linux-kernel - Re: [PATCH 5.10 1/1] ovl: fail on invalid uid/gid mapping at copy up

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Date:   Wed, 31 May 2023 18:12:32 +0300
From:   listdansp <listdansp@...l.ru>
To:     Amir Goldstein <amir73il@...il.com>
Cc:     stable@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Miklos Szeredi <miklos@...redi.hu>,
        linux-unionfs@...r.kernel.org, linux-kernel@...r.kernel.org,
        Miklos Szeredi <mszeredi@...hat.com>,
        Seth Forshee <sforshee@...nel.org>,
        Christian Brauner <brauner@...nel.org>
Subject: Re: [PATCH 5.10 1/1] ovl: fail on invalid uid/gid mapping at copy up


>> From: Miklos Szeredi <mszeredi@...hat.com>
>>
>> commit  4f11ada10d0ad3fd53e2bd67806351de63a4f9c3 upstream.
>>
>> If st_uid/st_gid doesn't have a mapping in the mounter's user_ns, then
>> copy-up should fail, just like it would fail if the mounter task was doing
>> the copy using "cp -a".
>>
>> There's a corner case where the "cp -a" would succeed but copy up fail: if
>> there's a mapping of the invalid uid/gid (65534 by default) in the user
>> namespace.  This is because stat(2) will return this value if the mapping
>> doesn't exist in the current user_ns and "cp -a" will in turn be able to
>> create a file with this uid/gid.
>>
>> This behavior would be inconsistent with POSIX ACL's, which return -1 for
>> invalid uid/gid which result in a failed copy.
>>
>> For consistency and simplicity fail the copy of the st_uid/st_gid are
>> invalid.
>>
>> Fixes: 459c7c565ac3 ("ovl: unprivieged mounts")
>> Cc: <stable@...r.kernel.org> # v5.11
>> Signed-off-by: Miklos Szeredi <mszeredi@...hat.com>
>> Reviewed-by: Christian Brauner <brauner@...nel.org>
>> Reviewed-by: Seth Forshee <sforshee@...nel.org>
>> Signed-off-by: Danila Chernetsov <listdansp@...l.ru>
>> ---
>>   fs/overlayfs/copy_up.c | 4 ++++
>>   1 file changed, 4 insertions(+)
>>
>> diff --git a/fs/overlayfs/copy_up.c b/fs/overlayfs/copy_up.c
>> index e466c58f9ec4..fd33abc0edc0 100644
>> --- a/fs/overlayfs/copy_up.c
>> +++ b/fs/overlayfs/copy_up.c
>> @@ -882,6 +882,10 @@ static int ovl_copy_up_one(struct dentry *parent, struct dentry *dentry,
>>          if (err)
>>                  return err;
>>
>> +       if (!kuid_has_mapping(current_user_ns(), ctx.stat.uid) ||
>> +           !kgid_has_mapping(current_user_ns(), ctx.stat.gid))
>> +               return -EOVERFLOW;
>> +
>>          ctx.metacopy = ovl_need_meta_copy_up(dentry, ctx.stat.mode, flags);
>>
>>          if (parent) {
>> --
>> 2.25.1
>>
> You are requesting to backport to kernel 5.10.y a fix to a bug that is tagged as
> introduced in 5.11 with overlayfs unprivileged mounts.
>
> IOW, in kernel 5.10, current_user_ns() would always be init_user_ns.
>
> Am I missing something?
>
> Thanks,
> Amir.
>
Sorry for this inconvientions, really this issue was introduced in 5.11

Danila