Hi,
Fedora got a bug report on 4.14.4 of a crash on
reboot https://bugzilla.redhat.com/show_bug.cgi?id=1525279
BUG: unable to handle kernel NULL pointer dereference at 0000000000000254
IP: __task_pid_nr_ns+0xc7/0xf0
PGD 0 P4D 0
Oops: 0000 [#1] SMP
Modules linked in: fuse vhost_net vhost tap xt_CHECKSUM ipt_MASQUERADE nf_nat_masquerade_ipv4 tun nf_conntrack_netbios_ns nf_conntrack_broadcast xt_CT xt_nat xt_mark ip6t_rpfilter ip6t_REJECT nf_reject_ipv6 xt_conntrack ip_set nfnetlink ebtable_nat ebtable_broute ip6table_nat nf_conntrack_ipv6 nf_defrag_ipv6 nf_nat_ipv6 ip6table_mangle ip6table_raw ip6table_security iptable_nat nf_conntrack_ipv4 nf_defrag_ipv4 nf_nat_ipv4 nf_nat nf_conntrack libcrc32c iptable_mangle bridge iptable_raw stp llc iptable_security ebtable_filter ebtables ip6table_filter ip6_tables sunrpc vfat fat edac_mce_amd eeepc_wmi kvm_amd asus_wmi wmi_bmof sparse_keymap rfkill video kvm snd_hda_codec_realtek snd_hda_codec_generic snd_hda_codec_hdmi irqbypass crct10dif_pclmul snd_hda_intel crc32_pclmul snd_hda_codec ghash_clmulni_intel
snd_hda_core snd_hwdep snd_seq joydev snd_seq_device snd_pcm snd_timer sp5100_tco i2c_piix4 ccp snd soundcore tpm_tis shpchp tpm_tis_core tpm wmi acpi_cpufreq binfmt_misc btrfs xor zstd_decompress zstd_compress xxhash raid6_pq hid_logitech_hidpp amdkfd amd_iommu_v2 amdgpu i2c_algo_bit drm_kms_helper ttm drm crc32c_intel r8169 hid_logitech_dj mii nvme nvme_core i2c_dev
CPU: 2 PID: 1 Comm: systemd Not tainted 4.14.4-200.fc26.x86_64 #1
Hardware name: System manufacturer System Product Name/PRIME B350M-A, BIOS 3203 11/09/2017
task: ffff88c4baee8000 task.stack: ffff96d5c3158000
RIP: 0010:__task_pid_nr_ns+0xc7/0xf0
RSP: 0018:ffff96d5c315bd50 EFLAGS: 00010206
RAX: 0000000000000000 RBX: ffff88c4b68c6000 RCX: 0000000000000250
RDX: ffffffffa5e47960 RSI: 0000000000000000 RDI: ffff88c490f6ab00
RBP: ffff96d5c315bd50 R08: 0000000000001000 R09: 0000000000000005
R10: ffff88c4be006b80 R11: ffff88c42f1b8004 R12: ffff96d5c315bf18
R13: ffff88c42d7dd200 R14: ffff88c490f6a510 R15: ffff88c4b68c6000
FS: 00007f9446f8ea00(0000) GS:ffff88c4be680000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000254 CR3: 00000007f956f000 CR4: 00000000003406e0
Call Trace:
cgroup_procs_show+0x19/0x30
cgroup_seqfile_show+0x4c/0xb0
kernfs_seq_show+0x21/0x30
seq_read+0x2ec/0x3f0
kernfs_fop_read+0x134/0x180
__vfs_read+0x37/0x160
? security_file_permission+0x9b/0xc0
vfs_read+0x8e/0x130
SyS_read+0x55/0xc0
entry_SYSCALL_64_fastpath+0x1a/0xa5
RIP: 0033:0x7f94455f942d
RSP: 002b:00007ffe81ba2d00 EFLAGS: 00000293 ORIG_RAX: 0000000000000000
RAX: ffffffffffffffda RBX: 00005574e2233f00 RCX: 00007f94455f942d
RDX: 0000000000001000 RSI: 00005574e2321a90 RDI: 000000000000002b
RBP: 0000000000000000 R08: 00005574e2321a90 R09: 00005574e231de60
R10: 00007f94458c8b38 R11: 0000000000000293 R12: 00007f94458c8ae0
R13: 00007ffe81ba3800 R14: 0000000000000000 R15: 00005574e2116560
Code: 04 74 0e 89 f6 48 8d 04 76 48 8d 04 c5 f0 05 00 00 48 8b bf b8 05 00 00 48 01 c7 31 c0 48 8b 0f 48 85 c9 74 18 8b b2 30 08 00 00 <3b> 71 04 77 0d 48 c1 e6 05 48 01 f1 48 3b 51 38 74 09 5d c3 8b
RIP: __task_pid_nr_ns+0xc7/0xf0 RSP: ffff96d5c315bd50
It looks like a few others reported similar problems
https://marc.info/?l=linux-kernel&m=151210295120013
https://marc.info/?l=linux-kernel&m=151151651301420
I haven't yet asked the reporter to try and bisect. Is there a fix queued
up for this yet?
