Re: commit_creds oops

[Eric W. Biederman]

Dave Jones <davej@xxxxxxxxxx> writes:

> Just hit this on Linus' current tree.
>
> [   89.621770] BUG: unable to handle kernel NULL pointer dereference at 00000000000000c8
> [   89.623111] IP: [<ffffffff810784b0>] commit_creds+0x250/0x2f0
> [   89.624062] PGD 122bfd067 PUD 122bfe067 PMD 0 
> [   89.624901] Oops: 0000 [#1] PREEMPT SMP 
> [   89.625678] Modules linked in: caif_socket caif netrom bridge hidp 8021q garp stp mrp rose llc2 af_rxrpc phonet af_key binfmt_misc bnep l2tp_ppp can_bcm l2tp_core pppoe pppox can_raw scsi_transport_iscsi ppp_generic slhc nfnetlink can ipt_ULOG ax25 decnet irda nfc rds x25 crc_ccitt appletalk atm ipx p8023 psnap p8022 llc lockd sunrpc ip6t_REJECT nf_conntrack_ipv6 nf_defrag_ipv6 xt_conntrack nf_conntrack ip6table_filter ip6_tables btusb bluetooth snd_hda_codec_realtek snd_hda_intel snd_hda_codec snd_pcm vhost_net snd_page_alloc snd_timer tun macvtap usb_debug snd rfkill microcode macvlan edac_core pcspkr serio_raw kvm_amd soundcore kvm r8169 mii
> [   89.637846] CPU 2 
> [   89.638175] Pid: 782, comm: trinity-main Not tainted 3.8.0+ #63 Gigabyte Technology Co., Ltd. GA-MA78GM-S2H/GA-MA78GM-S2H
> [   89.639850] RIP: 0010:[<ffffffff810784b0>]  [<ffffffff810784b0>] commit_creds+0x250/0x2f0
> [   89.641161] RSP: 0018:ffff880115657eb8  EFLAGS: 00010207
> [   89.641984] RAX: 00000000000003e8 RBX: ffff88012688b000 RCX: 0000000000000000
> [   89.643069] RDX: 0000000000000000 RSI: ffffffff81c32960 RDI: ffff880105839600
> [   89.644167] RBP: ffff880115657ed8 R08: 0000000000000000 R09: 0000000000000000
> [   89.645254] R10: 0000000000000001 R11: 0000000000000246 R12: ffff880105839600
> [   89.646340] R13: ffff88011beea490 R14: ffff88011beea490 R15: 0000000000000000
> [   89.647431] FS:  00007f3ac063b740(0000) GS:ffff88012b200000(0000) knlGS:0000000000000000
> [   89.648660] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
> [   89.649548] CR2: 00000000000000c8 CR3: 0000000122bfc000 CR4: 00000000000007e0
> [   89.650635] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> [   89.651723] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
> [   89.652812] Process trinity-main (pid: 782, threadinfo ffff880115656000, task ffff88011beea490)
> [   89.654128] Stack:
> [   89.654433]  0000000000000000 ffff8801058396a0 ffff880105839600 ffff88011beeaa78
> [   89.655769]  ffff880115657ef8 ffffffff812c7d9b ffffffff82079be0 0000000000000000
> [   89.657073]  ffff880115657f28 ffffffff8106c665 0000000000000002 ffff880115657f58
> [   89.658399] Call Trace:
> [   89.658822]  [<ffffffff812c7d9b>] key_change_session_keyring+0xfb/0x140
> [   89.659845]  [<ffffffff8106c665>] task_work_run+0xa5/0xd0
> [   89.660698]  [<ffffffff81002911>] do_notify_resume+0x71/0xb0
> [   89.661581]  [<ffffffff816c9a4a>] int_signal+0x12/0x17
> [   89.662385] Code: 24 90 00 00 00 48 8b b3 90 00 00 00 49 8b 4c 24 40 48 39 f2 75 08 e9 83 00 00 00 48 89 ca 48 81 fa 60 29 c3 81 0f 84 41 fe ff ff <48> 8b 8a c8 00 00 00 48 39 ce 75 e4 3b 82 d0 00 00 00 0f 84 4b 
> [   89.667778] RIP  [<ffffffff810784b0>] commit_creds+0x250/0x2f0
> [   89.668733]  RSP <ffff880115657eb8>
> [   89.669301] CR2: 00000000000000c8
>
> My fastest trinity induced oops yet!
>
>
> Appears to be..
>
>                 if ((set_ns == subset_ns->parent)  &&
>      850:       48 8b 8a c8 00 00 00    mov    0xc8(%rdx),%rcx
>
> from the inlined cred_cap_issubset

Interesting.  That line is protected with the check subset_ns !=
&init_user_ns so subset_ns->parent must be valid or subset_ns is not
a proper user namespace.

Ugh.  I think I see what is going on and it is just silly. 

It looks like by historical accident we have been reading trying to set
new->user_ns from new->user_ns.  Which is totally silly as new->user_ns
is NULL (as is every other field in new except session_keyring at that
point).

It looks like it is safe to sleep in key_change_session_keyring so why
we just don't use prepare_creds there like everywhere else is beyond
me.

The intent is clearly to copy all of the fields from old to new so what
we should be doing is is copying old->user_ns into new->user_ns.

Dave can you verify that this patch fixes the oops?

Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
---
diff --git a/security/keys/process_keys.c b/security/keys/process_keys.c
index 58dfe08..a571fad 100644
--- a/security/keys/process_keys.c
+++ b/security/keys/process_keys.c
@@ -839,7 +839,7 @@ void key_change_session_keyring(struct callback_head *twork)
        new-> sgid      = old-> sgid;
        new->fsgid      = old->fsgid;
        new->user       = get_uid(old->user);
-       new->user_ns    = get_user_ns(new->user_ns);
+       new->user_ns    = get_user_ns(old->user_ns);
        new->group_info = get_group_info(old->group_info);
 
        new->securebits = old->securebits;

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