Re: [PATCH] prctl.2: document PR_SET_PTRACER

[Pavel Emelyanov <xemul-bzQdu9zFT3WakBO8gow8eQ@public.gmane.org>]

On 01/11/2013 03:24 AM, Michael Kerrisk (man-pages) wrote:
> [CC+=Pavel]
> 
> Hi Kees,
> 
> On Thu, Jan 10, 2013 at 10:28 PM, Kees Cook <keescook-F7+t8E8rja9g9hUCZPvPmw@public.gmane.org> wrote:
>> This documents the Yama LSM's prctl handler that allows processes to
>> declare ptrace restriction exception relationships via PR_SET_PTRACER.
> 
> This is synchronicity! I was just looking  at what was undocumented in
> prctl() after a conversation with Pavel and wondering about pinging
> you about PR_SET_PTRACER.
> 
> Some questions/observations:
> 
> 1. From the look of it, each tracee can have only one tracer, so that
> PR_SET_PTRACER overwrites any previous PID, right. The text implies
> that, but I'll add a few more words to make it explicit.
> 
> 2. There is no PR_GET_PTRACER operation. I suppose it's not needed for
> your purposes. I wonder however if it might matter for CRIU to have
> PR_GET_PTRACER, hence I CC Pavel.

Since CRIU does PTRACE_SEIZE on task it wants to dump, it will fail to
do it with any task that's already being traced by someone else. Thus the
getter for this prctl is not required for C/R.

Thanks,
Pavel

> Your patch is applied.
> 
> Cheers,
> 
> Michael
> 
>> Signed-off-by: Kees Cook <keescook-F7+t8E8rja9g9hUCZPvPmw@public.gmane.org>
>> ---
>>  man2/prctl.2 |   18 +++++++++++++++++-
>>  1 file changed, 17 insertions(+), 1 deletion(-)
>>
>> diff --git a/man2/prctl.2 b/man2/prctl.2
>> index c8a4919..506c7b3 100644
>> --- a/man2/prctl.2
>> +++ b/man2/prctl.2
>> @@ -45,9 +45,10 @@
>>  .\" 2012-09-20 Kees Cook, document PR_SET_NO_NEW_PRIVS, PR_GET_NO_NEW_PRIVS
>>  .\" 2012-10-25 Michael Kerrisk, Document PR_SET_TIMERSLACK and
>>  .\"                             PR_GET_TIMERSLACK
>> +.\" 2013-01-10 Kees Cook, document PR_SET_PTRACER
>>  .\"
>>  .\"
>> -.TH PRCTL 2 2012-10-25 "Linux" "Linux Programmer's Manual"
>> +.TH PRCTL 2 2013-01-10 "Linux" "Linux Programmer's Manual"
>>  .SH NAME
>>  prctl \- operations on a process
>>  .SH SYNOPSIS
>> @@ -270,6 +271,21 @@ Return the current value of the parent process death signal,
>>  in the location pointed to by
>>  .IR "(int\ *) arg2" .
>>  .TP
>> +.BR PR_SET_PTRACER " (since Linux 3.4)"
>> +This is only meaningful when the Yama LSM is enabled and in mode 1
>> +("restricted ptrace", visible via
>> +.IR /proc/sys/kernel/yama/ptrace_scope ).
>> +When a "ptracer process id" is passed in \fIarg2\fP, the caller is declaring
>> +that the ptracer process can ptrace the current process as if it were a
>> +direct process ancestor. When set to 0, this relationship is removed. When
>> +set to
>> +.BR PR_SET_PTRACER_ANY,
>> +the ptrace restrictions introduced by Yama are effectively disabled for the
>> +current process.
>> +
>> +For further information, see the kernel source file
>> +.IR Documentation/security/Yama.txt .
>> +.TP
>>  .BR PR_SET_SECCOMP " (since Linux 2.6.23)"
>>  .\" See http://thread.gmane.org/gmane.linux.kernel/542632
>>  .\" [PATCH 0 of 2] seccomp updates
>> --
>> 1.7.9.5
>>
>> --
>> Kees Cook
>> Chrome OS Security
> 
> 
> 


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