Thanks,
Laura
Hello, Laura.
On Tue, Dec 12, 2017 at 04:38:33PM -0800, Laura Abbott wrote:
> Hi,
>
> Fedora got a bug report on 4.14.4 of a crash on
> reboot https://bugzilla.redhat.com/show_bug.cgi?id=1525279
>
> BUG: unable to handle kernel NULL pointer dereference at 0000000000000254
> IP: __task_pid_nr_ns+0xc7/0xf0
Any chance you can map this back to the source line with addr2line?
Thanks.
--
tejun
On 12/13/2017 07:29 AM, Tejun Heo wrote:
> Hello, Laura.
>
> On Tue, Dec 12, 2017 at 04:38:33PM -0800, Laura Abbott wrote:
>> Hi,
>>
>> Fedora got a bug report on 4.14.4 of a crash on
>> reboot https://bugzilla.redhat.com/show_bug.cgi?id=1525279
>>
>> BUG: unable to handle kernel NULL pointer dereference at 0000000000000254
>> IP: __task_pid_nr_ns+0xc7/0xf0
>
> Any chance you can map this back to the source line with addr2line?
>
> Thanks.
>
annotated with decode_stacktrace.sh
IP: __task_pid_nr_ns (/usr/src/debug/kernel-4.14.fc26/linux-4.14.4-200.fc26.x86_64/kernel/pid.c:506 /usr/src/debug/kernel-4.14.fc26/linux-4.14.4-200.fc26.x86_64/kernel/pid.c:535)
cgroup_procs_show (/usr/src/debug/kernel-4.14.fc26/linux-4.14.4-200.fc26.x86_64/kernel/cgroup/cgroup.c:4240)
cgroup_seqfile_show (/usr/src/debug/kernel-4.14.fc26/linux-4.14.4-200.fc26.x86_64/kernel/cgroup/cgroup.c:3413)
kernfs_seq_show (/usr/src/debug/kernel-4.14.fc26/linux-4.14.4-200.fc26.x86_64/fs/kernfs/file.c:169)
seq_read (/usr/src/debug/kernel-4.14.fc26/linux-4.14.4-200.fc26.x86_64/fs/seq_file.c:269)
kernfs_fop_read (/usr/src/debug/kernel-4.14.fc26/linux-4.14.4-200.fc26.x86_64/fs/kernfs/file.c:252)
__vfs_read (/usr/src/debug/kernel-4.14.fc26/linux-4.14.4-200.fc26.x86_64/fs/read_write.c:416)
? security_file_permission (/usr/src/debug/kernel-4.14.fc26/linux-4.14.4-200.fc26.x86_64/include/linux/fsnotify.h:56 /usr/src/debug/kernel-4.14.fc26/linux-4.14.4-200.fc26.x86_64/security/security.c:867)
vfs_read (/usr/src/debug/kernel-4.14.fc26/linux-4.14.4-200.fc26.x86_64/fs/read_write.c:448)
SyS_read (/usr/src/debug/kernel-4.14.fc26/linux-4.14.4-200.fc26.x86_64/fs/read_write.c:574 /usr/src/debug/kernel-4.14.fc26/linux-4.14.4-200.fc26.x86_64/fs/read_write.c:566)
entry_SYSCALL_64_fastpath (/usr/src/debug/kernel-4.14.fc26/linux-4.14.4-200.fc26.x86_64/arch/x86/entry/entry_64.S:206)
All code
========
0: 04 74 add $0x74,%al
2: 0e (bad)
3: 89 f6 mov %esi,%esi
5: 48 8d 04 76 lea (%rsi,%rsi,2),%rax
9: 48 8d 04 c5 f0 05 00 lea 0x5f0(,%rax,8),%rax
10: 00
11: 48 8b bf b8 05 00 00 mov 0x5b8(%rdi),%rdi
18: 48 01 c7 add %rax,%rdi
1b: 31 c0 xor %eax,%eax
1d: 48 8b 0f mov (%rdi),%rcx
20: 48 85 c9 test %rcx,%rcx
23: 74 18 je 0x3d
