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 fi…

Overview

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).

Affected

• linux_kernel >= 6.8, < 7.1.4

Remediation

Upgrade past the affected range:

• linux_kernel 7.1.4