[PATCH v2 0/2] tdx-guest: Make Quote buffer size dynamic

[PATCH v2 0/2] tdx-guest: Make Quote buffer size dynamic

[Peter Fang]

Hi,

This series changes the TDX attestation driver's Quote buffer size from
a fixed constant to a value queried from the TDX module. So effectively:

  s/FIXED_BUF_SIZE/queried_buf_size/g

...in the TDX guest driver.

Terminology
===========

A "TD Quote" is an attestation structure signed with a platform key. It
contains information about a TDX guest and the platform it's running on.

The "Quote buffer" in the TDX guest driver is a memory buffer shared
between the TDX guest and the host VMM to retrieve TD Quotes. It has a
header defined in the GHCI spec [1].

Device Identifier Composition Engine ("DICE") provides a framework for
layering attestation evidence. This replaces the SGX model of contacting
an Intel server to obtain a certificate.

Problem
=======

The fixed-size Quote buffer approach is not sustainable. As
cryptographic algorithms evolve, TD Quote sizes also grow. A previous
commit [2] increased the guest driver's fixed-size Quote buffer to 128
KB to accommodate DICE Quotes, but it may still be insufficient when
those Quotes use post-quantum cryptography (PQC). PQC certificate chains
are roughly 10x-15x larger than conventional ones, which can increase
Quote sizes significantly.

What's in this series
=====================

To avoid changing the driver whenever the Quote buffer becomes too
small, newer TDX modules report their maximum Quote size via a metadata
field. The guest driver uses this value for its Quote buffer when
available. Older TDX modules continue to use the 128 KB buffer.

The changes do not affect configfs-tsm-report ABIs.

Patch 1/2: Add a helper to read the QUOTE_MAX_SIZE metadata field.
Patch 2/2: Replace the fixed Quote buffer size with the queried value,
           when available.

AI use
======

I used Claude:claude-opus-4-8 to proofread this cover letter and the
changelogs. The series also underwent AI code review (Claude:claude-opus-4-7),
but the feedback was limited to style suggestions.

v1: https://lore.kernel.org/all/20260612110853.3188196-1-peter.fang@intel.com/

Changes in v2:
 - Collect Reviewed-by tags. [Kiryl, Xiaoyao, Binbin, Sathya]
 - Keep the explicit (u32) cast in tdx_get_max_quote_size(). [Binbin]
 - Calculate the Quote buffer size with struct_size_t(). [Kiryl, Binbin]
 - Add __GFP_NOWARN to the allocation since its size comes from the
   host. [sashiko]
 - Rename quote_data_size to quote_data_len. [Sathya]
 - Drop the Assisted-by tags, as AI was not used to write the code.

[1] Guest Hypervisor Communication Interface (GHCI) Specification,
    Version 1.5, Section "TDG.VP.VMCALL<GetQuote>"
[2] 43185067c6fd ("configfs-tsm-report: tdx_guest: Increase Quote buffer
    size to 128KB")

Kuppuswamy Sathyanarayanan (1):
  virt: tdx-guest: Allocate Quote buffer dynamically

Peter Fang (1):
  x86/tdx: Add helper to query maximum TD Quote size

 arch/x86/coco/tdx/tdx.c                 | 19 +++++++++
 arch/x86/include/asm/shared/tdx.h       |  1 +
 arch/x86/include/asm/tdx.h              |  2 +
 drivers/virt/coco/tdx-guest/tdx-guest.c | 55 ++++++++++++++++++-------
 4 files changed, 62 insertions(+), 15 deletions(-)


base-commit: a13c140cc289c0b7b3770bce5b3ad42ab35074aa
-- 
2.53.0

[PATCH v2 1/2] x86/tdx: Add helper to query maximum TD Quote size

[Peter Fang]

TDX attestation report ("Quote") sizes can grow with newer cryptographic
schemes, so guests can no longer rely on a fixed-size buffer for the
Quote.

Newer TDX modules report the maximum Quote size via a TD-scope metadata
field. Add a helper to query the size instead of exposing tdg_vm_rd()
directly, as it can read arbitrary metadata fields.

Thanks to Xu Yilun for suggesting this in an off-list discussion.