25: 8b b2 30 08 00 00 mov 0x830(%rdx),%esi
2b:* 3b 71 04 cmp 0x4(%rcx),%esi <-- trapping instruction
2e: 77 0d ja 0x3d
30: 48 c1 e6 05 shl $0x5,%rsi
34: 48 01 f1 add %rsi,%rcx
37: 48 3b 51 38 cmp 0x38(%rcx),%rdx
3b: 74 09 je 0x46
3d: 5d pop %rbp
3e: c3 retq
3f: 8b .byte 0x8b
Code starting with the faulting instruction
===========================================
0: 3b 71 04 cmp 0x4(%rcx),%esi
3: 77 0d ja 0x12
5: 48 c1 e6 05 shl $0x5,%rsi
9: 48 01 f1 add %rsi,%rcx
c: 48 3b 51 38 cmp 0x38(%rcx),%rdx
10: 74 09 je 0x1b
12: 5d pop %rbp
13: c3 retq
14: 8b .byte 0x8b
Hello, Laura.
On Wed, Dec 13, 2017 at 09:10:36AM -0800, Laura Abbott wrote:
> On 12/13/2017 07:29 AM, Tejun Heo wrote:
> >Hello, Laura.
> >
> >On Tue, Dec 12, 2017 at 04:38:33PM -0800, Laura Abbott wrote:
> >>Hi,
> >>
> >>Fedora got a bug report on 4.14.4 of a crash on
> >>reboot https://bugzilla.redhat.com/show_bug.cgi?id=1525279
> >>
> >> BUG: unable to handle kernel NULL pointer dereference at 0000000000000254
> >> IP: __task_pid_nr_ns+0xc7/0xf0
> >
> >Any chance you can map this back to the source line with addr2line?
> >
> >Thanks.
> >
>
> annotated with decode_stacktrace.sh
>
>
> IP: __task_pid_nr_ns (/usr/src/debug/kernel-4.14.fc26/linux-4.14.4-200.fc26.x86_64/kernel/pid.c:506 /usr/src/debug/kernel-4.14.fc26/linux-4.14.4-200.fc26.x86_64/kernel/pid.c:535)
Thank you very much for the trace. I *think* I found the place where
the css task iterator points to a task without holding its ref
allowing the task to go away before dereferenced. I'll update once I
learn more.
Thanks.
--
tejun
Hello,
Applied the following to cgroup/for-4.15-fixes. Will push out to
linus later this week. I could reproduce the problem reliably and am
pretty sure this is the right fix but I'd greatly appreciate if you
guys can confirm the fix too.
Thank you very much.
------ 8< ------
>From 74d0833c659a8a54735e5efdd44f4b225af68586 Mon Sep 17 00:00:00 2001
From: Tejun Heo <[email protected]>
Date: Wed, 20 Dec 2017 07:09:19 -0800
While teaching css_task_iter to handle skipping over tasks which
aren't group leaders, bc2fb7ed089f ("cgroup: add @flags to
css_task_iter_start() and implement CSS_TASK_ITER_PROCS") introduced a
silly bug.
CSS_TASK_ITER_PROCS is implemented by repeating
css_task_iter_advance() while the advanced cursor is pointing to a
non-leader thread. However, the cursor variable, @l, wasn't updated
when the iteration has to advance to the next css_set and the
following repetition would operate on the terminal @l from the
previous iteration which isn't pointing to a valid task leading to
oopses like the following or infinite looping.
