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Donenfeld" To: "H. Peter Anvin" Cc: Borislav Petkov , pbonzini@redhat.com, ebiggers@kernel.org, x86@kernel.org, linux-kernel@vger.kernel.org, qemu-devel@nongnu.org, ardb@kernel.org, kraxel@redhat.com, philmd@linaro.org Subject: Re: [PATCH qemu] x86: don't let decompressed kernel image clobber setup_data Message-ID: References: <46466e54-25c3-3194-8546-a57cd4a80d9d@zytor.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <46466e54-25c3-3194-8546-a57cd4a80d9d@zytor.com> X-Spam-Status: No, score=-6.8 required=5.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, RCVD_IN_DNSWL_HI,SPF_HELO_NONE,SPF_PASS autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on lindbergh.monkeyblade.net Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Dec 30, 2022 at 05:06:55PM -0800, H. Peter Anvin wrote: > > > On 12/30/22 14:10, Jason A. Donenfeld wrote: > > On Fri, Dec 30, 2022 at 01:58:39PM -0800, H. Peter Anvin wrote: > >> See the other thread fork. They have identified the problem already. > > > > Not sure I follow. Is there another thread where somebody worked out why > > this 62meg limit was happening? > > > > Note that I sent v2/v3, to fix the original problem in a different way, > > and if that looks good to the QEMU maintainers, then we can all be happy > > with that. But I *haven't* addressed and still don't fully understand > > why the 62meg limit applied to my v1 in the way it does. Did you find a > > bug there to fix? If so, please do CC me. > > > > Yes, you yourself posted the problem: > > > Then build qemu. Run it with `-kernel bzImage`, based on the kernel > > built with the .config I attached. > > > > You'll see that the CPU triple faults when hitting this line: > > > > sd = (struct setup_data *)boot_params->hdr.setup_data; > > while (sd) { > > unsigned long sd_addr = (unsigned long)sd; > > > > kernel_add_identity_map(sd_addr, sd_addr + sizeof(*sd) + sd->len); <---- > > sd = (struct setup_data *)sd->next; > > } > > > > , because it dereferences *sd. This does not happen if the decompressed > > size of the kernel is < 62 megs. > > > > So that's the "big and pretty serious" bug that might be worthy of > > investigation. > > This needs to be something like: > > kernel_add_identity_map(sd_addr, sd_addr + sizeof(*sd)); > kernel_add_identity_map(sd_addr + sizeof(*sd), > sd_addr + sizeof(*sd) + sd->len); > Oh, right, duh. Thanks for spelling it out. Jason