linux-kernel - Re: [PATCH v2] ovl: drop CAP_SYS_RESOURCE from saved mounter's credentials

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Date:   Sat, 22 Jul 2017 12:30:39 +0300
From:   Amir Goldstein <amir73il@...il.com>
To:     Vivek Goyal <vgoyal@...hat.com>
Cc:     Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        overlayfs <linux-unionfs@...r.kernel.org>,
        Miklos Szeredi <miklos@...redi.hu>,
        "stable [v4.8]" <stable@...r.kernel.org>
Subject: Re: [PATCH v2] ovl: drop CAP_SYS_RESOURCE from saved mounter's credentials

Bumped into this patch (Now upstream commit 51f8f3c4e225) and realized
it is missing cc: stable # v4.8

At least this docker PR suggests that regression introduced in v4.8 will not be
appreciated down the road:
https://github.com/moby/moby/issues/29364


On Tue, Jan 10, 2017 at 9:17 PM, Vivek Goyal <vgoyal@...hat.com> wrote:
> On Tue, Jan 10, 2017 at 09:30:21PM +0300, Konstantin Khlebnikov wrote:
>> If overlay was mounted by root then quota set for upper layer does not work
>> because overlay now always use mounter's credentials for operations.
>> Also overlay might deplete reserved space and inodes in ext4.
>>
>> This patch drops capability SYS_RESOURCE from saved credentials.
>> This affects creation new files, whiteouts, and copy-up operations.
>>
>
> I am not an expert in this area, but I thought previous patch was
> better. I am not sure why overlay internal operations should be
> done without CAP_SYS_RESOURCES when caller has CAP_SYS_RESOURCES. That
> might be counter-intuitive.
>
> If some task is allowed to bypass quota limitations on a file system
> then same should be true when task is working on overlay.
>
> Similary if a task is allowed to use reserved space on filesystem, then same
> task should be allowed to use reserved space on underlying filesystem
> when doing overlay.  It should not be overlay's job to prevent that?
>
> May be it is just me....
>

Vivek,

Since your question was not answered in this thread, IMO, your concern
is just, but in practice I think that:
1. It's going to be harder to implement for every operation to combine the
    mounter's creds with the process capabilities... weird
2. The use case of ext4 reserved blocks is to allow sys admin some slack
    for disk allocations that are needed in order to free up disk space or for
    other critical tasks to prevent the system from hanging. It doesn't sound
    like this use case fits an overlayfs mount that well.
3. FYI, xfs project quota (which as you know can be applied to docker
    overlayfs container) does not check CAP_SYS_RESOURCES at all.
    and if and when ext4 project quotas can also be applied to docker
    overlayfs container, I am sure that containers admin will not appreciate
    a container exceeding its quota, even if that was a privileged process
    writing to that container

So IMO that fix as it is is good for all practical purpose.

Cheers,
Amir.

>
>
>> Signed-off-by: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
>> Fixes: 1175b6b8d963 ("ovl: do operations on underlying file system in mounter's context")
>> Cc: Vivek Goyal <vgoyal@...hat.com>
>> Cc: Miklos Szeredi <mszeredi@...hat.com>
>> ---
>>  fs/overlayfs/super.c |    9 +++++++--
>>  1 file changed, 7 insertions(+), 2 deletions(-)
>>
>> diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
>> index 20f48abbb82f..8dba982e1af5 100644
>> --- a/fs/overlayfs/super.c
>> +++ b/fs/overlayfs/super.c
>> @@ -701,6 +701,7 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
>>       unsigned int stacklen = 0;
>>       unsigned int i;
>>       bool remote = false;
>> +     struct cred *cred;
>>       int err;
>>
>>       err = -ENOMEM;
>> @@ -870,10 +871,14 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
>>       else
>>               sb->s_d_op = &ovl_dentry_operations;
>>
>> -     ufs->creator_cred = prepare_creds();
>> -     if (!ufs->creator_cred)
>> +     cred = prepare_creds();
>> +     if (!cred)
>>               goto out_put_lower_mnt;
>>
>> +     /* Never override disk quota limits or use reserved space */
>> +     cap_lower(cred->cap_effective, CAP_SYS_RESOURCE);
>> +     ufs->creator_cred = cred;
>> +
>>       err = -ENOMEM;
>>       oe = ovl_alloc_entry(numlower);
>>       if (!oe)