BUG: unable to handle kernel NULL pointer dereference at 0000000000000254
IP: __task_pid_nr_ns+0xc7/0xf0
PGD 0 P4D 0
Oops: 0000 [#1] SMP
...
CPU: 2 PID: 1 Comm: systemd Not tainted 4.14.4-200.fc26.x86_64 #1
Hardware name: System manufacturer System Product Name/PRIME B350M-A, BIOS 3203 11/09/2017
task: ffff88c4baee8000 task.stack: ffff96d5c3158000
RIP: 0010:__task_pid_nr_ns+0xc7/0xf0
RSP: 0018:ffff96d5c315bd50 EFLAGS: 00010206
RAX: 0000000000000000 RBX: ffff88c4b68c6000 RCX: 0000000000000250
RDX: ffffffffa5e47960 RSI: 0000000000000000 RDI: ffff88c490f6ab00
RBP: ffff96d5c315bd50 R08: 0000000000001000 R09: 0000000000000005
R10: ffff88c4be006b80 R11: ffff88c42f1b8004 R12: ffff96d5c315bf18
R13: ffff88c42d7dd200 R14: ffff88c490f6a510 R15: ffff88c4b68c6000
FS: 00007f9446f8ea00(0000) GS:ffff88c4be680000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000254 CR3: 00000007f956f000 CR4: 00000000003406e0
Call Trace:
cgroup_procs_show+0x19/0x30
cgroup_seqfile_show+0x4c/0xb0
kernfs_seq_show+0x21/0x30
seq_read+0x2ec/0x3f0
kernfs_fop_read+0x134/0x180
__vfs_read+0x37/0x160
? security_file_permission+0x9b/0xc0
vfs_read+0x8e/0x130
SyS_read+0x55/0xc0
entry_SYSCALL_64_fastpath+0x1a/0xa5
RIP: 0033:0x7f94455f942d
RSP: 002b:00007ffe81ba2d00 EFLAGS: 00000293 ORIG_RAX: 0000000000000000
RAX: ffffffffffffffda RBX: 00005574e2233f00 RCX: 00007f94455f942d
RDX: 0000000000001000 RSI: 00005574e2321a90 RDI: 000000000000002b
RBP: 0000000000000000 R08: 00005574e2321a90 R09: 00005574e231de60
R10: 00007f94458c8b38 R11: 0000000000000293 R12: 00007f94458c8ae0
R13: 00007ffe81ba3800 R14: 0000000000000000 R15: 00005574e2116560
Code: 04 74 0e 89 f6 48 8d 04 76 48 8d 04 c5 f0 05 00 00 48 8b bf b8 05 00 00 48 01 c7 31 c0 48 8b 0f 48 85 c9 74 18 8b b2 30 08 00 00 <3b> 71 04 77 0d 48 c1 e6 05 48 01 f1 48 3b 51 38 74 09 5d c3 8b
RIP: __task_pid_nr_ns+0xc7/0xf0 RSP: ffff96d5c315bd50
Fix it by moving the initialization of the cursor below the repeat
label. While at it, rename it to @next for readability.
Signed-off-by: Tejun Heo <[email protected]>
Fixes: bc2fb7ed089f ("cgroup: add @flags to css_task_iter_start() and implement CSS_TASK_ITER_PROCS")
Cc: [email protected] # v4.14+
Reported-by: Laura Abbott <[email protected]>
Reported-by: Bronek Kozicki <[email protected]>
Reported-by: George Amanakis <[email protected]>
Signed-off-by: Tejun Heo <[email protected]>
---
kernel/cgroup/cgroup.c | 14 ++++++--------
1 file changed, 6 insertions(+), 8 deletions(-)
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index f4c2f8c..2cf06c2 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -4125,26 +4125,24 @@ static void css_task_iter_advance_css_set(struct css_task_iter *it)
static void css_task_iter_advance(struct css_task_iter *it)
{
- struct list_head *l = it->task_pos;
+ struct list_head *next;
lockdep_assert_held(&css_set_lock);
- WARN_ON_ONCE(!l);
-
repeat:
/*
* Advance iterator to find next entry. cset->tasks is consumed
* first and then ->mg_tasks. After ->mg_tasks, we move onto the
* next cset.
