linux-kernel - Re: [PATCH 1/3] mm/vmalloc: fix size check for remap_vmalloc_range

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Date:   Thu, 3 Jan 2019 16:13:57 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Roman Penyaev <rpenyaev@...e.de>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Joe Perches <joe@...ches.com>,
        "Luis R. Rodriguez" <mcgrof@...nel.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH 1/3] mm/vmalloc: fix size check for
 remap_vmalloc_range_partial()

On Thu 03-01-19 15:59:52, Roman Penyaev wrote:
> area->size can include adjacent guard page but get_vm_area_size()
> returns actual size of the area.
> 
> This fixes possible kernel crash when userspace tries to map area
> on 1 page bigger: size check passes but the following vmalloc_to_page()
> returns NULL on last guard (non-existing) page.

Can this actually happen? I am not really familiar with all the callers
of this API but VM_NO_GUARD is not really used wildly in the kernel.
All I can see is kasan na arm64 which doesn't really seem to use it
for vmalloc.

So is the problem real or this is a mere cleanup?
 
> Signed-off-by: Roman Penyaev <rpenyaev@...e.de>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Michal Hocko <mhocko@...e.com>
> Cc: Andrey Ryabinin <aryabinin@...tuozzo.com>
> Cc: Joe Perches <joe@...ches.com>
> Cc: "Luis R. Rodriguez" <mcgrof@...nel.org>
> Cc: linux-mm@...ck.org
> Cc: linux-kernel@...r.kernel.org
> Cc: stable@...r.kernel.org
> ---
>  mm/vmalloc.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 871e41c55e23..2cd24186ba84 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -2248,7 +2248,7 @@ int remap_vmalloc_range_partial(struct vm_area_struct *vma, unsigned long uaddr,
>  	if (!(area->flags & VM_USERMAP))
>  		return -EINVAL;
>  
> -	if (kaddr + size > area->addr + area->size)
> +	if (kaddr + size > area->addr + get_vm_area_size(area))
>  		return -EINVAL;
>  
>  	do {
> -- 
> 2.19.1

-- 
Michal Hocko
SUSE Labs