PT-2026-64504 · Linux · Linux
CVE-2026-64283
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Published
2026-07-25
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Updated
2026-07-25
None
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In the Linux kernel, the following vulnerability has been resolved:
KVM: guest memfd: Treat memslot binding offset+size as unsigned values
When binding a memslot to a guest memfd file, treat the offset and size as
unsigned values to fix a bug where the sum of the two can result in a false
negative when checking for overflow against the size of the file. Passing
unsigned values also avoids relying on somewhat obscure checks in other
flows for safety, and tracks the offset and size as they are intended to be
tracked, as unsigned values.
On 64-bit kernels, the number of pages a memslot contains and thus the size
(and offset) of its guest memfd binding are unsigned 64-bit values. Taking
the offset+size as an loff t instead of a uoff t inadvertently converts
the unsigned value to a signed value if the offset and/or size is massive.
Locally storing the offset and size as signed values is benign in and of
itself (though even that is extremely difficult to discern), but
operating on their sum is not.
For the offset, KVM explicitly checks against a negative value, which might
seem like a bug as KVM could incorrectly reject a legitimate binding, but
that's not actually the case as KVM CREATE GUEST MEMFD takes a signed value
for its size, i.e. a would-be-negative offset is also greater than the
maximum possible size of any guest memfd file.
Regarding the size, while KVM lacks an explicit check for a negative value,
i.e. seemingly has a flawed overflow check, KVM restricts the number of
pages in a single memslot to the largest positive signed 32-bit value:
if (id < KVM USER MEM SLOTS &&
(mem->memory size >> PAGE SHIFT) > KVM MEM MAX NR PAGES)
return -EINVAL;and so that maximum "size" will ever be is 0x7fffffff000.
The sum of the two is, however, problematic. While the size is restricted
by KVM's memslot logic, the offset is not, i.e. the offset is completely
unchecked until the "offset + size > i size read(inode)" check. If the
offset is the (nearly) largest possible positive value, then adding size
to the offset can result in a signed, negative 64-bit value. When compared
against the size of the file (guaranteed to be positive), the negative sum
is always smaller, and KVM incorrectly allows the absurd offset.
Opportunistically add missing includes in kvm mm.h (instead of relying on
its parents).
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Related Identifiers
Affected Products
Linux