*/
- l = l->next;
+ next = it->task_pos->next;
- if (l == it->tasks_head)
- l = it->mg_tasks_head->next;
+ if (next == it->tasks_head)
+ next = it->mg_tasks_head->next;
- if (l == it->mg_tasks_head)
+ if (next == it->mg_tasks_head)
css_task_iter_advance_css_set(it);
else
- it->task_pos = l;
+ it->task_pos = next;
/* if PROCS, skip over tasks which aren't group leaders */
if ((it->flags & CSS_TASK_ITER_PROCS) && it->task_pos &&
--
2.9.5
This resolves the bug for me.
Thank you,
George
On Wed, Dec 20, 2017 at 10:13 AM, Tejun Heo <[email protected]> wrote:
> Hello,
>
> Applied the following to cgroup/for-4.15-fixes. Will push out to
> linus later this week. I could reproduce the problem reliably and am
> pretty sure this is the right fix but I'd greatly appreciate if you
> guys can confirm the fix too.
>
> Thank you very much.
>
> ------ 8< ------
> From 74d0833c659a8a54735e5efdd44f4b225af68586 Mon Sep 17 00:00:00 2001
> From: Tejun Heo <[email protected]>
> Date: Wed, 20 Dec 2017 07:09:19 -0800
>
> While teaching css_task_iter to handle skipping over tasks which
> aren't group leaders, bc2fb7ed089f ("cgroup: add @flags to
> css_task_iter_start() and implement CSS_TASK_ITER_PROCS") introduced a
> silly bug.
>
> CSS_TASK_ITER_PROCS is implemented by repeating
> css_task_iter_advance() while the advanced cursor is pointing to a
> non-leader thread. However, the cursor variable, @l, wasn't updated
> when the iteration has to advance to the next css_set and the
> following repetition would operate on the terminal @l from the
> previous iteration which isn't pointing to a valid task leading to
> oopses like the following or infinite looping.
>
> BUG: unable to handle kernel NULL pointer dereference at 0000000000000254
> IP: __task_pid_nr_ns+0xc7/0xf0
> PGD 0 P4D 0
> Oops: 0000 [#1] SMP
> ...
> CPU: 2 PID: 1 Comm: systemd Not tainted 4.14.4-200.fc26.x86_64 #1
> Hardware name: System manufacturer System Product Name/PRIME B350M-A, BIOS 3203 11/09/2017
> task: ffff88c4baee8000 task.stack: ffff96d5c3158000
> RIP: 0010:__task_pid_nr_ns+0xc7/0xf0
> RSP: 0018:ffff96d5c315bd50 EFLAGS: 00010206
> RAX: 0000000000000000 RBX: ffff88c4b68c6000 RCX: 0000000000000250
> RDX: ffffffffa5e47960 RSI: 0000000000000000 RDI: ffff88c490f6ab00
> RBP: ffff96d5c315bd50 R08: 0000000000001000 R09: 0000000000000005
> R10: ffff88c4be006b80 R11: ffff88c42f1b8004 R12: ffff96d5c315bf18
> R13: ffff88c42d7dd200 R14: ffff88c490f6a510 R15: ffff88c4b68c6000
> FS: 00007f9446f8ea00(0000) GS:ffff88c4be680000(0000) knlGS:0000000000000000
> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000000000000254 CR3: 00000007f956f000 CR4: 00000000003406e0
> Call Trace:
> cgroup_procs_show+0x19/0x30
> cgroup_seqfile_show+0x4c/0xb0
> kernfs_seq_show+0x21/0x30
> seq_read+0x2ec/0x3f0
> kernfs_fop_read+0x134/0x180
> __vfs_read+0x37/0x160
> ? security_file_permission+0x9b/0xc0
> vfs_read+0x8e/0x130
> SyS_read+0x55/0xc0
> entry_SYSCALL_64_fastpath+0x1a/0xa5
> RIP: 0033:0x7f94455f942d
> RSP: 002b:00007ffe81ba2d00 EFLAGS: 00000293 ORIG_RAX: 0000000000000000
> RAX: ffffffffffffffda RBX: 00005574e2233f00 RCX: 00007f94455f942d
> RDX: 0000000000001000 RSI: 00005574e2321a90 RDI: 000000000000002b
> RBP: 0000000000000000 R08: 00005574e2321a90 R09: 00005574e231de60
> R10: 00007f94458c8b38 R11: 0000000000000293 R12: 00007f94458c8ae0
> R13: 00007ffe81ba3800 R14: 0000000000000000 R15: 00005574e2116560
> Code: 04 74 0e 89 f6 48 8d 04 76 48 8d 04 c5 f0 05 00 00 48 8b bf b8 05 00 00 48 01 c7 31 c0 48 8b 0f 48 85 c9 74 18 8b b2 30 08 00 00 <3b> 71 04 77 0d 48 c1 e6 05 48 01 f1 48 3b 51 38 74 09 5d c3 8b
> RIP: __task_pid_nr_ns+0xc7/0xf0 RSP: ffff96d5c315bd50
>
> Fix it by moving the initialization of the cursor below the repeat
> label. While at it, rename it to @next for readability.