Reviewed-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Signed-off-by: Peter Fang <peter.fang@intel.com>
---
v2:
 - Keep the explicit (u32) cast to document the metadata field width. [Binbin]
 - Note that Xu Yilun's suggestion was made in an off-list discussion. [Sathya]
 - Drop the Assisted-by tags, as the code was not written by AI.
---
 arch/x86/coco/tdx/tdx.c           | 19 +++++++++++++++++++
 arch/x86/include/asm/shared/tdx.h |  1 +
 arch/x86/include/asm/tdx.h        |  2 ++
 3 files changed, 22 insertions(+)

diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
index 29b6f1ed59ec..fa2ed012e736 100644
--- a/arch/x86/coco/tdx/tdx.c
+++ b/arch/x86/coco/tdx/tdx.c
@@ -198,6 +198,25 @@ u64 tdx_hcall_get_quote(u8 *buf, size_t size)
 }
 EXPORT_SYMBOL_GPL(tdx_hcall_get_quote);
 
+/**
+ * tdx_get_max_quote_size() - Get the maximum TD Quote size
+ *
+ * Read the maximum size of a TD Quote from a 4-byte TD metadata field. The TDX
+ * guest driver uses it to size the buffer for Quote retrieval. Older TDX
+ * modules do not support this field and return an error.
+ *
+ * Return: Maximum Quote size in bytes on success, or 0 on failure.
+ */
+u32 tdx_get_max_quote_size(void)
+{
+	u64 val, ret;
+
+	ret = tdg_vm_rd(TDCS_QUOTE_MAX_SIZE, &val);
+
+	return ret ? 0 : (u32)val;
+}
+EXPORT_SYMBOL_GPL(tdx_get_max_quote_size);
+
 static void __noreturn tdx_panic(const char *msg)
 {
 	struct tdx_module_args args = {
diff --git a/arch/x86/include/asm/shared/tdx.h b/arch/x86/include/asm/shared/tdx.h
index f20e91d7ac35..a58751321613 100644
--- a/arch/x86/include/asm/shared/tdx.h
+++ b/arch/x86/include/asm/shared/tdx.h
@@ -50,6 +50,7 @@
 /* TDX TD-Scope Metadata. To be used by TDG.VM.WR and TDG.VM.RD */
 #define TDCS_CONFIG_FLAGS		0x1110000300000016
 #define TDCS_TD_CTLS			0x1110000300000017
+#define TDCS_QUOTE_MAX_SIZE		0x9010000200000008
 #define TDCS_NOTIFY_ENABLES		0x9100000000000010
 #define TDCS_TOPOLOGY_ENUM_CONFIGURED	0x9100000000000019
 
diff --git a/arch/x86/include/asm/tdx.h b/arch/x86/include/asm/tdx.h
index 89e97d5761d8..a75dbbe004bd 100644
--- a/arch/x86/include/asm/tdx.h
+++ b/arch/x86/include/asm/tdx.h
@@ -83,6 +83,8 @@ int tdx_mcall_extend_rtmr(u8 index, u8 *data);
 
 u64 tdx_hcall_get_quote(u8 *buf, size_t size);
 
+u32 tdx_get_max_quote_size(void);
+
 void __init tdx_dump_attributes(u64 td_attr);
 void __init tdx_dump_td_ctls(u64 td_ctls);
 
-- 
2.53.0

[PATCH v2 2/2] virt: tdx-guest: Allocate Quote buffer dynamically

[Peter Fang]

From: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>

The TDX attestation driver currently uses a fixed 128 KB Quote buffer
shared with the host VMM. This may be too small for Quotes using schemes
such as post-quantum cryptography (PQC), where larger certificate chains
can increase the Quote size significantly.

Allocate the Quote buffer based on the size reported by the TDX module
instead of always reserving a fixed-size buffer. This avoids wasting
memory on platforms that do not require larger Quotes. Older platforms
fall back to the default 128 KB buffer.

Because the Quote buffer must be physically contiguous, its size is
bound by the buddy allocator's maximum page order (4 MB), which should
be sufficient for current attestation needs.

struct tdx_quote_buf has a trailing flexible array, so use struct_size_t()
to calculate the buffer size.

Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Reviewed-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Signed-off-by: Peter Fang <peter.fang@intel.com>
---
v2:
 - Use struct_size_t() instead of offsetof() to compute the buffer size. [Kiryl, Binbin]
 - Pass __GFP_NOWARN to alloc_pages_exact() so an oversized size fails quietly. [sashiko]
 - Reword the description to avoid implying Quotes can exceed the 4 MB limit. [Binbin]
 - Rename quote_data_size to quote_data_len. [Sathya]
 - Drop the Assisted-by tags, as the code was not written by AI.
---
 drivers/virt/coco/tdx-guest/tdx-guest.c | 55 ++++++++++++++++++-------
 1 file changed, 40 insertions(+), 15 deletions(-)

