ghsa-p437-j6g9-3f2f
Vulnerability from github
In the Linux kernel, the following vulnerability has been resolved:
KVM: x86/mmu: x86: Don't overflow lpage_info when checking attributes
Fix KVM_SET_MEMORY_ATTRIBUTES to not overflow lpage_info array and trigger KASAN splat, as seen in the private_mem_conversions_test selftest.
When memory attributes are set on a GFN range, that range will have specific properties applied to the TDP. A huge page cannot be used when the attributes are inconsistent, so they are disabled for those the specific huge pages. For internal KVM reasons, huge pages are also not allowed to span adjacent memslots regardless of whether the backing memory could be mapped as huge.
What GFNs support which huge page sizes is tracked by an array of arrays 'lpage_info' on the memslot, of ‘kvm_lpage_info’ structs. Each index of lpage_info contains a vmalloc allocated array of these for a specific supported page size. The kvm_lpage_info denotes whether a specific huge page (GFN and page size) on the memslot is supported. These arrays include indices for unaligned head and tail huge pages.
Preventing huge pages from spanning adjacent memslot is covered by incrementing the count in head and tail kvm_lpage_info when the memslot is allocated, but disallowing huge pages for memory that has mixed attributes has to be done in a more complicated way. During the KVM_SET_MEMORY_ATTRIBUTES ioctl KVM updates lpage_info for each memslot in the range that has mismatched attributes. KVM does this a memslot at a time, and marks a special bit, KVM_LPAGE_MIXED_FLAG, in the kvm_lpage_info for any huge page. This bit is essentially a permanently elevated count. So huge pages will not be mapped for the GFN at that page size if the count is elevated in either case: a huge head or tail page unaligned to the memslot or if KVM_LPAGE_MIXED_FLAG is set because it has mixed attributes.
To determine whether a huge page has consistent attributes, the KVM_SET_MEMORY_ATTRIBUTES operation checks an xarray to make sure it consistently has the incoming attribute. Since level - 1 huge pages are aligned to level huge pages, it employs an optimization. As long as the level - 1 huge pages are checked first, it can just check these and assume that if each level - 1 huge page contained within the level sized huge page is not mixed, then the level size huge page is not mixed. This optimization happens in the helper hugepage_has_attrs().
Unfortunately, although the kvm_lpage_info array representing page size 'level' will contain an entry for an unaligned tail page of size level, the array for level - 1 will not contain an entry for each GFN at page size level. The level - 1 array will only contain an index for any unaligned region covered by level - 1 huge page size, which can be a smaller region. So this causes the optimization to overflow the level - 1 kvm_lpage_info and perform a vmalloc out of bounds read.
In some cases of head and tail pages where an overflow could happen, callers skip the operation completely as KVM_LPAGE_MIXED_FLAG is not required to prevent huge pages as discussed earlier. But for memslots that are smaller than the 1GB page size, it does call hugepage_has_attrs(). In this case the huge page is both the head and tail page. The issue can be observed simply by compiling the kernel with CONFIG_KASAN_VMALLOC and running the selftest “private_mem_conversions_test”, which produces the output like the following:
BUG: KASAN: vmalloc-out-of-bounds in hugepage_has_attrs+0x7e/0x110 Read of size 4 at addr ffffc900000a3008 by task private_mem_con/169 Call Trace: dump_stack_lvl print_report ? __virt_addr_valid ? hugepage_has_attrs ? hugepage_has_attrs kasan_report ? hugepage_has_attrs hugepage_has_attrs kvm_arch_post_set_memory_attributes kvm_vm_ioctl
It is a little ambiguous whether the unaligned head page (in the bug case also the tail page) should be expected to have KVM_LPAGE_MIXED_FLAG set. It is not functionally required, as the unal ---truncated---
{
"affected": [],
"aliases": [
"CVE-2024-26991"
],
"database_specific": {
"cwe_ids": [],
"github_reviewed": false,
"github_reviewed_at": null,
"nvd_published_at": "2024-05-01T06:15:16Z",
"severity": "MODERATE"
},
