CVE-2022-50471
Unknown Unknown - Not Provided
BaseFortify

Publication date: 2025-10-04

Last updated on: 2025-10-06

Assigner: kernel.org

Description
In the Linux kernel, the following vulnerability has been resolved: xen/gntdev: Accommodate VMA splitting Prior to this commit, the gntdev driver code did not handle the following scenario correctly with paravirtualized (PV) Xen domains: * User process sets up a gntdev mapping composed of two grant mappings (i.e., two pages shared by another Xen domain). * User process munmap()s one of the pages. * User process munmap()s the remaining page. * User process exits. In the scenario above, the user process would cause the kernel to log the following messages in dmesg for the first munmap(), and the second munmap() call would result in similar log messages: BUG: Bad page map in process doublemap.test pte:... pmd:... page:0000000057c97bff refcount:1 mapcount:-1 \ mapping:0000000000000000 index:0x0 pfn:... ... page dumped because: bad pte ... file:gntdev fault:0x0 mmap:gntdev_mmap [xen_gntdev] readpage:0x0 ... Call Trace: <TASK> dump_stack_lvl+0x46/0x5e print_bad_pte.cold+0x66/0xb6 unmap_page_range+0x7e5/0xdc0 unmap_vmas+0x78/0xf0 unmap_region+0xa8/0x110 __do_munmap+0x1ea/0x4e0 __vm_munmap+0x75/0x120 __x64_sys_munmap+0x28/0x40 do_syscall_64+0x38/0x90 entry_SYSCALL_64_after_hwframe+0x61/0xcb ... For each munmap() call, the Xen hypervisor (if built with CONFIG_DEBUG) would print out the following and trigger a general protection fault in the affected Xen PV domain: (XEN) d0v... Attempt to implicitly unmap d0's grant PTE ... (XEN) d0v... Attempt to implicitly unmap d0's grant PTE ... As of this writing, gntdev_grant_map structure's vma field (referred to as map->vma below) is mainly used for checking the start and end addresses of mappings. However, with split VMAs, these may change, and there could be more than one VMA associated with a gntdev mapping. Hence, remove the use of map->vma and rely on map->pages_vm_start for the original start address and on (map->count << PAGE_SHIFT) for the original mapping size. Let the invalidate() and find_special_page() hooks use these. Also, given that there can be multiple VMAs associated with a gntdev mapping, move the "mmu_interval_notifier_remove(&map->notifier)" call to the end of gntdev_put_map, so that the MMU notifier is only removed after the closing of the last remaining VMA. Finally, use an atomic to prevent inadvertent gntdev mapping re-use, instead of using the map->live_grants atomic counter and/or the map->vma pointer (the latter of which is now removed). This prevents the userspace from mmap()'ing (with MAP_FIXED) a gntdev mapping over the same address range as a previously set up gntdev mapping. This scenario can be summarized with the following call-trace, which was valid prior to this commit: mmap gntdev_mmap mmap (repeat mmap with MAP_FIXED over the same address range) gntdev_invalidate unmap_grant_pages (sets 'being_removed' entries to true) gnttab_unmap_refs_async unmap_single_vma gntdev_mmap (maps the shared pages again) munmap gntdev_invalidate unmap_grant_pages (no-op because 'being_removed' entries are true) unmap_single_vma (For PV domains, Xen reports that a granted page is being unmapped and triggers a general protection fault in the affected domain, if Xen was built with CONFIG_DEBUG) The fix for this last scenario could be worth its own commit, but we opted for a single commit, because removing the gntdev_grant_map structure's vma field requires guarding the entry to gntdev_mmap(), and the live_grants atomic counter is not sufficient on its own to prevent the mmap() over a pre-existing mapping.
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Meta Information
Published
2025-10-04
Last Modified
2025-10-06
Generated
2026-05-07
AI Q&A
2025-10-04
EPSS Evaluated
2026-05-05
NVD
CVE-2022-50471
Affected Vendors & Products
Showing 1 associated CPE
Vendor Product Version / Range
xen xen *
Helpful Resources
Exploitability
CWE
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KEV
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CWE ID Description
CWE-UNKNOWN
Attack-Flow Graph
AI Powered Q&A
Can you explain this vulnerability to me?

This vulnerability in the Linux kernel's xen/gntdev driver occurs when a user process sets up a gntdev mapping composed of two grant mappings shared by another Xen domain, then unmaps these pages one by one and exits. The driver did not correctly handle this scenario, leading to kernel log errors and, if Xen is built with CONFIG_DEBUG, a general protection fault in the affected paravirtualized Xen domain. The root cause is improper handling of split VMAs (virtual memory areas) and reuse of gntdev mappings, which could cause bad page mappings and faults during munmap() calls.


How can this vulnerability impact me? :

This vulnerability can cause kernel errors and crashes (general protection faults) in paravirtualized Xen domains when unmapping shared grant pages. This can lead to instability or denial of service in affected virtual machines running on Xen hypervisors with the vulnerable gntdev driver, especially if Xen is built with debugging enabled.


How can this vulnerability be detected on my network or system? Can you suggest some commands?

This vulnerability can be detected by monitoring the kernel logs (dmesg) for specific error messages related to gntdev mappings and munmap calls. Look for messages such as 'BUG: Bad page map in process', 'page dumped because: bad pte', and Xen hypervisor messages like '(XEN) Attempt to implicitly unmap d0's grant PTE'. You can use the command 'dmesg | grep -i gntdev' or 'dmesg | grep -E "BUG: Bad page map|Attempt to implicitly unmap"' to find these logs.


What immediate steps should I take to mitigate this vulnerability?

Immediate mitigation involves updating the Linux kernel to a version that includes the fix for this vulnerability, which properly handles VMA splitting in the gntdev driver. Until then, avoid using gntdev mappings in paravirtualized Xen domains in the problematic manner described (e.g., avoid munmap() calls on split grant mappings and repeated mmap() with MAP_FIXED over the same address range).


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