Re: [PATCH v3 1/2] mm/vmalloc: Add a safer version of find_vm_area() for debug

From: Vlastimil Babka <vbabka@suse.cz>
To: "Joel Fernandes (Google)" <joel@joelfernandes.org>,
	linux-kernel@vger.kernel.org,
	Andrew Morton <akpm@linux-foundation.org>,
	Uladzislau Rezki <urezki@gmail.com>,
	Christoph Hellwig <hch@infradead.org>,
	Lorenzo Stoakes <lstoakes@gmail.com>,
	"Paul E. McKenney" <paulmck@kernel.org>
Cc: Zhen Lei <thunder.leizhen@huaweicloud.com>,
	rcu@vger.kernel.org, Zqiang <qiang.zhang1211@gmail.com>,
	stable@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH v3 1/2] mm/vmalloc: Add a safer version of find_vm_area() for debug
Date: Thu, 7 Sep 2023 08:53:14 +0200	[thread overview]
Message-ID: <49ff5505-5a4a-1ad5-8552-6e79a91ee8c9@suse.cz> (raw)
In-Reply-To: <20230904180806.1002832-1-joel@joelfernandes.org>

Hi,

On 9/4/23 20:08, 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().

I was a bit confused by the subject which suggests a new function is added,
but it seems open-coded in its only caller. I assume it's due to evolution
of the series. Something like:

mm/vmalloc: use trylock for vmap_area_lock in vmalloc_dump_obj()

?

I also notice it's trying hard to copy everything from "vm" to temporary
variables before unlocking, presumably to prevent use-after-free, so should
that be also mentioned in the changelog?

> [applied test robot feedback on unused function fix.]
> [applied Uladzislau feedback on locking.]
> 
> Reported-by: Zhen Lei <thunder.leizhen@huaweicloud.com>
> Cc: Paul E. McKenney <paulmck@kernel.org>
> Cc: rcu@vger.kernel.org
> Cc: Zqiang <qiang.zhang1211@gmail.com>
> Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> Fixes: 98f180837a89 ("mm: Make mem_dump_obj() handle vmalloc() memory")
> Cc: stable@vger.kernel.org
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> ---
>  mm/vmalloc.c | 26 ++++++++++++++++++++++----
>  1 file changed, 22 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 93cf99aba335..2c6a0e2ff404 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -4274,14 +4274,32 @@ void pcpu_free_vm_areas(struct vm_struct **vms, int nr_vms)
>  #ifdef CONFIG_PRINTK
>  bool vmalloc_dump_obj(void *object)
>  {
> -	struct vm_struct *vm;
>  	void *objp = (void *)PAGE_ALIGN((unsigned long)object);
> +	const void *caller;
> +	struct vm_struct *vm;
> +	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)objp, &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 = (unsigned long)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