Re: [PATCH] mm/vmalloc: Don't allow VM_NO_GUARD on vmap()

[Will Deacon <will@kernel.org>]

On Thu, Sep 16, 2021 at 12:41:56PM +0200, Peter Zijlstra wrote:
> 
> The vmalloc guard pages are added on top of each allocation, thereby
> isolating any two allocations from one another. The top guard of the
> lower allocation is the bottom guard guard of the higher allocation
> etc.
> 
> Therefore VM_NO_GUARD is dangerous; it breaks the basic premise of
> isolating separate allocations.
> 
> There are only two in-tree users of this flag, neither of which use it
> through the exported interface. Ensure it stays this way.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
>  include/linux/vmalloc.h | 2 +-
>  mm/vmalloc.c            | 7 +++++++
>  2 files changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
> index 671d402c3778..10e9571ff0b2 100644
> --- a/include/linux/vmalloc.h
> +++ b/include/linux/vmalloc.h
> @@ -22,7 +22,7 @@ struct notifier_block;		/* in notifier.h */
>  #define VM_USERMAP		0x00000008	/* suitable for remap_vmalloc_range */
>  #define VM_DMA_COHERENT		0x00000010	/* dma_alloc_coherent */
>  #define VM_UNINITIALIZED	0x00000020	/* vm_struct is not fully initialized */
> -#define VM_NO_GUARD		0x00000040      /* don't add guard page */
> +#define VM_NO_GUARD		0x00000040      /* ***DANGEROUS*** don't add guard page */
>  #define VM_KASAN		0x00000080      /* has allocated kasan shadow memory */
>  #define VM_FLUSH_RESET_PERMS	0x00000100	/* reset direct map and flush TLB on unmap, can't be freed in atomic context */
>  #define VM_MAP_PUT_PAGES	0x00000200	/* put pages and free array in vfree */
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index d77830ff604c..01927ebea267 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -2743,6 +2743,13 @@ void *vmap(struct page **pages, unsigned int count,
>  
>  	might_sleep();
>  
> +	/*
> +	 * Your top guard is someone else's bottom guard. Not having a top
> +	 * guard compromises someone else's mappings too.
> +	 */
> +	if (WARN_ON_ONCE(flags & VM_NO_GUARD))
> +		flags &= ~VM_NO_GUARD;
> +
>  	if (count > totalram_pages())
>  		return NULL;

Acked-by: Will Deacon <will@kernel.org>

Thanks!

Will