diff --git a/drivers/virt/coco/tdx-guest/tdx-guest.c b/drivers/virt/coco/tdx-guest/tdx-guest.c
index d0303e31e816..ce2c2ef74676 100644
--- a/drivers/virt/coco/tdx-guest/tdx-guest.c
+++ b/drivers/virt/coco/tdx-guest/tdx-guest.c
@@ -162,7 +162,7 @@ static void tdx_mr_deinit(const struct attribute_group *mr_grp)
  * DICE-based attestation uses layered evidence that requires
  * larger Quote size (~100K).
  */
-#define GET_QUOTE_BUF_SIZE		SZ_128K
+#define GET_QUOTE_DEFAULT_BUF_SIZE	SZ_128K
 
 #define GET_QUOTE_CMD_VER		1
 
@@ -170,7 +170,7 @@ static void tdx_mr_deinit(const struct attribute_group *mr_grp)
 #define GET_QUOTE_SUCCESS		0
 #define GET_QUOTE_IN_FLIGHT		0xffffffffffffffff
 
-#define TDX_QUOTE_MAX_LEN		(GET_QUOTE_BUF_SIZE - sizeof(struct tdx_quote_buf))
+#define TDX_QUOTE_BUF_LEN(n)		struct_size_t(struct tdx_quote_buf, data, n)
 
 /* struct tdx_quote_buf: Format of Quote request buffer.
  * @version: Quote format version, filled by TD.
@@ -191,8 +191,9 @@ struct tdx_quote_buf {
 	u8 data[];
 };
 
-/* Quote data buffer */
+/* Quote data buffer and length */
 static void *quote_data;
+static size_t quote_data_len;
 
 /* Lock to streamline quote requests */
 static DEFINE_MUTEX(quote_lock);
@@ -209,9 +210,8 @@ static long tdx_get_report0(struct tdx_report_req __user *req)
 			     USER_SOCKPTR(req->tdreport));
 }
 
-static void free_quote_buf(void *buf)
+static void free_quote_buf(void *buf, size_t len)
 {
-	size_t len = PAGE_ALIGN(GET_QUOTE_BUF_SIZE);
 	unsigned int count = len >> PAGE_SHIFT;
 
 	if (set_memory_encrypted((unsigned long)buf, count)) {
@@ -222,19 +222,44 @@ static void free_quote_buf(void *buf)
 	free_pages_exact(buf, len);
 }
 
-static void *alloc_quote_buf(void)
+static size_t get_quote_buf_size(void)
 {
-	size_t len = PAGE_ALIGN(GET_QUOTE_BUF_SIZE);
-	unsigned int count = len >> PAGE_SHIFT;
+	size_t buf_size = GET_QUOTE_DEFAULT_BUF_SIZE;
+	u32 quote_size;
+
+	quote_size = tdx_get_max_quote_size();
+
+	if (quote_size)
+		/* Reported size does not include GetQuote header */
+		buf_size = TDX_QUOTE_BUF_LEN(quote_size);
+
+	return PAGE_ALIGN(buf_size);
+}
+
+static void *alloc_quote_buf(size_t *buflen)
+{
+	unsigned int count;
+	size_t len;
 	void *addr;
 
-	addr = alloc_pages_exact(len, GFP_KERNEL | __GFP_ZERO);
+	len = get_quote_buf_size();
+
+	/*
+	 * This fails if the requested size exceeds the buddy allocator's
+	 * maximum order. Use __GFP_NOWARN since the size comes from the host
+	 * and should fail quietly rather than warn.
+	 */
+	addr = alloc_pages_exact(len, GFP_KERNEL | __GFP_ZERO | __GFP_NOWARN);
 	if (!addr)
 		return NULL;
 
+	count = len >> PAGE_SHIFT;
+
 	if (set_memory_decrypted((unsigned long)addr, count))
 		return NULL;
 
+	*buflen = len;
+
 	return addr;
 }
 
@@ -285,7 +310,7 @@ static int tdx_report_new_locked(struct tsm_report *report, void *data)
 	if (desc->inblob_len != TDX_REPORTDATA_LEN)
 		return -EINVAL;
 
-	memset(quote_data, 0, GET_QUOTE_BUF_SIZE);
+	memset(quote_data, 0, quote_data_len);
 
 	/* Update Quote buffer header */
 	quote_buf->version = GET_QUOTE_CMD_VER;
@@ -296,7 +321,7 @@ static int tdx_report_new_locked(struct tsm_report *report, void *data)
 	if (ret)
 		return ret;
 