>
> Signed-off-by: Tejun Heo <[email protected]>
> Fixes: bc2fb7ed089f ("cgroup: add @flags to css_task_iter_start() and implement CSS_TASK_ITER_PROCS")
> Cc: [email protected] # v4.14+
> Reported-by: Laura Abbott <[email protected]>
> Reported-by: Bronek Kozicki <[email protected]>
> Reported-by: George Amanakis <[email protected]>
> Signed-off-by: Tejun Heo <[email protected]>
> ---
> kernel/cgroup/cgroup.c | 14 ++++++--------
> 1 file changed, 6 insertions(+), 8 deletions(-)
>
> diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
> index f4c2f8c..2cf06c2 100644
> --- a/kernel/cgroup/cgroup.c
> +++ b/kernel/cgroup/cgroup.c
> @@ -4125,26 +4125,24 @@ static void css_task_iter_advance_css_set(struct css_task_iter *it)
>
> static void css_task_iter_advance(struct css_task_iter *it)
> {
> - struct list_head *l = it->task_pos;
> + struct list_head *next;
>
> lockdep_assert_held(&css_set_lock);
> - WARN_ON_ONCE(!l);
> -
> repeat:
> /*
> * Advance iterator to find next entry. cset->tasks is consumed
> * first and then ->mg_tasks. After ->mg_tasks, we move onto the
> * next cset.
> */
> - l = l->next;
> + next = it->task_pos->next;
>
> - if (l == it->tasks_head)
> - l = it->mg_tasks_head->next;
> + if (next == it->tasks_head)
> + next = it->mg_tasks_head->next;
>
> - if (l == it->mg_tasks_head)
> + if (next == it->mg_tasks_head)
> css_task_iter_advance_css_set(it);
> else
> - it->task_pos = l;
> + it->task_pos = next;
>
> /* if PROCS, skip over tasks which aren't group leaders */
> if ((it->flags & CSS_TASK_ITER_PROCS) && it->task_pos &&
> --
> 2.9.5
>
Tejun Heo <[email protected]> writes:
> Hello,
>
> Applied the following to cgroup/for-4.15-fixes. Will push out to
> linus later this week. I could reproduce the problem reliably and am
> pretty sure this is the right fix but I'd greatly appreciate if you
> guys can confirm the fix too.
Fixed here as well, 4.14.8 stable with your fix on top.
Thanks,
Rouven
Hello,
Tejun Heo <[email protected]> writes:
> Applied the following to cgroup/for-4.15-fixes. Will push out to
> linus later this week. [...]
I can't find your fixes in master yet, did you perhaps forget to push
out?
Thanks and a happy new year,
Rouven
On Mon, Jan 08, 2018 at 11:12:47AM +0100, Rouven Czerwinski wrote:
> Hello,
>
> Tejun Heo <[email protected]> writes:
>
> > Applied the following to cgroup/for-4.15-fixes. Will push out to
> > linus later this week. [...]
>
> I can't find your fixes in master yet, did you perhaps forget to push
> out?
Thanks for reminding me. Sent out the pull request.
Happy new year!
--
tejun