"details": "In the Linux kernel, the following vulnerability has been resolved:\n\nKVM: x86/mmu: x86: Don\u0027t overflow lpage_info when checking attributes\n\nFix KVM_SET_MEMORY_ATTRIBUTES to not overflow lpage_info array and trigger\nKASAN splat, as seen in the private_mem_conversions_test selftest.\n\nWhen memory attributes are set on a GFN range, that range will have\nspecific properties applied to the TDP. A huge page cannot be used when\nthe attributes are inconsistent, so they are disabled for those the\nspecific huge pages. For internal KVM reasons, huge pages are also not\nallowed to span adjacent memslots regardless of whether the backing memory\ncould be mapped as huge.\n\nWhat GFNs support which huge page sizes is tracked by an array of arrays\n\u0027lpage_info\u0027 on the memslot, of \u2018kvm_lpage_info\u2019 structs. Each index of\nlpage_info contains a vmalloc allocated array of these for a specific\nsupported page size. The kvm_lpage_info denotes whether a specific huge\npage (GFN and page size) on the memslot is supported. These arrays include\nindices for unaligned head and tail huge pages.\n\nPreventing huge pages from spanning adjacent memslot is covered by\nincrementing the count in head and tail kvm_lpage_info when the memslot is\nallocated, but disallowing huge pages for memory that has mixed attributes\nhas to be done in a more complicated way. During the\nKVM_SET_MEMORY_ATTRIBUTES ioctl KVM updates lpage_info for each memslot in\nthe range that has mismatched attributes. KVM does this a memslot at a\ntime, and marks a special bit, KVM_LPAGE_MIXED_FLAG, in the kvm_lpage_info\nfor any huge page. This bit is essentially a permanently elevated count.\nSo huge pages will not be mapped for the GFN at that page size if the\ncount is elevated in either case: a huge head or tail page unaligned to\nthe memslot or if KVM_LPAGE_MIXED_FLAG is set because it has mixed\nattributes.\n\nTo determine whether a huge page has consistent attributes, the\nKVM_SET_MEMORY_ATTRIBUTES operation checks an xarray to make sure it\nconsistently has the incoming attribute. Since level - 1 huge pages are\naligned to level huge pages, it employs an optimization. As long as the\nlevel - 1 huge pages are checked first, it can just check these and assume\nthat if each level - 1 huge page contained within the level sized huge\npage is not mixed, then the level size huge page is not mixed. This\noptimization happens in the helper hugepage_has_attrs().\n\nUnfortunately, although the kvm_lpage_info array representing page size\n\u0027level\u0027 will contain an entry for an unaligned tail page of size level,\nthe array for level - 1 will not contain an entry for each GFN at page\nsize level. The level - 1 array will only contain an index for any\nunaligned region covered by level - 1 huge page size, which can be a\nsmaller region. So this causes the optimization to overflow the level - 1\nkvm_lpage_info and perform a vmalloc out of bounds read.\n\nIn some cases of head and tail pages where an overflow could happen,\ncallers skip the operation completely as KVM_LPAGE_MIXED_FLAG is not\nrequired to prevent huge pages as discussed earlier. But for memslots that\nare smaller than the 1GB page size, it does call hugepage_has_attrs(). In\nthis case the huge page is both the head and tail page. The issue can be\nobserved simply by compiling the kernel with CONFIG_KASAN_VMALLOC and\nrunning the selftest \u201cprivate_mem_conversions_test\u201d, which produces the\noutput like the following:\n\nBUG: KASAN: vmalloc-out-of-bounds in hugepage_has_attrs+0x7e/0x110\nRead of size 4 at addr ffffc900000a3008 by task private_mem_con/169\nCall Trace:\n dump_stack_lvl\n print_report\n ? __virt_addr_valid\n ? hugepage_has_attrs\n ? hugepage_has_attrs\n kasan_report\n ? hugepage_has_attrs\n hugepage_has_attrs\n kvm_arch_post_set_memory_attributes\n kvm_vm_ioctl\n\nIt is a little ambiguous whether the unaligned head page (in the bug case\nalso the tail page) should be expected to have KVM_LPAGE_MIXED_FLAG set.\nIt is not functionally required, as the unal\n---truncated---",
"id": "GHSA-p437-j6g9-3f2f",
"modified": "2024-11-07T21:31:37Z",
"published": "2024-05-01T06:31:43Z",
"references": [
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2024-26991"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/048cc4a028e635d339687ed968985d2d1669494c"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/992b54bd083c5bee24ff7cc35991388ab08598c4"
},
{
"type": "WEB",
"url": "https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/4EZ6PJW7VOZ224TD7N4JZNU6KV32ZJ53"
},
{
"type": "WEB",
"url": "https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/DAMSOZXJEPUOXW33WZYWCVAY7Z5S7OOY"
},
{
"type": "WEB",
"url": "https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/GCBZZEC7L7KTWWAS2NLJK6SO3IZIL4WW"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H",
"type": "CVSS_V3"
}
]
}
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