Re: [PATCH 04/10] swiotlb: Use free_decrypted_pages()

[Petr Tesařík]

On Tue, 17 Oct 2023 13:24:59 -0700
Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx> wrote:

> On TDX it is possible for the untrusted host to cause
> set_memory_encrypted() or set_memory_decrypted() to fail such that an
> error is returned and the resulting memory is shared. Callers need to take
> care to handle these errors to avoid returning decrypted (shared) memory to
> the page allocator, which could lead to functional or security issues.
> 
> Swiotlb could free decrypted/shared pages if set_memory_decrypted() fails.
> Use the recently added free_decrypted_pages() to avoid this.
> 
> In swiotlb_exit(), check for set_memory_encrypted() errors manually,
> because the pages are not nessarily going to the page allocator.
> 
> Cc: Christoph Hellwig <hch@xxxxxx>
> Cc: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
> Cc: Robin Murphy <robin.murphy@xxxxxxx>
> Cc: iommu@xxxxxxxxxxxxxxx
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
> ---
>  kernel/dma/swiotlb.c | 11 +++++++----
>  1 file changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> index 394494a6b1f3..ad06786c4f98 100644
> --- a/kernel/dma/swiotlb.c
> +++ b/kernel/dma/swiotlb.c
> @@ -524,6 +524,7 @@ void __init swiotlb_exit(void)
>  	unsigned long tbl_vaddr;
>  	size_t tbl_size, slots_size;
>  	unsigned int area_order;
> +	int ret;
>  
>  	if (swiotlb_force_bounce)
>  		return;
> @@ -536,17 +537,19 @@ void __init swiotlb_exit(void)
>  	tbl_size = PAGE_ALIGN(mem->end - mem->start);
>  	slots_size = PAGE_ALIGN(array_size(sizeof(*mem->slots), mem->nslabs));
>  
> -	set_memory_encrypted(tbl_vaddr, tbl_size >> PAGE_SHIFT);
> +	ret = set_memory_encrypted(tbl_vaddr, tbl_size >> PAGE_SHIFT);
>  	if (mem->late_alloc) {
>  		area_order = get_order(array_size(sizeof(*mem->areas),
>  			mem->nareas));
>  		free_pages((unsigned long)mem->areas, area_order);
> -		free_pages(tbl_vaddr, get_order(tbl_size));
> +		if (!ret)
> +			free_pages(tbl_vaddr, get_order(tbl_size));
>  		free_pages((unsigned long)mem->slots, get_order(slots_size));
>  	} else {
>  		memblock_free_late(__pa(mem->areas),
>  			array_size(sizeof(*mem->areas), mem->nareas));
> -		memblock_free_late(mem->start, tbl_size);
> +		if (!ret)
> +			memblock_free_late(mem->start, tbl_size);
>  		memblock_free_late(__pa(mem->slots), slots_size);
>  	}
>  
> @@ -581,7 +584,7 @@ static struct page *alloc_dma_pages(gfp_t gfp, size_t bytes)
>  	return page;
>  
>  error:
> -	__free_pages(page, order);
> +	free_decrypted_pages((unsigned long)vaddr, order);
>  	return NULL;
>  }

I admit I'm not familiar with the encryption/decryption API, but if a
__free_pages() is not sufficient here, then it is quite confusing.
The error label is reached only if set_memory_decrypted() returns
non-zero. My naive expectation is that the memory is *not* decrypted in
that case and does not require special treatment. Is this assumption
wrong?

OTOH I believe there is a bug in the logic. The subsequent
__free_pages() in swiotlb_alloc_tlb() would have to be changed to a
free_decrypted_pages(). However, I'm proposing a different approach to
address the latter issue here:

https://lore.kernel.org/linux-iommu/20231026095123.222-1-petrtesarik@xxxxxxxxxxxxxxx/T/

Petr T