-	err = tdx_hcall_get_quote(quote_data, GET_QUOTE_BUF_SIZE);
+	err = tdx_hcall_get_quote(quote_data, quote_data_len);
 	if (err) {
 		pr_err("GetQuote hypercall failed, status:%llx\n", err);
 		return -EIO;
@@ -315,7 +340,7 @@ static int tdx_report_new_locked(struct tsm_report *report, void *data)
 
 	out_len = READ_ONCE(quote_buf->out_len);
 
-	if (out_len > TDX_QUOTE_MAX_LEN)
+	if (TDX_QUOTE_BUF_LEN(out_len) > quote_data_len)
 		return -EFBIG;
 
 	buf = kvmemdup(quote_buf->data, out_len, GFP_KERNEL);
@@ -417,7 +442,7 @@ static int __init tdx_guest_init(void)
 	if (ret)
 		goto deinit_mr;
 
-	quote_data = alloc_quote_buf();
+	quote_data = alloc_quote_buf(&quote_data_len);
 	if (!quote_data) {
 		pr_err("Failed to allocate Quote buffer\n");
 		ret = -ENOMEM;
@@ -431,7 +456,7 @@ static int __init tdx_guest_init(void)
 	return 0;
 
 free_quote:
-	free_quote_buf(quote_data);
+	free_quote_buf(quote_data, quote_data_len);
 free_misc:
 	misc_deregister(&tdx_misc_dev);
 deinit_mr:
@@ -444,7 +469,7 @@ module_init(tdx_guest_init);
 static void __exit tdx_guest_exit(void)
 {
 	tsm_report_unregister(&tdx_tsm_ops);
-	free_quote_buf(quote_data);
+	free_quote_buf(quote_data, quote_data_len);
 	misc_deregister(&tdx_misc_dev);
 	tdx_mr_deinit(tdx_attr_groups[0]);
 }
-- 
2.53.0

Re: [PATCH v2 2/2] virt: tdx-guest: Allocate Quote buffer dynamically

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [High] Premature exposure of the misc device allows a Use-After-Free (UAF) on the module initialization error path if `alloc_quote_buf()` fails.
--

--- Patch [2]: [PATCH v2 2/2] virt: tdx-guest: Allocate Quote buffer dynamically ---
commit e37e54ca6bcf9f24fca7285847baa3251ff5dfba
Author: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>

virt: tdx-guest: Allocate Quote buffer dynamically

This modifies the TDX attestation driver to dynamically allocate the Quote
buffer based on the size reported by the TDX module. It uses alloc_pages_exact()
to allocate up to a 4MB contiguous buffer and fails quietly if the allocation
cannot be satisfied.

> diff --git a/drivers/virt/coco/tdx-guest/tdx-guest.c b/drivers/virt/coco/tdx-guest/tdx-guest.c
> index d0303e31e8161..ce2c2ef746762 100644
> --- a/drivers/virt/coco/tdx-guest/tdx-guest.c
> +++ b/drivers/virt/coco/tdx-guest/tdx-guest.c

[ ... ]

> @@ -417,7 +442,7 @@ static int __init tdx_guest_init(void)
>  	if (ret)
>  		goto deinit_mr;
>  
> -	quote_data = alloc_quote_buf();
> +	quote_data = alloc_quote_buf(&quote_data_len);

[Severity: High]
This is a pre-existing issue, but this patch makes it significantly more
likely to trigger by introducing a large contiguous memory allocation (up to
4MB) that can easily fail on fragmented systems.

Can this lead to a Use-After-Free on the initialization error path?

If misc_register(&tdx_misc_dev) (called just before this hunk) succeeds, 
the /dev/tdx_guest device is immediately exposed to userspace. A userspace 
process can open it and call ioctl(TDX_CMD_GET_REPORT0), entering
tdx_do_report() which uses tdx_report_buf.

If alloc_quote_buf() then fails, the error path calls misc_deregister() and 
tdx_mr_deinit(). 
    
tdx_mr_deinit() executes:

    kfree(tdx_report_buf);
    
This freeing happens without acquiring mr_lock, meaning it can race with the 
ongoing ioctl. Could the concurrent ioctl in tdx_do_report() then access the 
freed tdx_report_buf?

Should misc_register() be moved to the end of tdx_guest_init() after all
resource allocations succeed?

>  	if (!quote_data) {
>  		pr_err("Failed to allocate Quote buffer\n");
>  		ret = -ENOMEM;

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260717214349.4075994-1-peter.fang@intel.com?part=2