x86/mm/ident_map: Use gbpages only where full GB page should be mapped.

In the Linux kernel, the following vulnerability has been resolved: x86/mm/ident_map: Use gbpages only where full GB page should be mapped. When ident_pud_init() uses only GB pages to create identity maps, large ranges of addresses not actually requested can be included in the resulting table; a 4K request will map a full GB. This can include a lot of extra address space past that requested, including areas marked reserved by the BIOS. That allows processor speculation into reserved regions, that on UV systems can cause system halts. Only use GB pages when map creation requests include the full GB page of space. Fall back to using smaller 2M pages when only portions of a GB page are included in the request. No attempt is made to coalesce mapping requests. If a request requires a map entry at the 2M (pmd) level, subsequent mapping requests within the same 1G region will also be at the pmd level, even if adjacent or overlapping such requests could have been combined to map a full GB page. Existing usage starts with larger regions and then adds smaller regions, so this should not have any great consequence.

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Linux kernel 安全漏洞

Linux kernel是美国Linux基金会的开源操作系统Linux所使用的内核。 Linux kernel存在安全漏洞，该漏洞源于在创建身份映射时错误地使用GB页面导致的处理器推测进入保留区域。

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