != -ENOPARAM)
> >>>>       return ret;
> >>>>   }
> >>> Hi Paul,
> >>>
> >>> You are right.
> >>> I almost connected the dots, but not exactly.
> >>> Now that I read more code around, setting "param->string = NULL" in
> >>> smack_fs_context_parse_param() looks correct to me (the fs copies and
> >>> takes ownership of the string).
> >>>
> >>> I don't see how the crash happened...
> >> FWIW, I poked around a bit too and couldn't see anything obvious
> >> either, but I can't pretend to know as much about the VFS layer as the
> >> VFS folks.  Hopefully they might have better luck.
> > I'm not sure that's right.
> > If the smack hook runs first, it will set
> >
> > param->string = NULL
> >
> > now the selinux hook runs. But the selinux param hook doesn't end up in
> > selinux_add_opt() instead it will fail before
> > opt = fs_parse(fc, selinux_fs_parameters, param, &result);
> > which will return -ENOPARAM since it's not a selinux option subsequently
> > causing the crash.
> >
> > Does that sound plausible?
> 
> No. You can't (currently) have both Smack and SELinux enabled at

Ah, I thought that already worked. :)

I'm EOD here but I'll try to look closer tomorrow or after the weekend.

Christian