[CFT,00/10] Making new mounts of proc and sysfs as safe as bind mounts (take 2)

Richard Weinberger <richard@nod.at> writes:

> [CC'ing libvirt-lxc folks]
>
> Am 28.05.2015 um 23:32 schrieb Eric W. Biederman:
>> Richard Weinberger <richard@nod.at> writes:
>> 
>>> Am 28.05.2015 um 21:57 schrieb Eric W. Biederman:
>>>>> FWIW, it breaks also libvirt-lxc:
>>>>> Error: internal error: guest failed to start: Failed to re-mount /proc/sys on /proc/sys flags=1021: Operation not permitted
>>>>
>>>> Interesting.  I had not anticipated a failure there?  And it is failing
>>>> in remount?  Oh that is interesting.
>>>>
>>>> That implies that there is some flag of the original mount of /proc that
>>>> the remount of /proc/sys is clearing, and that previously 
>>>>
>>>> The flags specified are current rdonly,remount,bind so I expect there
>>>> are some other flags on proc that libvirt-lxc is clearing by accident
>>>> and we did not fail before because the kernel was not enforcing things.
>>>
>>> Please see:
>>> http://libvirt.org/git/?p=libvirt.git;a=blob;f=src/lxc/lxc_container.c;h=9a9ae5c2aaf0f90ff472f24fda43c077b44998c7;hb=HEAD#l933
>>> lxcContainerMountBasicFS()
>>>
>>> and:
>>> http://libvirt.org/git/?p=libvirt.git;a=blob;f=src/lxc/lxc_container.c;h=9a9ae5c2aaf0f90ff472f24fda43c077b44998c7;hb=HEAD#l850
>>> lxcBasicMounts
>>>
>>>> What are the mount flags in a working libvirt-lxc?
>>>
>>> See:
>>> test1:~ # cat /proc/self/mountinfo
>>> 149 147 0:56 / /proc rw,nosuid,nodev,noexec,relatime - proc proc rw
>>> 150 149 0:56 /sys /proc/sys ro,nodev,relatime - proc proc rw
>> 
>>> If you need more info, please let me know. :-)
>> 
>> Oh interesting I had not realized libvirt-lxc had grown an unprivileged
>> mode using user namespaces.
>> 
>> This does appear to be a classic remount bug, where you are not
>> preserving the permissions.  It appears the fact that the code
>> failed to enforce locked permissions on the fresh mount of proc
>> was hiding this bug until now.
>> 
>> I expect what you actually want is the code below:
>> 
>> diff --git a/src/lxc/lxc_container.c b/src/lxc/lxc_container.c
>> index 9a9ae5c2aaf0..f008a7484bfe 100644
>> --- a/src/lxc/lxc_container.c
>> +++ b/src/lxc/lxc_container.c
>> @@ -850,7 +850,7 @@ typedef struct {
>>  
>>  static const virLXCBasicMountInfo lxcBasicMounts[] = {
>>      { "proc", "/proc", "proc", MS_NOSUID|MS_NOEXEC|MS_NODEV, false, false, false },
>> -    { "/proc/sys", "/proc/sys", NULL, MS_BIND|MS_RDONLY, false, false, false },
>> +    { "/proc/sys", "/proc/sys", NULL, MS_BIND|MS_NOSUID|MS_NOEXEC|MS_NODEV|MS_RDONLY, false, false, false },
>>      { "/.oldroot/proc/sys/net/ipv4", "/proc/sys/net/ipv4", NULL, MS_BIND, false, false, true },
>>      { "/.oldroot/proc/sys/net/ipv6", "/proc/sys/net/ipv6", NULL, MS_BIND, false, false, true },
>>      { "sysfs", "/sys", "sysfs", MS_NOSUID|MS_NOEXEC|MS_NODEV|MS_RDONLY, false, false, false },
>> 
>> Or possibly just:
>> 
>> diff --git a/src/lxc/lxc_container.c b/src/lxc/lxc_container.c
>> index 9a9ae5c2aaf0..a60ccbd12bfc 100644
>> --- a/src/lxc/lxc_container.c
>> +++ b/src/lxc/lxc_container.c
>> @@ -850,7 +850,7 @@ typedef struct {
>>  
>>  static const virLXCBasicMountInfo lxcBasicMounts[] = {
>>      { "proc", "/proc", "proc", MS_NOSUID|MS_NOEXEC|MS_NODEV, false, false, false },
>> -    { "/proc/sys", "/proc/sys", NULL, MS_BIND|MS_RDONLY, false, false, false },
>> +    { "/proc/sys", "/proc/sys", NULL, MS_BIND|MS_RDONLY, true, false, false },
>>      { "/.oldroot/proc/sys/net/ipv4", "/proc/sys/net/ipv4", NULL, MS_BIND, false, false, true },
>>      { "/.oldroot/proc/sys/net/ipv6", "/proc/sys/net/ipv6", NULL, MS_BIND, false, false, true },
>>      { "sysfs", "/sys", "sysfs", MS_NOSUID|MS_NOEXEC|MS_NODEV|MS_RDONLY, false, false, false },
>> 
>> As the there is little point in making /proc/sys read-only in a
>> user-namespace, as the permission checks are uid based and no-one should
>> have the global uid 0 in your container.  Making mounting /proc/sys
>> read-only rather pointless.
>
> Eric, using the patch below I was able to spawn a user-namespace enabled container
> using libvirt-lxc. :-)
>
> I had to:
> 1. Disable the read-only mount of /proc/sys which is anyway useless in the user-namespace case.
> 2. Disable the /proc/sys/net/ipv{4,6} bind mounts, this ugly hack is only needed for the non user-namespace case.
> 3. Remove MS_RDONLY from the sysfs mount (For the non user-namespace case we'd have to keep this, though).
>
> Daniel, I'd take this as a chance to disable all the MS_RDONLY games if user-namespace are configured.
> With Eric's fixes they hurt us. And as I wrote many times before if root within the user-namespace
> is able to do nasty things in /sys and /proc that's a plain kernel bug which needs fixing. There is no
> point in mounting these read-only. Except for the case then no user-namespace is used.
>

For clarity the patch below appears to be the minimal change needed to
fix this security issue.

AKA add mnt_mflags in when remounting something read-only.

/proc/sys needed to be updated so it had the proper flags to be added
back in.

I hope this helps.

Eric
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[CFT,00/10] Making new mounts of proc and sysfs as safe as bind mounts (take 2)

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