linux-kernel - Re: [PATCH 1/4] kernel/kcmp.c: Use new infrastructure to fix deadlocks in execve

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Date:   Tue, 10 Mar 2020 20:42:53 +0100
From:   Bernd Edlinger <bernd.edlinger@...mail.de>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Christian Brauner <christian.brauner@...ntu.com>,
        Kees Cook <keescook@...omium.org>,
        Jann Horn <jannh@...gle.com>, Jonathan Corbet <corbet@....net>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Oleg Nesterov <oleg@...hat.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Andrei Vagin <avagin@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Yuyang Du <duyuyang@...il.com>,
        David Hildenbrand <david@...hat.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Anshuman Khandual <anshuman.khandual@....com>,
        David Howells <dhowells@...hat.com>,
        James Morris <jamorris@...ux.microsoft.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Christian Kellner <christian@...lner.me>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Aleksa Sarai <cyphar@...har.com>,
        "Dmitry V. Levin" <ldv@...linux.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>
Subject: Re: [PATCH 1/4] kernel/kcmp.c: Use new infrastructure to fix
 deadlocks in execve

On 3/10/20 8:01 PM, Eric W. Biederman wrote:
> Bernd Edlinger <bernd.edlinger@...mail.de> writes:
> 
>> This changes kcmp_epoll_target to use the new exec_update_mutex
>> instead of cred_guard_mutex.
>>
>> This should be safe, as the credentials are only used for reading,
>> and furthermore ->mm and ->sighand are updated on execve,
>> but only under the new exec_update_mutex.
>>
> 
> Can you add a comment that the exec_update_mutex is not needed for
> KCMP_FILE?  As both sets of credentials during exec are valid
> for accessing the files so exec_update_mutex does not matter.
> 

some files are closed by do_close_on_exec,
so in theory this allows you to examine files that
were open in the old user but closed for the new user
with either credential.

It is not a race condition, but it may be a security
concern.

> I don't think exec_update_mutex is needed for KCMP_SYSVSEM
> or KCMP_EPOLL_TFD either.  As I don't think exec changes either
> one of those.
> 

KCMP_EPOLL_TFD is also accessing file pointers,
that is possible.

It might be that KCMP_SYSVSEM is a missed optimization, but
I may have overlooked something.
I'd rather err on the safe side.

> Eric
> 
> 
>> Signed-off-by: Bernd Edlinger <bernd.edlinger@...mail.de>
>> ---
>>  kernel/kcmp.c | 8 ++++----
>>  1 file changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/kernel/kcmp.c b/kernel/kcmp.c
>> index a0e3d7a..b3ff928 100644
>> --- a/kernel/kcmp.c
>> +++ b/kernel/kcmp.c
>> @@ -173,8 +173,8 @@ static int kcmp_epoll_target(struct task_struct *task1,
>>  	/*
>>  	 * One should have enough rights to inspect task details.
>>  	 */
>> -	ret = kcmp_lock(&task1->signal->cred_guard_mutex,
>> -			&task2->signal->cred_guard_mutex);
>> +	ret = kcmp_lock(&task1->signal->exec_update_mutex,
>> +			&task2->signal->exec_update_mutex);
>>  	if (ret)
>>  		goto err;
>>  	if (!ptrace_may_access(task1, PTRACE_MODE_READ_REALCREDS) ||
>> @@ -229,8 +229,8 @@ static int kcmp_epoll_target(struct task_struct *task1,
>>  	}
>>  
>>  err_unlock:
>> -	kcmp_unlock(&task1->signal->cred_guard_mutex,
>> -		    &task2->signal->cred_guard_mutex);
>> +	kcmp_unlock(&task1->signal->exec_update_mutex,
>> +		    &task2->signal->exec_update_mutex);
>>  err:
>>  	put_task_struct(task1);
>>  	put_task_struct(task2);