Re: [syzbot] general protection fault in legacy_parse_param

[Dmitry Vyukov <dvyukov@google.com>]

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...



> > >>  vfs_fsconfig_locked fs/fsopen.c:265 [inline]
> > >>  __do_sys_fsconfig fs/fsopen.c:439 [inline]
> > >>  __se_sys_fsconfig+0xba9/0xff0 fs/fsopen.c:314
> > >>  do_syscall_64+0x3f/0xb0 arch/x86/entry/common.c:47
> > >>  entry_SYSCALL_64_after_hwframe+0x44/0xae
> > >> RIP: 0033:0x4665d9
> > >> Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 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 bc ff ff ff f7 d8 64 89 01 48
> > >> RSP: 002b:00007fe01ae27188 EFLAGS: 00000246 ORIG_RAX: 00000000000001af
> > >> RAX: ffffffffffffffda RBX: 000000000056bf80 RCX: 00000000004665d9
> > >> RDX: 00000000200002c0 RSI: 0000000000000001 RDI: 0000000000000003
> > >> RBP: 00000000004bfcb9 R08: 0000000000000000 R09: 0000000000000000
> > >> R10: 0000000020000300 R11: 0000000000000246 R12: 000000000056bf80
> > >> R13: 00007ffd4bb7c5bf R14: 00007fe01ae27300 R15: 0000000000022000
> > >> Modules linked in:
> > >> ---[ end trace 5d7119165725bd63 ]---
>
> --
> paul moore
> www.paul-moore.com