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Subject: Re: [PATCH] ksys_mount: check for permissions before resource
 allocation
From:   Ilya Matveychikov <matvejchikov@gmail.com>
In-Reply-To: <87o9gpatxs.fsf@xmission.com>
Date:   Wed, 6 Jun 2018 13:32:12 +0400
Cc:     linux-kernel@vger.kernel.org,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        linux-fsdevel@vger.kernel.org
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Message-Id: <64021AF2-81EE-439C-91D4-9A33AB0D08F7@gmail.com>
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 <BD48252B-65D7-4238-BDA2-71BAC1CEA855@gmail.com>
 <87o9gpatxs.fsf@xmission.com>
To:     "Eric W. Biederman" <ebiederm@xmission.com>
Sender: linux-kernel-owner@vger.kernel.org
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> On Jun 5, 2018, at 11:56 PM, Eric W. Biederman <ebiederm@xmission.com> =
wrote:
>=20
> Ilya Matveychikov <matvejchikov@gmail.com> writes:
>=20
>> Just CC=E2=80=99ed to some of maintainers.
>>=20
>> $ perl scripts/get_maintainer.pl =
fs/0001-ksys_mount-check-for-permissions-before-resource-all.patch
>> Alexander Viro <viro@zeniv.linux.org.uk> (maintainer:FILESYSTEMS (VFS =
and infrastructure))
>> linux-fsdevel@vger.kernel.org (open list:FILESYSTEMS (VFS and =
infrastructure))
>> linux-kernel@vger.kernel.org (open list)
>>=20
>>> On Jun 5, 2018, at 6:00 AM, Ilya Matveychikov =
<matvejchikov@gmail.com> wrote:
>>>=20
>>> Early check for mount permissions prevents possible allocation of 3
>>> pages from kmalloc() pool by unpriveledged user which can be used =
for
>>> spraying the kernel heap.
>=20
> *Snort*
>=20
> You clearly have not read may_mount.  Your modified code still
> let's unprivileged users in.  So even if all of Al's good objections
> were not applicable this change would still be buggy and wrong.
>=20
> Nacked-by: "Eric W. Biederman" <ebiederm@xmission.com>


Don=E2=80=99t get me wrong but may_mount() is:

static inline bool may_mount(void)
{
        return ns_capable(current->nsproxy->mnt_ns->user_ns, =
CAP_SYS_ADMIN);
}

What do you mean by "You clearly have not read may_mount=E2=80=9D? The =
only thing that
can affect may_mount result (as mentioned earlier) is that task=E2=80=99s =
NS capability
might be changed by security_sb_mount() hook.

So, do you think that is=E2=80=99s possible to NOT have CAP_SYS_ADMIN =
while entering to
ksys_mount() but getting it with the security_sb_mount() hook?

This is the only case I see that using may_mount() before =
security_sb_mount()
is wrong. This was the point?