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Thu, 07 Nov 2024 01:25:04 -0800 (PST) Message-ID: Date: Thu, 7 Nov 2024 10:25:02 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [XEN PATCH] xen/vpci: Fix UB in mask_write To: Mykyta Poturai Cc: "xen-devel@lists.xenproject.org" , =?UTF-8?Q?Roger_Pau_Monn=C3=A9?= References: <559dfac91b8f097bc59c4de194fd2ae2b5b4144c.1730880005.git.mykyta_poturai@epam.com> <6d3f322f-7047-4033-95b5-86751a58cc70@suse.com> <65d2043f-5e6f-4d84-8241-eb28dd94fc45@epam.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: <65d2043f-5e6f-4d84-8241-eb28dd94fc45@epam.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 06.11.2024 13:26, Mykyta Poturai wrote: > On 06.11.2024 10:07, Roger Pau Monné wrote:> >> Wait - how can msi->vectors ever be 0? AFAICT there's no way in the >> MSI logic to configure 0 vectors, there will always be at least 1 vector >> enabled. >> >> Maybe what you want, if this fix is for compliance reasons, is an >> assert unreachable that msi->vectors > 0? > > I did some investigation and figured out that the value of 0 is being > set by guest writing to msi_control_reg. As far as I understand, the > control_write() function only checks that vectors are not greater than > the maximum allowed value, but does not check for 0. How that? How could it even check for 0, when 0 isn't possible? Quoting the code there: unsigned int vectors = min_t(uint8_t, 1u << MASK_EXTR(val, PCI_MSI_FLAGS_QSIZE), pdev->msi_maxvec); "val" in the guest written value. As that's used as a shift count, how could 0 result there? The only way I can see 0 ending up in vectors is when pdev->msi_maxvec was still zero. Yet that's then a bug in device initialization. Jan