Re: [PATCH 2/2] fix sys_unshare()+SEM_UNDO: perform an implicit CLONE_SYSVSEM in CLONE_NEWIPC

["Serge E. Hallyn" <serue@us.ibm.com>]

Quoting Manfred Spraul (manfred@colorfullife.com):
> Serge E. Hallyn wrote:
>> Quoting Manfred Spraul (manfred@colorfullife.com):
>>   
>>> sys_unshare(CLONE_NEWIPC) doesn't handle the undo lists properly, this 
>>> can
>>> cause a kernel memory corruption. CLONE_NEWIPC must detach from the 
>>> existing
>>> undo lists.
>>> Fix, part 2: perform an implicit CLONE_SYSVSEM in CLONE_NEWIPC.
>>> CLONE_NEWIPC creates a new IPC namespace, the task cannot access the
>>> existing semaphore arrays after the unshare syscall. Thus the task
>>> can/must detach from the existing undo list entries, too.
>>>
>>> This fixes the kernel corruption, because it makes it impossible that
>>> undo records from two different namespaces are in sysvsem.undo_list.
>>>     
>>
>> So this was never an issue with clone(CLONE_NEWIPC|CLONE_SYSVSEM), which
>> should have had the same result as unshare(CLONE_NEWIPC&~CLONE_SYSVSEM)?
>>
>>   
> Actually, the story is slightly different:
> unshare(CLONE_NEWIPC|CLONE_SYSVSEM) returns -EINVAL right now.

Right, but it's feasible with clone (where CLONE_SYSVSEM has the
opposite meaning of with unshare) right?  According to the
comment, CLONE_SYSVSEM was disabled mainly out of fear that it
would be confusing to enable it since it does have the inverse meaning
in unshare as in clone.

> Thus all apps right now call unshare(CLONE_NEWIPC|&~CLONE_SYSVSEM).
> This combination doesn't make much sense. Even worse - it easily causes a 
> kernel oops.
> Thus my fix is twofold:
> - add support for unshare(CLONE_SYSVSEM).
> - implicitely add CLONE_SYSVSEM to all calls that set CLONE_NEWIPC.

Right but your fix ignores the fact that you can achieve the same
result as unshare(CLONE_NEWIPC&~CLONE_SYSVSEM) by doing
clone(CLONE_NEWIPC|CLONE_SYSVSEM).

> It's not really pretty: If a pivot_namespace syscall is ever added, then 
> CLONE_NEWIPC&~CLONE_SYSVSEM would make sense again.
> What do you think? Can we break backward compatibility and add
>    if ( (unshare_flags & CLONE_NEWIPC) && !(unshare_flags & CLONE_SYSVSEM) 
> )
>       return -EINVAL;
> into sys_unshare()?

I don't think we need to, I think what you're doing makes perfect sense.
If you asked for CLONE_NEWIPC, then you wanted to do CLONE_SYSVSEM (uh, or,
in clone parlance, !CLONE_SYSVSEM :)

> I have decided against that, it breaks the current ABI.
> And we gain virtually nothing - most if not all unshare users will be 
> single threaded apps that do not use sysvsem at all, and even most sysvsem 
> users do not use SEM_UNDO.

And most importantly sharing your semundo list but not your sems with
your parent is silly!

-serge