linux-kernel - Re: [PATCH] mm: vmalloc: Prevent use after free in _vm_unmap

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Date:   Wed, 24 Mar 2021 14:32:42 +0100
From:   Uladzislau Rezki <urezki@...il.com>
To:     Vijayanand Jitta <vjitta@...eaurora.org>
Cc:     Uladzislau Rezki <urezki@...il.com>, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        vinmenon@...eaurora.org
Subject: Re: [PATCH] mm: vmalloc: Prevent use after free in _vm_unmap_aliases

> 
> On 3/18/2021 10:29 PM, Uladzislau Rezki wrote:
> > On Thu, Mar 18, 2021 at 03:38:25PM +0530, vjitta@...eaurora.org wrote:
> >> From: Vijayanand Jitta <vjitta@...eaurora.org>
> >>
> >> A potential use after free can occur in _vm_unmap_aliases
> >> where an already freed vmap_area could be accessed, Consider
> >> the following scenario:
> >>
> >> Process 1						Process 2
> >>
> >> __vm_unmap_aliases					__vm_unmap_aliases
> >> 	purge_fragmented_blocks_allcpus				rcu_read_lock()
> >> 		rcu_read_lock()
> >> 			list_del_rcu(&vb->free_list)
> >> 									list_for_each_entry_rcu(vb .. )
> >> 	__purge_vmap_area_lazy
> >> 		kmem_cache_free(va)
> >> 										va_start = vb->va->va_start
> > Or maybe we should switch to kfree_rcu() instead of kmem_cache_free()?
> > 
> > --
> > Vlad Rezki
> > 
> 
> Thanks for suggestion.
> 
> I see free_vmap_area_lock (spinlock) is taken in __purge_vmap_area_lazy
> while it loops through list and calls kmem_cache_free on va's. So, looks
> like we can't replace it with kfree_rcu as it might cause scheduling
> within atomic context.
> 
A double argument of the kfree_rcu() is a safe way to be used from atomic
contexts, it does not use any sleeping primitives, so it can be replaced.

>From the other hand i see that per-cpu KVA allocator is only one user of
the RCU and your change fixes it. Feel free to use:

Reviewed-by: Uladzislau Rezki (Sony) <urezki@...il.com>

Thanks.

--
Vlad Rezki