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National Vulnerability Database

National Vulnerability Database

NVD

National Vulnerability Database
  1. Vulnerabilities

CVE-2021-47277 Detail

Description

In the Linux kernel, the following vulnerability has been resolved: kvm: avoid speculation-based attacks from out-of-range memslot accesses KVM's mechanism for accessing guest memory translates a guest physical address (gpa) to a host virtual address using the right-shifted gpa (also known as gfn) and a struct kvm_memory_slot. The translation is performed in __gfn_to_hva_memslot using the following formula: hva = slot->userspace_addr + (gfn - slot->base_gfn) * PAGE_SIZE It is expected that gfn falls within the boundaries of the guest's physical memory. However, a guest can access invalid physical addresses in such a way that the gfn is invalid. __gfn_to_hva_memslot is called from kvm_vcpu_gfn_to_hva_prot, which first retrieves a memslot through __gfn_to_memslot. While __gfn_to_memslot does check that the gfn falls within the boundaries of the guest's physical memory or not, a CPU can speculate the result of the check and continue execution speculatively using an illegal gfn. The speculation can result in calculating an out-of-bounds hva. If the resulting host virtual address is used to load another guest physical address, this is effectively a Spectre gadget consisting of two consecutive reads, the second of which is data dependent on the first. Right now it's not clear if there are any cases in which this is exploitable. One interesting case was reported by the original author of this patch, and involves visiting guest page tables on x86. Right now these are not vulnerable because the hva read goes through get_user(), which contains an LFENCE speculation barrier. However, there are patches in progress for x86 uaccess.h to mask kernel addresses instead of using LFENCE; once these land, a guest could use speculation to read from the VMM's ring 3 address space. Other architectures such as ARM already use the address masking method, and would be susceptible to this same kind of data-dependent access gadgets. Therefore, this patch proactively protects from these attacks by masking out-of-bounds gfns in __gfn_to_hva_memslot, which blocks speculation of invalid hvas. Sean Christopherson noted that this patch does not cover kvm_read_guest_offset_cached. This however is limited to a few bytes past the end of the cache, and therefore it is unlikely to be useful in the context of building a chain of data dependent accesses.


Metrics

 
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CVSS 4.0 Severity and Vector Strings:

NIST CVSS score
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References to Advisories, Solutions, and Tools

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URL Source(s) Tag(s)
https://git.kernel.org/stable/c/22b87fb17a28d37331bb9c1110737627b17f6781 CVE, kernel.org Patch 
https://git.kernel.org/stable/c/3098b86390a6b9ea52657689f08410baf130ceff CVE, kernel.org Patch 
https://git.kernel.org/stable/c/361ce3b917aff93123e9e966d8608655c967f438 CVE, kernel.org Patch 
https://git.kernel.org/stable/c/740621309b25bbf619b8a0ba5fd50a8e58989441 CVE, kernel.org Patch 
https://git.kernel.org/stable/c/7af299b97734c7e7f465b42a2139ce4d77246975 CVE, kernel.org Patch 
https://git.kernel.org/stable/c/bff1fbf0cf0712686f1df59a83fba6e31d2746a0 CVE, kernel.org Patch 
https://git.kernel.org/stable/c/da27a83fd6cc7780fea190e1f5c19e87019da65c CVE, kernel.org Patch 
https://git.kernel.org/stable/c/ed0e2a893092c7fcb4ff7ba74e5efce53a6f5940 CVE, kernel.org Patch 

Weakness Enumeration

CWE-ID CWE Name Source
CWE-125 Out-of-bounds Read cwe source acceptance level NIST  

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Change History

4 change records found show changes

Quick Info

CVE Dictionary Entry:
CVE-2021-47277
NVD Published Date:
05/21/2024
NVD Last Modified:
04/30/2025
Source:
kernel.org