Re: [RFC PATCH v1 2/3] x86/virt/seamldr: Support large TDX module binaries

[Edgecombe, Rick P]

On Fri, 2026-10-30 at 01:52 -0700, Chao Gao wrote:
> The module binary is handed to P-SEAMLDR as a list of page physical
> addresses in 'struct seamldr_params'. The list has 496 entries, which
> caps a module at 496 pages.
> 
> TDX modules are gaining features and are about to outgrow that limit.
> 
> The P-SEAMLDR ABI was extended for this. An upcoming revision of the
> SEAMLDR specification adds the following to the description of the
> module page address list in 'struct seamldr_params'.
> 
>   For TDX module image with larger than 495 pages, the last entry will
>   point to a new 4 KB page that contains up to 511 additional HPAs. Each
>   page will use its last HPA to reference another page if required. This
>   chain continues until all module pages are referenced.
> 
> Allocate the needed chain pages together with 'struct seamldr_params',
> as a flexible array at its end. Link them up at allocation time. Then
> fill in the module page addresses as before, but skip the link entries.
> A single allocation leaves nothing to unwind on failure and keeps the
> existing kfree() as the only cleanup.
> 
> Do not query P-SEAMLDR for chaining support. Userspace selects the update
> image and is responsible for ensuring the module is compatible with
> P-SEAMLDR.
> 
> Chaining removes the 496 page cap, but the module page count still needs
> a bound. Without one, a crafted header can overflow the u32 module length
> ('module_nr_pages * PAGE_SIZE') and pass the image size check. It can
> also make the chain page allocation exceed KMALLOC_MAX_SIZE, which warns
> in the page allocator.
> 
> Raise SEAMLDR_MAX_NR_MODULE_PAGES to a 1GB software limit to rule out
> both problems. 1GB is far above the few MB of current modules.
> 
> The size of module_pages_pa_list[] no longer reflects the maximum module
> page count. Define a new SEAMLDR_NR_MODULE_PA_ENTRIES for it.
> 
> Thanks to Yilun for the idea of allocating the chain pages together
> with 'struct seamldr_params'.
> 
> AI was used under supervision to review code and workshop logs. It
> spotted both problems that the software limit addresses.
> 
> Signed-off-by: Chao Gao <chao.gao@xxxxxxxxx>
> ---
>  arch/x86/virt/vmx/tdx/seamldr.c | 95 +++++++++++++++++++++++++++++++--
>  1 file changed, 90 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/virt/vmx/tdx/seamldr.c b/arch/x86/virt/vmx/tdx/seamldr.c
> index 3513fd28536a..09c8f6e30e9f 100644
> --- a/arch/x86/virt/vmx/tdx/seamldr.c
> +++ b/arch/x86/virt/vmx/tdx/seamldr.c
> @@ -24,7 +24,14 @@
>  #define P_SEAMLDR_INFO			0x8000000000000000
>  #define P_SEAMLDR_INSTALL		0x8000000000000001
>  
> -#define SEAMLDR_MAX_NR_MODULE_PAGES	496
> +/*
> + * Arbitrary software limit, far above the few MB of current modules.
> + * It keeps the module length within a u32, and 'struct seamldr_params'
> + * plus its chain pages within KMALLOC_MAX_SIZE.
> + */
> +#define SEAMLDR_MAX_NR_MODULE_PAGES	(SZ_1G / PAGE_SIZE)
> +
> +#define SEAMLDR_NR_MODULE_PA_ENTRIES	496
>  #define SEAMLDR_MAX_NR_SIG_PAGES	1
>  
>  /*
> @@ -34,6 +41,19 @@
>   */
>  #define SEAMLDR_SCENARIO_UPDATE		1
>  
> +#define SEAMLDR_NR_CHAIN_PA_ENTRIES	(PAGE_SIZE / sizeof(u64) - 1)
> +/*
> + * A 4KB page holding the physical addresses of the module pages that do
> + * not fit in module_pages_pa_list[]. @next_page_pa points to the next
> + * page, forming a singly linked list.
> + */
> +struct chain_page {
> +	u64	pa[SEAMLDR_NR_CHAIN_PA_ENTRIES];
> +	u64	next_page_pa;
> +} __packed;
> +
> +static_assert(sizeof(struct chain_page) == 4096);
> +
>  /*
>   * This is the "SEAMLDR_PARAMS" data structure defined in the
>   * "SEAM Loader (SEAMLDR) Interface Specification".
> @@ -48,10 +68,13 @@ struct seamldr_params {
>  	u64	sigstruct_pages_pa_list[SEAMLDR_MAX_NR_SIG_PAGES];
>  	u8	reserved[104];
>  	u64	module_nr_pages;
> -	u64	module_pages_pa_list[SEAMLDR_MAX_NR_MODULE_PAGES];
> +	u64	module_pages_pa_list[SEAMLDR_NR_MODULE_PA_ENTRIES];
> +
> +	/* Hold module pages that cannot fit in module_pages_pa_list[]. */
> +	struct chain_page	chain_pages[];
>  } __packed;
>  
> -static_assert(sizeof(struct seamldr_params) == 4096);
> +static_assert(offsetof(struct seamldr_params, chain_pages) == 4096);
>  
>  /*
>   * Serialize P-SEAMLDR calls since the hardware only allows a single CPU to
> @@ -143,6 +166,68 @@ static void populate_pa_list(u64 *pa_list, const u8
> *vmalloc_addr, u32 vmalloc_l
>  	}
>  }
>  
> +/*
> + * Like populate_pa_list(), but skip entries pointing to the next page.
> + */
> +static void populate_module_pa_list(struct seamldr_params *params,
> +				    const u8 *vmalloc_addr, u32
> vmalloc_len_pages)
> +{
> +	u64 *pa_list = params->module_pages_pa_list;
> +	int i, j;
> +
> +	for (i = 0, j = 0; i < vmalloc_len_pages; i++, j++) {
> +		unsigned long offset = i * PAGE_SIZE;
> +		unsigned long pfn = vmalloc_to_pfn(&vmalloc_addr[offset]);
> +
> +		/*
> +		 * Skip non-zero entries. They point to the next page and
> +		 * are populated during allocation.
> +		 */
> +		if (pa_list[j])
> +			j++;

