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Wed, 09 Apr 2025 23:27:50 -0700 (PDT) Message-ID: Date: Thu, 10 Apr 2025 08:27:49 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 2/2] x86/hvm: fix write emulation of RO ranges To: =?UTF-8?Q?Roger_Pau_Monn=C3=A9?= Cc: Marek Marczykowski , Andrew Cooper , xen-devel@lists.xenproject.org References: <20250408093156.83277-3-roger.pau@citrix.com> <16c73cae-2ac0-4811-97d3-b25c95ed5abc@suse.com> <2df78a5d-2f9f-4866-81cc-03ae09c76d50@suse.com> <8709e7af-5827-4c96-9f6b-1f548045040d@suse.com> Content-Language: en-US From: Jan Beulich Autocrypt: addr=jbeulich@suse.com; keydata= xsDiBFk3nEQRBADAEaSw6zC/EJkiwGPXbWtPxl2xCdSoeepS07jW8UgcHNurfHvUzogEq5xk hu507c3BarVjyWCJOylMNR98Yd8VqD9UfmX0Hb8/BrA+Hl6/DB/eqGptrf4BSRwcZQM32aZK 7Pj2XbGWIUrZrd70x1eAP9QE3P79Y2oLrsCgbZJfEwCgvz9JjGmQqQkRiTVzlZVCJYcyGGsD /0tbFCzD2h20ahe8rC1gbb3K3qk+LpBtvjBu1RY9drYk0NymiGbJWZgab6t1jM7sk2vuf0Py O9Hf9XBmK0uE9IgMaiCpc32XV9oASz6UJebwkX+zF2jG5I1BfnO9g7KlotcA/v5ClMjgo6Gl MDY4HxoSRu3i1cqqSDtVlt+AOVBJBACrZcnHAUSuCXBPy0jOlBhxPqRWv6ND4c9PH1xjQ3NP nxJuMBS8rnNg22uyfAgmBKNLpLgAGVRMZGaGoJObGf72s6TeIqKJo/LtggAS9qAUiuKVnygo 3wjfkS9A3DRO+SpU7JqWdsveeIQyeyEJ/8PTowmSQLakF+3fote9ybzd880fSmFuIEJldWxp Y2ggPGpiZXVsaWNoQHN1c2UuY29tPsJgBBMRAgAgBQJZN5xEAhsDBgsJCAcDAgQVAggDBBYC AwECHgECF4AACgkQoDSui/t3IH4J+wCfQ5jHdEjCRHj23O/5ttg9r9OIruwAn3103WUITZee e7Sbg12UgcQ5lv7SzsFNBFk3nEQQCACCuTjCjFOUdi5Nm244F+78kLghRcin/awv+IrTcIWF hUpSs1Y91iQQ7KItirz5uwCPlwejSJDQJLIS+QtJHaXDXeV6NI0Uef1hP20+y8qydDiVkv6l IreXjTb7DvksRgJNvCkWtYnlS3mYvQ9NzS9PhyALWbXnH6sIJd2O9lKS1Mrfq+y0IXCP10eS FFGg+Av3IQeFatkJAyju0PPthyTqxSI4lZYuJVPknzgaeuJv/2NccrPvmeDg6Coe7ZIeQ8Yj t0ARxu2xytAkkLCel1Lz1WLmwLstV30g80nkgZf/wr+/BXJW/oIvRlonUkxv+IbBM3dX2OV8 AmRv1ySWPTP7AAMFB/9PQK/VtlNUJvg8GXj9ootzrteGfVZVVT4XBJkfwBcpC/XcPzldjv+3 HYudvpdNK3lLujXeA5fLOH+Z/G9WBc5pFVSMocI71I8bT8lIAzreg0WvkWg5V2WZsUMlnDL9 mpwIGFhlbM3gfDMs7MPMu8YQRFVdUvtSpaAs8OFfGQ0ia3LGZcjA6Ik2+xcqscEJzNH+qh8V m5jjp28yZgaqTaRbg3M/+MTbMpicpZuqF4rnB0AQD12/3BNWDR6bmh+EkYSMcEIpQmBM51qM EKYTQGybRCjpnKHGOxG0rfFY1085mBDZCH5Kx0cl0HVJuQKC+dV2ZY5AqjcKwAxpE75MLFkr wkkEGBECAAkFAlk3nEQCGwwACgkQoDSui/t3IH7nnwCfcJWUDUFKdCsBH/E5d+0ZnMQi+G0A nAuWpQkjM1ASeQwSHEeAWPgskBQL In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 09.04.2025 17:33, Roger Pau Monné wrote: > On Wed, Apr 09, 2025 at 04:08:47PM +0200, Jan Beulich wrote: >> On 09.04.2025 16:01, Roger Pau Monné wrote: >>> As I understand the checks done in >>> mmio_ro_emulated_write() are to ensure correctness, but carrying the >>> access even when the %cr2 check fail wouldn't cause issues to Xen >>> itself? >> >> Well, "wouldn't" is too strong for my taste, "shouldn't" would fit. The >> checking is there to avoid guests playing games. Whether that prevents >> merely in-guest just-bugs or actual XSAs we can't know until we find a >> case where the game playing might make us do something wrong. I expect >> it's unlikely for Xen itself to be affected. But an in-guest privilege >> escalation would already be bad enough. > > I see. That was kind of my understanding of the checks. At least on > HVM it feels a bit weird to handle r/o regions this way. It would IMO > be more natural to use an hvm_io_handler, but that's maybe because I'm > more familiar with those. I guess this would be an option; I assume it's the way it is because PVHv1 inherited it from PV, and PVHv2 inherited it from PVHv1. > And in that regard, hvm_io_handler don't seem to do any of the extra > checking that mmio_ro_emulated_write() does with the %cr2, but maybe > that's done by some higher layer? AFAICT that would ultimately get > called by hvmemul_read(), and there are no checks there at all. That more general framework isn't page-fault specific, and hence there's no CR2 recorded to check against. Jan