linux-kernel - Re: [PATCH] fs: Make /proc/sys inodes be owned by global root.

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Message-ID: <87k1kzjdff.fsf@xmission.com>
Date:   Mon, 26 Nov 2018 23:29:40 -0600
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Luis Chamberlain <mcgrof@...nel.org>
Cc:     Radoslaw Burny <rburny@...gle.com>,
        Seth Forshee <seth.forshee@...onical.com>,
        Kees Cook <keescook@...omium.org>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        jsperbeck@...gle.com
Subject: Re: [PATCH] fs: Make /proc/sys inodes be owned by global root.

Luis Chamberlain <mcgrof@...nel.org> writes:

> On Mon, Nov 26, 2018 at 06:26:07PM +0100, Radoslaw Burny wrote:
>> Due to a recent commit (d151ddc00498 - fs: Update i_[ug]id_(read|write)
>> to translate relative to s_user_ns),
>
> Recent? This is commit is from 2014 and present upstream since v4.8.
> And the commit ID you mentioned in your commit log seems to be
> incorrect. I get:
>
> 81754357770ebd900801231e7bc8d151ddc00498a fs: Update i_[ug]id_(read|write) to translate relative to s_user_ns
>
>> inodes under /proc/sys have -1
>> written to their i_uid/i_gid members if a containing userns does not
>> have entries for root in the uid/gid_map.
>
> Thanks for the description of how to run into the issue described but
> is there also a practical use case today where this is happening? I ask
> as it would be good to know the severity of the issue in the real world
> today.

People trying to run containers without a root user in the container.
It atypical but something doable.  

>> This wouldn't normally matter, because these values are not used for
>> access checks. However, a later change (0bd23d09b874 - Don't modify
>> inodes with a uid or gid unknown to the vfs) changes the kernel to
>> prevent opens for write if the i_uid/i_gid field in the inode is -1,
>> even if the /proc/sys-specific access checks would otherwise pass.
>> 
>> This causes a problem: in a userns without root mapping, even the
>> namespace creator cannot write to e.g. /proc/sys/kernel/shmmax.
>> This change fixes the problem by overriding i_uid/i_gid back to
>> GLOBAL_ROOT_UID/GID.
>
> We really need Seth and Eric to provide guidance here as they were
> the ones devising this long ago, but to me your solution seems backward.
> Why allow any namespace to muck with /proc/sys/ seettings?

There are many per namespace sysctls.  Most of them are in the
networking stack.

> Let's recall that this case was a corner case, and writeback was the
> biggest concern, and for that it was decided that you'd simply not get
> write access, and so its read only. Its not clear to me if things like
> proc were considered. For the regular file case the situation can be
> addressed with  chown, however we can't chown proc files.
>
>> Tested: Used a repro program that creates a user namespace without any
>> mapping and stat'ed /proc/$PID/root/proc/sys/kernel/shmmax from outside.
>> Before the change, it shows uid/gid of 65534,
>
> I thought you said it would be uid/gid -1 without your patch?

It is INVALID_UID/INVALID_GID.  It is an over simplifcation to call
them -1.   As they are not a valid value and are never mapped in any
user namespace they are displayed as the overflow_uid or overflow_gid
which is 65534 by default.

>> with the change it's 0.
>
> Note that a good way to also test issues is with the lib/test_sysctl.c
> module and the tools/testing/selftests/sysctl/sysctl.sh script, so if
> you can device a test there, once we decide what to do that would be
> appreciated.

We spoke about this at LPC.  And this is the correct behavioral change.

The problem is there is a default value for i_uid and i_gid that is
correct in the general case.  That default value is not corect for
sysctl, because proc is weird.  As the sysctl permission check in
test_perm are all against GLOBAL_ROOT_UID and GLOBAL_ROOT_GID we did not
notice that i_uid and i_gid were being set wrong.

So all this patch does is fix the default values i_uid and i_gid.

The commit comment seems worth cleaning up.  But for the
content of the code.

I expect when I have a few moments I will pick this change up.

Reviewed-by: "Eric W. Biederman" <ebiederm@...ssion.com>

Eric

>> Signed-off-by: Radoslaw Burny <rburny@...gle.com>
>> ---
>>  fs/proc/proc_sysctl.c | 4 ++++
>>  1 file changed, 4 insertions(+)
>> 
>> diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
>> index c5cbbdff3c3d..67379a389658 100644
>> --- a/fs/proc/proc_sysctl.c
>> +++ b/fs/proc/proc_sysctl.c
>> @@ -499,6 +499,10 @@ static struct inode *proc_sys_make_inode(struct super_block *sb,
>>  
>>  	if (root->set_ownership)
>>  		root->set_ownership(head, table, &inode->i_uid, &inode->i_gid);
>> +	else {
>> +		inode->i_uid = GLOBAL_ROOT_UID;
>> +		inode->i_gid = GLOBAL_ROOT_GID;
>> +	}
>>  
>>  out:
>>  	return inode;
>> -- 
>> 2.20.0.rc0.387.gc7a69e6b6c-goog
>>