linux-kernel - Re: [PATCH] kernel/signal: Signal-based pre-coredump notification

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Message-ID: <3e3728f7-1370-586c-c5ac-4ba0604976a5@cisco.com>
Date:   Mon, 15 Oct 2018 14:31:57 -0700
From:   Enke Chen <enkechen@...co.com>
To:     Alan Cox <gnomes@...rguk.ukuu.org.uk>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Arnd Bergmann <arnd@...db.de>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Khalid Aziz <khalid.aziz@...cle.com>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        Helge Deller <deller@....de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christian Brauner <christian@...uner.io>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Dave Martin <Dave.Martin@....com>,
        Mauro Carvalho Chehab <mchehab+samsung@...nel.org>,
        Michal Hocko <mhocko@...nel.org>,
        Rik van Riel <riel@...riel.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Roman Gushchin <guro@...com>,
        Marcos Paulo de Souza <marcos.souza.org@...il.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Dominik Brodowski <linux@...inikbrodowski.net>,
        Cyrill Gorcunov <gorcunov@...nvz.org>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        Jann Horn <jannh@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        "Victor Kamensky (kamensky)" <kamensky@...co.com>,
        xe-linux-external@...co.com, Stefan Strogin <sstrogin@...co.com>,
        Enke Chen <enkechen@...co.com>
Subject: Re: [PATCH] kernel/signal: Signal-based pre-coredump notification

Hi, Alan:

As I replied earlier, I will remove the logic that allows setting on others.
This function "set_predump_signal_perm()" will be gone too.

Thanks.  -- Enke

On 10/15/18 2:21 PM, Alan Cox wrote:
>> +/*
>> + * Returns true if current's euid is same as p's uid or euid,
>> + * or has CAP_SYS_ADMIN.
>> + *
>> + * Called with rcu_read_lock, creds are safe.
>> + *
>> + * Adapted from set_one_prio_perm().
>> + */
>> +static bool set_predump_signal_perm(struct task_struct *p)
>> +{
>> +	const struct cred *cred = current_cred(), *pcred = __task_cred(p);
>> +
>> +	return uid_eq(pcred->uid, cred->euid) ||
>> +	       uid_eq(pcred->euid, cred->euid) ||
>> +	       capable(CAP_SYS_ADMIN);
>> +}
> 
> This makes absolutely no security sense whatsoever. The uid and euid of
> the parent and child can both change between the test and the signal
> delivery.
> 
> There are reasons that the child signal control code is incredibly
> careful about either the parent or child using execve or doing a
> privilege change that might pose a risk.
> 
> Until this code gets the same protections I don't believe it's safe.
> 
> Alan
>