Re: [PATCH v2 1/2] mm/vmalloc: Add a safer version of find_vm_area() for debug

[Joel Fernandes]

On Thu, Aug 31, 2023 at 09:47:52PM +0200, Uladzislau Rezki wrote:
> On Thu, Aug 31, 2023 at 05:18:25PM +0000, Joel Fernandes (Google) wrote:
> > It is unsafe to dump vmalloc area information when trying to do so from
> > some contexts. Add a safer trylock version of the same function to do a
> > best-effort VMA finding and use it from vmalloc_dump_obj().
> > 
> > [apply test robot feedback on unused function fix.]
> > 
> > Reported-by: Zhen Lei <thunder.leizhen@xxxxxxxxxxxxxxx>
> > Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>
> > Cc: rcu@xxxxxxxxxxxxxxx
> > Cc: Zqiang <qiang.zhang1211@xxxxxxxxx>
> > Reviewed-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> > ---
> > v1->v2: Apply review tags and test robot feedback.
> > 
> >  mm/vmalloc.c | 39 ++++++++++++++++++++++++++++++++++++++-
> >  1 file changed, 38 insertions(+), 1 deletion(-)
> > 
> > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > index 93cf99aba335..f09e882ae3b8 100644
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> > @@ -1865,6 +1865,20 @@ struct vmap_area *find_vmap_area(unsigned long addr)
> >  	return va;
> >  }
> >  
> > +#ifdef CONFIG_PRINTK
> > +static struct vmap_area *find_vmap_area_trylock(unsigned long addr)
> > +{
> > +	struct vmap_area *va;
> > +
> > +	if (!spin_trylock(&vmap_area_lock))
> > +		return NULL;
> > +	va = __find_vmap_area(addr, &vmap_area_root);
> > +	spin_unlock(&vmap_area_lock);
> > +
> > +	return va;
> > +}
> > +#endif
> > +
> >  static struct vmap_area *find_unlink_vmap_area(unsigned long addr)
> >  {
> >  	struct vmap_area *va;
> > @@ -2671,6 +2685,29 @@ struct vm_struct *find_vm_area(const void *addr)
> >  	return va->vm;
> >  }
> >  
> > +/**
> > + * try_to_find_vm_area - find a continuous kernel virtual area
> > + * @addr:	  base address
> > + *
> > + * This function is the same as find_vm_area() except that it is
> > + * safe to call if vmap_area_lock is already held and returns NULL
> > + * if it is. See comments in find_vmap_area() for other details.
> > + *
> > + * Return: the area descriptor on success or %NULL on failure.
> > + */
> > +#ifdef CONFIG_PRINTK
> > +static struct vm_struct *try_to_find_vm_area(const void *addr)
> > +{
> > +	struct vmap_area *va;
> > +
> > +	va = find_vmap_area_trylock((unsigned long)addr);
> > +	if (!va)
> > +		return NULL;
> > +
> > +	return va->vm;
> > +}
> > +#endif
> > +
> >  /**
> >   * remove_vm_area - find and remove a continuous kernel virtual area
> >   * @addr:	    base address
> > @@ -4277,7 +4314,7 @@ bool vmalloc_dump_obj(void *object)
> >  	struct vm_struct *vm;
> >  	void *objp = (void *)PAGE_ALIGN((unsigned long)object);
> >  
> > -	vm = find_vm_area(objp);
> > +	vm = try_to_find_vm_area(objp);
> >  	if (!vm)
> >  		return false;
> >  	pr_cont(" %u-page vmalloc region starting at %#lx allocated at %pS\n",

Hi Vlad,
Thanks for taking a look.

> I am not sure if this patch makes a lot of sense. I agree, this is a
> problem and it mitigates it. But it is broken in terms of once you drop
> the lock, the VA should not be accessed.

Just to note the lockless-access issue you are referring to is not introduced
by this patch but is rather in the existing code. Also just to note this is
debug code.

> Is that a real issue or it gets triggered due to some syntetic test case?

It is a real issue. See 2/2.

> If i were you, i would go with open-coded version of trylock. Because
> there is only one user so far.

Taking your open coding and locking suggestions, I came up with the below
which actually results in a smaller patch. Does it look good to you?

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 93cf99aba335..aaf6bad997a7 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -4274,14 +4274,31 @@ void pcpu_free_vm_areas(struct vm_struct **vms, int nr_vms)
 #ifdef CONFIG_PRINTK
 bool vmalloc_dump_obj(void *object)
 {
+	void *caller, *objp = (void *)PAGE_ALIGN((unsigned long)object);
 	struct vm_struct *vm;
-	void *objp = (void *)PAGE_ALIGN((unsigned long)object);
+	struct vmap_area *va;
+	unsigned long addr;
+	unsigned int nr_pages;
 
-	vm = find_vm_area(objp);
-	if (!vm)
+	if (!spin_trylock(&vmap_area_lock))
+		return false;
+	va = __find_vmap_area((unsigned long)addr, &vmap_area_root);
+	if (!va) {
+		spin_unlock(&vmap_area_lock);
 		return false;
+	}
+
+	vm = va->vm;
+	if (!vm) {
+		spin_unlock(&vmap_area_lock);
+		return false;
+	}
+	addr = vm->addr;
+	caller = vm->caller;
+	nr_pages = vm->nr_pages;
+	spin_unlock(&vmap_area_lock);
 	pr_cont(" %u-page vmalloc region starting at %#lx allocated at %pS\n",
-		vm->nr_pages, (unsigned long)vm->addr, vm->caller);
+		nr_pages, addr, caller);
 	return true;
 }
 #endif