Re: [PATCH v8 02/11] x86/virt/tdx: Allocate page bitmap for Dynamic PAMT

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] The 16-bit TDX metadata field for 'pamt_page_bitmap_entry_bits' is silently truncated to 8 bits, potentially causing severe memory under-allocation on future TDX modules.
--

--- Patch [2]: [PATCH v8 02/11] x86/virt/tdx: Allocate page bitmap for Dynamic PAMT ---
commit 88c7f3b3a7c606e18ce66049ee20b819855d481e
Author: Kiryl Shutsemau <kas@kernel.org>
x86/virt/tdx: Allocate page bitmap for Dynamic PAMT

This commit begins implementing Dynamic PAMT by reading the bits-per-page
metadata required for Dynamic PAMT and calculating the needed bitmap size.
It then allocates a page bitmap instead of the normal 4KB PAMT if Dynamic
PAMT is supported by the TDX module.

> diff --git a/arch/x86/include/asm/tdx_global_metadata.h b/arch/x86/include/asm/tdx_global_metadata.h
> index 41150d546589c..2a42551fc33cd 100644
> --- a/arch/x86/include/asm/tdx_global_metadata.h
> +++ b/arch/x86/include/asm/tdx_global_metadata.h

[ ... ]

> @@ -21,6 +21,9 @@ struct tdx_sys_info_tdmr {
>  	u16 pamt_4k_entry_size;
>  	u16 pamt_2m_entry_size;
>  	u16 pamt_1g_entry_size;
> +
> +	/* Optional metadata, if Dynamic PAMT is supported */
> +	u8  pamt_page_bitmap_entry_bits;
>  };

[ ... ]

> diff --git a/arch/x86/virt/vmx/tdx/tdx_global_metadata.c b/arch/x86/virt/vmx/tdx/tdx_global_metadata.c
> index e49c300f23d43..8393d2aa59dbe 100644
> --- a/arch/x86/virt/vmx/tdx/tdx_global_metadata.c
> +++ b/arch/x86/virt/vmx/tdx/tdx_global_metadata.c

[ ... ]

> @@ -33,6 +33,18 @@ static __init int get_tdx_sys_info_features(struct tdx_sys_info_features *sysinf
>  	return ret;
>  }
>  
> +static __init int get_tdx_sys_info_tdmr_dpamt(struct tdx_sys_info_tdmr *sysinfo_tdmr)
> +{
> +	int ret;
> +	u64 val;
> +
> +	ret = read_sys_metadata_field(0x9100000100000013, &val);
> +	if (!ret)
> +		sysinfo_tdmr->pamt_page_bitmap_entry_bits = val;

[Severity: Medium]
Does assigning val to pamt_page_bitmap_entry_bits result in an unsafe
truncation?

Since pamt_page_bitmap_entry_bits is declared as a u8 in
struct tdx_sys_info_tdmr, and the metadata field ID 0x9100000100000013
specifies a 16-bit value according to the TDX ABI (bits 33:32 are '01'),
a value from the hardware might be silently truncated.

If a future TDX module increases the required bitmap entry size to 256 bits
or more, wouldn't this truncation cause severe memory under-allocation for
the physically contiguous PAMT memory?

> +
> +	return ret;
> +}

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260725002302.3337017-1-rick.p.edgecombe@intel.com?part=2