BTRFS losing SE Linux labels on power failure or "reboot -nffd"

The command "reboot -nffd" (kernel reboot without flushing kernel buffers or writing status) when run on a BTRFS system with SE Linux will often result in /var/log/audit/audit.log being unlabeled.

It also results in some systemd-
journald files like /var/log/journal/c195779d29154ed8bcb4e8444c4a1728/ system.journal being unlabeled but that is rarer. I think that the same problem afflicts both systemd-journald and auditd but it's a race condition that on my systems (both production and test) is more likely to affect auditd.

root@stretch:/# xattr -l /var/log/audit/audit.log 
security.selinux: 
0000   73 79 73 74 65 6D 5F 75 3A 6F 62 6A 65 63 74 5F    system_u:object_ 
0010   72 3A 61 75 64 69 74 64 5F 6C 6F 67 5F 74 3A 73    r:auditd_log_t:s 
0020   30 00                                              0.

SE Linux uses the xattr "security.selinux", you can see what it's doing with xattr(1) but generally using "ls -Z" is easiest.

If this issue just affected "reboot -nffd" then a solution might be to just not run that command. However this affects systems after a power outage.

I have reproduced this bug with kernel 4.9.0-6-amd64 (the latest security update for Debian/Stretch which is the latest supported release of Debian). I have also reproduced it in an identical manner with kernel 4.16.0-1-amd64 (the latest from Debian/Unstable). For testing I reproduced this with a 4G filesystem in a VM, but in production it has happened on BTRFS RAID-1 arrays, both SSD and HDD.

#!/bin/bash
set -e
COUNT=$(ps aux|grep [s]bin/auditd|wc -l)
date
if [ "$COUNT" = "1" ]; then
echo "all good"
else
echo "failed"
exit 1
fi

Firstly the above is the script /usr/local/sbin/testit, I test for auditd running because it aborts if the context on it's log file is wrong. When SE Linux is in enforcing mode an incorrect/missing label on the audit.log file causes auditd to abort.

root@stretch:~# ls -liZ /var/log/audit/audit.log 
37952 -rw-------. 1 root root system_u:object_r:auditd_log_t:s0 4385230 Jun  1 
12:23 /var/log/audit/audit.log
Above is before I do the tests.

while ssh stretch /usr/local/sbin/testit ; do
ssh btrfs-local "reboot -nffd" > /dev/null 2>&1 &
sleep 20
done
Above is the shell code I run to do the tests. Note that the VM in question runs on SSD storage which is why it can consistently boot in less than 20 seconds.

Fri 1 Jun 12:26:13 UTC 2018
all good
Fri 1 Jun 12:26:33 UTC 2018
failed
Above is the output from the shell code in question. After the first reboot it fails. The probability of failure on my test system is greater than 50%.

root@stretch:~# ls -liZ /var/log/audit/audit.log  
37952 -rw-------. 1 root root system_u:object_r:unlabeled_t:s0 4396803 Jun  1 
12:26 /var/log/audit/audit.log
Now the result.  Note that the Inode has not changed.  I could understand a 
newly created file missing an xattr, but this is an existing file which 
shouldn't have had it's xattr changed.  But somehow it gets corrupted.

The first possibility I considered was that SE Linux code might be at fault. I asked on the SE Linux mailing list (I haven't been involved in SE Linux kernel code for about 15 years) and was informed that this isn't likely at all. There have been no problems like this reported with other filesystems.

Does anyone have any ideas of other tests I should run? Anyone want me to try a different kernel? I can give root on a VM to anyone who wants to poke at it.

Hi,

Quoted 3 lines · Russell Coker

The command "reboot -nffd" (kernel reboot without flushing kernel buffers or writing status) when run on a BTRFS system with SE Linux will often result in /var/log/audit/audit.log being unlabeled.

This recent fix might be what you're looking for:

https://www.spinics.net/lists/linux-btrfs/msg77927.html

The command "reboot -nffd" (kernel reboot without flushing kernel buffers or writing status) when run on a BTRFS system with SE Linux will often result in /var/log/audit/audit.log being unlabeled. It also results in some systemd- journald files like /var/log/journal/c195779d29154ed8bcb4e8444c4a1728/ system.journal being unlabeled but that is rarer. I think that the same problem afflicts both systemd-journald and auditd but it's a race condition that on my systems (both production and test) is more likely to affect auditd.

root@stretch:/# xattr -l /var/log/audit/audit.log
security.selinux:
0000   73 79 73 74 65 6D 5F 75 3A 6F 62 6A 65 63 74 5F    system_u:object_
0010   72 3A 61 75 64 69 74 64 5F 6C 6F 67 5F 74 3A 73    r:auditd_log_t:s
0020   30 00                                              0.

SE Linux uses the xattr "security.selinux", you can see what it's doing with xattr(1) but generally using "ls -Z" is easiest.

If this issue just affected "reboot -nffd" then a solution might be to just not run that command. However this affects systems after a power outage.

I have reproduced this bug with kernel 4.9.0-6-amd64 (the latest security update for Debian/Stretch which is the latest supported release of Debian). I have also reproduced it in an identical manner with kernel 4.16.0-1-amd64 (the latest from Debian/Unstable). For testing I reproduced this with a 4G filesystem in a VM, but in production it has happened on BTRFS RAID-1 arrays, both SSD and HDD.

