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(). 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 tail page should be
expected to have KVM_LPAGE_MIXED_FLAG set. It is not functionally
required, as the unaligned tail pages will already have their
kvm_lpage_info count incremented. The comments imply not setting it on
unaligned head pages is intentional, so fix the callers to skip trying to
set KVM_LPAGE_MIXED_FLAG in this case, and in doing so not call
hugepage_has_attrs().

Also rename hugepage_has_attrs() to __slot_hugepage_has_attrs() because it
is a delicate function that should not be widely used, and only is valid
for ranges covered by the passed slot.

Cc: stable@vger.kernel.org
Fixes: 90b4fe17981e ("KVM: x86: Disallow hugepages when memory attributes are mixed")
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
---
Hi,

I added cc stable because I didn't rule out a way to trigger a non-kasan
crash from userspace on non-x86. But of course this is a testing only
feature at this point and shouldn't cause a crash for normal users.

Testing was just the upstream selftests and a TDX guest boot on out of tree
branch.

Rick
---
 arch/x86/kvm/mmu/mmu.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

base-commit: 5abf6dceb066f2b02b225fd561440c98a8062681

Message ID	20240312173334.2484335-1-rick.p.edgecombe@intel.com (mailing list archive)
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KVM: x86/mmu: x86: Don't overflow lpage_info when checking attributes

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