We know what position in the page this should be right? Can we do some modulus
and skip it that way? Feels less hacky.

> +
> +		pa_list[j] = pfn << PAGE_SHIFT;
> +	}
> +}
> +
> +static struct seamldr_params *alloc_seamldr_params(u32 nr_module_pages)
> +{
> +	struct seamldr_params *params;
> +	u32 nr_chain_pages;
> +	u32 nr_direct;
> +	u64 *link;
> +
> +	/*
> +	 * Number of module pages that fit directly in

"fit directly" feels like adding jargon where there doesn't need to be.

> +	 * module_pages_pa_list[]. Minus one as the last entry is
> +	 * reserved to point to the first chain page.
> +	 */
> +	nr_direct = SEAMLDR_NR_MODULE_PA_ENTRIES - 1;
> +
> +	/* Calculate the number of extra chain pages needed. */
> +	nr_chain_pages = 0;
> +	if (nr_module_pages > nr_direct) {
> +		u32 nr_left = nr_module_pages - nr_direct;
> +
> +		nr_chain_pages = DIV_ROUND_UP(nr_left,
> SEAMLDR_NR_CHAIN_PA_ENTRIES);

Left pages, module pages, direct pages and chain pages. It's a lot...

> +	}
> +
> +	params = kzalloc_flex(*params, chain_pages, nr_chain_pages);
> +	if (!params)
> +		return NULL;
> +
> +	/* Link all the chain pages. */
> +	link = &params->module_pages_pa_list[nr_direct];
> +	for (int i = 0; i < nr_chain_pages; i++) {
> +		struct chain_page *page = &params->chain_pages[i];
> +
> +		*link = __pa(page);
> +		link = &page->next_page_pa;
> +	}
> +
> +	return params;
> +}

This all seems like more than should be needed.

> +
>  static void populate_seamldr_params(struct seamldr_params *params,
>  				    const u8 *sig, u32 sig_nr_pages,
>  				    const u8 *mod, u32 mod_nr_pages)
> @@ -152,7 +237,7 @@ static void populate_seamldr_params(struct seamldr_params
> *params,
>  	params->module_nr_pages	= mod_nr_pages;
>  
>  	populate_pa_list(params->sigstruct_pages_pa_list, sig, sig_nr_pages);
> -	populate_pa_list(params->module_pages_pa_list,	  mod, mod_nr_pages);
> +	populate_module_pa_list(params, mod, mod_nr_pages);
>  }
>  
>  /*
> @@ -195,7 +280,7 @@ static struct seamldr_params *build_seamldr_params(const
> struct tdx_image *image
>  	if (memchr_inv(header->reserved, 0, sizeof(header->reserved)))
>  		return ERR_PTR(-EINVAL);
>  
> -	params = kzalloc_obj(*params);
> +	params = alloc_seamldr_params(header->module_nr_pages);
>  	if (!params)
>  		return ERR_PTR(-ENOMEM);
>