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There are issues with restartable PV type change operations. To avoid using shadow pagetables for PV guests, Xen exposes the actual hardware pagetables to the guest. In order to prevent the guest from modifying these page tables directly, Xen keeps track of how pages are used using a type system; pages must be \"promoted\" before being used as a pagetable, and \"demoted\" before being used for any other type. Xen also allows for \"recursive\" promotions: i.e., an operating system promoting a page to an L4 pagetable may end up causing pages to be promoted to L3s, which may in turn cause pages to be promoted to L2s, and so on. These operations may take an arbitrarily large amount of time, and so must be re-startable. 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Para evitar el uso de tablas de p\u00e1ginas shadow para invitados PV, Xen expone las tablas de p\u00e1ginas de hardware actuales al invitado. Para impedir que el invitado modifique directamente estas tablas de p\u00e1ginas, Xen realiza un seguimiento de c\u00f3mo son usadas las p\u00e1ginas utilizando un sistema de tipos; las p\u00e1ginas deben ser \"promoted\" antes de ser usadas como una tabla de p\u00e1ginas y \"demoted\" antes de ser usadas por cualquier otro tipo. Xen tambi\u00e9n permite promociones \"recursive\": es decir, un sistema operativo que promociona una p\u00e1gina en una tabla de p\u00e1ginas L4 puede terminar haciendo que las p\u00e1ginas se promocionen a L3, lo que a su vez puede causar que las p\u00e1ginas se promocionen a L2, y as\u00ed sucesivamente. Estas operaciones pueden tomar una cantidad de tiempo arbitrariamente grande y, por lo tanto, deben ser reiniciadas. Desafortunadamente, hacer reiniciables las operaciones de promoci\u00f3n y degradaci\u00f3n de tablas de p\u00e1ginas recursivas es incre\u00edblemente complejo, y el c\u00f3digo contiene varias carreras que, si son activadas, pueden hacer que Xen elimine o retenga conteos de tipos adicionales, lo que potencialmente permite a los invitados conseguir acceso de escritura a las tablas de p\u00e1ginas en uso. Un administrador invitado malicioso de PV puede escalar sus privilegios a los del host. Todos los sistemas x86 con invitados PV no confiables son vulnerables. 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{"bugzilla": {"description": "xen: a malicious PV guest administrator may be able to escalate their privilege to that of the host", "id": "1765043", "url": "https://bugzilla.redhat.com/show_bug.cgi?id=1765043"}, "csaw": false, "cvss3": {"cvss3_base_score": "8.0", "cvss3_scoring_vector": "CVSS:3.0/AV:A/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:H", "status": "draft"}, "cwe": "CWE-268", "details": ["An issue was discovered in Xen through 4.12.x allowing x86 PV guest OS users to gain host OS privileges by leveraging race conditions in pagetable promotion and demotion operations. There are issues with restartable PV type change operations. To avoid using shadow pagetables for PV guests, Xen exposes the actual hardware pagetables to the guest. In order to prevent the guest from modifying these page tables directly, Xen keeps track of how pages are used using a type system; pages must be \"promoted\" before being used as a pagetable, and \"demoted\" before being used for any other type. Xen also allows for \"recursive\" promotions: i.e., an operating system promoting a page to an L4 pagetable may end up causing pages to be promoted to L3s, which may in turn cause pages to be promoted to L2s, and so on. These operations may take an arbitrarily large amount of time, and so must be re-startable. Unfortunately, making recursive pagetable promotion and demotion operations restartable is incredibly complicated, and the code contains several races which, if triggered, can cause Xen to drop or retain extra type counts, potentially allowing guests to get write access to in-use pagetables. A malicious PV guest administrator may be able to escalate their privilege to that of the host. All x86 systems with untrusted PV guests are vulnerable. HVM and PVH guests cannot exercise this vulnerability.", "A flaw was found in xen. Several race conditions exist that, if triggered, can cause Xen to drop or retain extra type counts allowing guests to get write access to in-use pagetables. The highest threat from this vulnerability is to data confidentiality and integrity as well as to system availability."], "name": "CVE-2019-18421", "package_state": [{"cpe": "cpe:/o:redhat:enterprise_linux:5", "fix_state": "Out of support scope", "package_name": "kernel-xen", "product_name": "Red Hat Enterprise Linux 5"}], "public_date": "2019-10-31T00:00:00Z", "references": ["https://www.cve.org/CVERecord?id=CVE-2019-18421\nhttps://nvd.nist.gov/vuln/detail/CVE-2019-18421"], "threat_severity": "Important"}