Re: [v2 PATCH] mm: introduce arg_lock to protect arg_start|end and env_start|end in mm_struct

[Cyrill Gorcunov <gorcunov@gmail.com>]

On Mon, Mar 26, 2018 at 11:37:25AM -0700, Matthew Wilcox wrote:
> On Tue, Mar 27, 2018 at 02:20:39AM +0800, Yang Shi wrote:
> > +++ b/kernel/sys.c
> > @@ -1959,7 +1959,7 @@ static int prctl_set_mm_map(int opt, const void __user *addr, unsigned long data
> >  			return error;
> >  	}
> >  
> > -	down_write(&mm->mmap_sem);
> > +	down_read(&mm->mmap_sem);
> >  
> >  	/*
> >  	 * We don't validate if these members are pointing to
> > @@ -1980,10 +1980,13 @@ static int prctl_set_mm_map(int opt, const void __user *addr, unsigned long data
> >  	mm->start_brk	= prctl_map.start_brk;
> >  	mm->brk		= prctl_map.brk;
> >  	mm->start_stack	= prctl_map.start_stack;
> > +
> > +	spin_lock(&mm->arg_lock);
> >  	mm->arg_start	= prctl_map.arg_start;
> >  	mm->arg_end	= prctl_map.arg_end;
> >  	mm->env_start	= prctl_map.env_start;
> >  	mm->env_end	= prctl_map.env_end;
> > +	spin_unlock(&mm->arg_lock);
> >  
> >  	/*
> >  	 * Note this update of @saved_auxv is lockless thus
> 
> I see the argument for the change to a write lock was because of a BUG
> validating arg_start and arg_end, but more generally, we are updating these
> values, so a write-lock is probably a good idea, and this is a very rare
> operation to do, so we don't care about making this more parallel.  I would
> not make this change (but if other more knowledgable people in this area
> disagree with me, I will withdraw my objection to this part).

Say we've two syscalls running prctl_set_mm_map in parallel, and imagine
one have @start_brk = 20 @brk = 10 and second caller has @start_brk = 30
and @brk = 20. Since now the call is guarded by _read_ the both calls
unlocked and due to OO engine it may happen then when both finish
we have @start_brk = 30 and @brk = 10. In turn "write" semaphore
has been take to have consistent data on exit, either you have [20;10]
or [30;20] assigned not something mixed.

That said I think using read-lock here would be a bug.

	Cyrill