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

Bernd Edlinger
2020-03-10T19:43:07Z

On 3/10/20 8:01 PM, Eric W. Biederman wrote:
> Bernd Edlinger <bernd.edlinger@hotmail.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@hotmail.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);