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Peter Anvin" , linux-kernel@vger.kernel.org, linux-coco@lists.linux.dev, kvm@vger.kernel.org, Binbin Wu Subject: Re: [PATCH v3 4/4] virt: tdx-guest: Allocate Quote buffer dynamically Message-ID: References: <20260729122939.1340412-1-peter.fang@intel.com> <20260729122939.1340412-5-peter.fang@intel.com> Precedence: bulk X-Mailing-List: linux-coco@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Fri, Sep 04, 2026 at 01:59:35PM -0700, Peter Fang wrote: > On Thu, Sep 03, 2026 at 09:33:17AM +0300, Tony Lindgren wrote: > > On Thu, Sep 03, 2026 at 02:21:35PM +0800, Xiaoyao Li wrote: > > > On 7/29/2026 8:29 PM, Peter Fang wrote: > > > > A new TDX module ABI reports the TD Quote size limit in a metadata > > > > field. This size used to be fixed at 128 KB. > > > > > > > > The guest driver's Quote buffer is shared with the host VMM. The current > > > > fixed size 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 reported limit. This avoids > > > > wasting memory on platforms that do not require larger Quotes. Older > > > > platforms fall back to the default 128 KB buffer. > > > > > > I think we need to justify why using the value reported by TDX module > > > (i.e., QUOTE_MAX_SIZE) is safe. > > > > > > From "Intel TDX Module Extension for Quoting" spec, > > > > > > QUOTE_MAX_SIZE that specifies the maximum size of the buffer that must be > > > allocated to contain the Intel TDX Quote received from Intel TDX Module. > > > > > > So, TDX module only guarantees that QUOTE_MAX_SIZE is big enough to hold > > > the quote that is generated by the TDX module. But for the current tdx > > > guest driver, it doesn't know how host will generate the quote. Host can > > > generate the SGX-based quote and we need to justify the QUOTE_MAX_SIZE will > > > be enough for such case. > > Thanks for pointing this out. I confirmed with the TDX security folks. > The reported QUOTE_MAX_SIZE covers all possible quote types on the > platform. If GetQuote gets routed to SGX and QUOTE_MAX_SIZE is too small > for that, it's a TDX module bug. > > I'll add a comment and update the changelog. > > > > > And can we assume the TDX module provided size does not change after init > > with a TDX module update? > > Yep, it's safe to assume that. Thanks for checking the above: Reviewed-by: Tony Lindgren