#!/bin/bash
set -e
COUNT=$(ps aux|grep [s]bin/auditd|wc -l)
date
if [ "$COUNT" = "1" ]; then
echo "all good"
else
echo "failed"
exit 1
fi

Firstly the above is the script /usr/local/sbin/testit, I test for auditd running because it aborts if the context on it's log file is wrong. When SE Linux is in enforcing mode an incorrect/missing label on the audit.log file causes auditd to abort.

root@stretch:~# ls -liZ /var/log/audit/audit.log
37952 -rw-------. 1 root root system_u:object_r:auditd_log_t:s0 4385230 Jun 
1 12:23 /var/log/audit/audit.log
Above is before I do the tests.

while ssh stretch /usr/local/sbin/testit ; do
ssh btrfs-local "reboot -nffd" > /dev/null 2>&1 &
sleep 20
done
Above is the shell code I run to do the tests. Note that the VM in question runs on SSD storage which is why it can consistently boot in less than 20 seconds.

Fri 1 Jun 12:26:13 UTC 2018
all good
Fri 1 Jun 12:26:33 UTC 2018
failed
Above is the output from the shell code in question. After the first reboot it fails. The probability of failure on my test system is greater than 50%.

root@stretch:~# ls -liZ /var/log/audit/audit.log
37952 -rw-------. 1 root root system_u:object_r:unlabeled_t:s0 4396803 Jun 
1 12:26 /var/log/audit/audit.log
Now the result.  Note that the Inode has not changed.  I could understand a
newly created file missing an xattr, but this is an existing file which
shouldn't have had it's xattr changed.  But somehow it gets corrupted.

The first possibility I considered was that SE Linux code might be at fault. I asked on the SE Linux mailing list (I haven't been involved in SE Linux kernel code for about 15 years) and was informed that this isn't likely at all. There have been no problems like this reported with other filesystems.

Does anyone have any ideas of other tests I should run? Anyone want me to try a different kernel? I can give root on a VM to anyone who wants to poke at it.

The command "reboot -nffd" (kernel reboot without flushing kernel buffers or writing status) when run on a BTRFS system with SE Linux will often result in /var/log/audit/audit.log being unlabeled. It also results in some systemd- journald files like /var/log/journal/c195779d29154ed8bcb4e8444c4a1728/ system.journal being unlabeled but that is rarer. I think that the same problem afflicts both systemd-journald and auditd but it's a race condition that on my systems (both production and test) is more likely to affect auditd.

root@stretch:/# xattr -l /var/log/audit/audit.log
security.selinux:
0000   73 79 73 74 65 6D 5F 75 3A 6F 62 6A 65 63 74 5F    system_u:object_
0010   72 3A 61 75 64 69 74 64 5F 6C 6F 67 5F 74 3A 73    r:auditd_log_t:s
0020   30 00                                              0.

SE Linux uses the xattr "security.selinux", you can see what it's doing with xattr(1) but generally using "ls -Z" is easiest.

If this issue just affected "reboot -nffd" then a solution might be to just not run that command. However this affects systems after a power outage.

I have reproduced this bug with kernel 4.9.0-6-amd64 (the latest security update for Debian/Stretch which is the latest supported release of Debian). I have also reproduced it in an identical manner with kernel 4.16.0-1-amd64 (the latest from Debian/Unstable). For testing I reproduced this with a 4G filesystem in a VM, but in production it has happened on BTRFS RAID-1 arrays, both SSD and HDD.

#!/bin/bash
set -e
COUNT=$(ps aux|grep [s]bin/auditd|wc -l)
date
if [ "$COUNT" = "1" ]; then
echo "all good"
else
echo "failed"
exit 1
fi

Firstly the above is the script /usr/local/sbin/testit, I test for auditd running because it aborts if the context on it's log file is wrong. When SE Linux is in enforcing mode an incorrect/missing label on the audit.log file causes auditd to abort.

root@stretch:~# ls -liZ /var/log/audit/audit.log
37952 -rw-------. 1 root root system_u:object_r:auditd_log_t:s0 4385230 Jun 
1 12:23 /var/log/audit/audit.log
Above is before I do the tests.

while ssh stretch /usr/local/sbin/testit ; do
ssh btrfs-local "reboot -nffd" > /dev/null 2>&1 &
sleep 20
done
Above is the shell code I run to do the tests. Note that the VM in question runs on SSD storage which is why it can consistently boot in less than 20 seconds.

Fri 1 Jun 12:26:13 UTC 2018
all good
Fri 1 Jun 12:26:33 UTC 2018
failed
Above is the output from the shell code in question. After the first reboot it fails. The probability of failure on my test system is greater than 50%.

root@stretch:~# ls -liZ /var/log/audit/audit.log
37952 -rw-------. 1 root root system_u:object_r:unlabeled_t:s0 4396803 Jun 
1 12:26 /var/log/audit/audit.log
Now the result.  Note that the Inode has not changed.  I could understand a
newly created file missing an xattr, but this is an existing file which
shouldn't have had it's xattr changed.  But somehow it gets corrupted.

The first possibility I considered was that SE Linux code might be at fault. I asked on the SE Linux mailing list (I haven't been involved in SE Linux kernel code for about 15 years) and was informed that this isn't likely at all. There have been no problems like this reported with other filesystems.

Does anyone have any ideas of other tests I should run? Anyone want me to try a different kernel? I can give root on a VM to anyone who wants to poke at it.