Wind River Support Network

Description

In the Linux kernel, the following vulnerability has been resolved:  KVM: Reject wrapped offset in kvm_reset_dirty_gfn()  kvm_reset_dirty_gfn() guards the gfn range with  	if (!memslot || (offset + __fls(mask)) >= memslot->npages) 		return;  but offset is u64 and the addition is unchecked.  The check can be silently bypassed by a u64 wrap.  The dirty ring backing those entries is MAP_SHARED at KVM_DIRTY_LOG_PAGE_OFFSET of the vcpu fd, so the VMM can rewrite the slot and offset fields of any entry between when the kernel pushes them and when KVM_RESET_DIRTY_RINGS consumes them.  On reset, kvm_dirty_ring_reset() re-reads the values via READ_ONCE() and feeds them straight back into this check; only the flags handshake is treated as the handover, the slot/offset payload is taken on trust.  Crafting two entries  	entry[i].offset   = 0xffffffffffffffc1 	entry[i+1].offset = 0  makes the coalescing loop in kvm_dirty_ring_reset() compute  	delta = (s64)(0 - 0xffffffffffffffc1) = 63  which falls in [0, BITS_PER_LONG), so it folds entry[i+1] into the existing mask by setting bit 63.  The trailing kvm_reset_dirty_gfn() call then sees offset = 0xffffffffffffffc1 and __fls(mask) = 63; the sum is 0 in u64 and the bounds check passes.  That offset propagates into kvm_arch_mmu_enable_log_dirty_pt_masked() unchanged.  On the legacy MMU path -- kvm_memslots_have_rmaps() == true, i.e. shadow paging, any VM that has allocated shadow roots, or a write-tracked slot -- it reaches gfn_to_rmap(), which indexes slot->arch.rmap[0][] with a near-U64_MAX gfn.  That is an out-of-bounds load of a kvm_rmap_head, followed by a conditional clear of PT_WRITABLE_MASK in whatever the loaded pointer points at. The path is reachable from any process holding /dev/kvm.  Range-check offset on its own first, so the addition cannot wrap. memslot->npages is bounded well below U64_MAX, so once offset < npages holds, offset + __fls(mask) (with __fls(mask) < BITS_PER_LONG) stays in range.