kvm_vm_set_mem_attributes() reserves an xarray entry for every GFN in the range before storing anything, so that the store phase cannot fail partway through. When a reservation fails, the loop jumps to out_unlock and the reservations already made are abandoned: nothing in the call releases them. A later request that clears a range covering them does erase them, as the clear stores NULL over the reservation, but nothing obliges userspace to issue one and a caller exploiting this will not; absent such a call the entries live until kvm_destroy_vm(). An unprivileged user with /dev/kvm on a VM with private-memory support can therefore leak kernel memory across calls (a 576-byte xa_node per 64 GFNs) for the life of the VM fd. The abandoned entries are not inert. A bare reservation is an XA_ZERO_ENTRY. kvm_range_has_memory_attributes() is inconsistent about it: the end == start + 1 path and the general loop treat it as absent (matching kvm_get_memory_attributes(), which maps it to NULL via xa_load()), but the !attrs fast path calls xas_find() directly, which returns the zero entry as present. Via hugepage_has_attrs(), that makes kvm_arch_post_set_memory_attributes() mark a straddling head/tail hugepage "mixed" for a range whose attributes are in fact uniform, so KVM stops using a hugepage there until a later request re-covers it. Release the reservations this call made on the failure path. xa_release() erases an entry only while it is still a reservation, so value entries that predate this call are left untouched; it takes no gfp and cannot fail. Only [start, i) is walked, i being the index whose reservation failed (no entry was created at or beyond it). Runtime-verified on v6.18.48 (isolated sw-protected VM, no KASAN, no fault injection; the reservations are left behind by real memcg pressure via clone(CLONE_VM), not by fault injection). Unpatched, a failed request retains on the order of 450000 xa_nodes (~250 MiB), and with one of them inside a 2 MiB region a clear of the region's head page leaves pages_2m unchanged (the reservation is invisible to xa_load), a clear of two pages drops pages_2m by one and raises pages_4k by 512 (the hugepage is degraded), and a clear covering the whole 2 MiB restores it. With this patch the same run retains under 2000 nodes -- three orders of magnitude less, at the level of run-to-run noise -- and pages_2m stays at 16 across all three clears: the reservations are released, so the hugepage is never degraded. Both arms used the same kernel config and the same test binary. The bug reproduces with XA_FLAGS_ACCOUNT applied (patch 3/3), i.e. accounting alone does not fix it. Found by an AI-assisted security audit. Fixes: 5a475554db1e ("KVM: Introduce per-page memory attributes") Cc: stable@vger.kernel.org Assisted-by: Claude-Code:claude-opus-5 Signed-off-by: David Ballesteros --- virt/kvm/kvm_main.c | 24 +++++++++++++++++++++++- 1 file changed, 23 insertions(+), 1 deletion(-) diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c @@ -2575,7 +2575,7 @@ static int kvm_vm_set_mem_attributes(struct kvm *kvm, gfn_t start, gfn_t end, for (i = start; i < end; i++) { r = xa_reserve(&kvm->mem_attr_array, i, GFP_KERNEL_ACCOUNT); if (r) - goto out_unlock; + goto out_release; cond_resched(); } @@ -2594,6 +2594,28 @@ static int kvm_vm_set_mem_attributes(struct kvm *kvm, gfn_t start, gfn_t end, out_unlock: mutex_unlock(&kvm->slots_lock); + return r; + +out_release: + /* + * The reservation loop failed at @i; the entries in [start, i) were + * reserved by this call and, without releasing them here, would be + * retained until userspace happens to clear a range covering them, or + * until the VM is destroyed. The retained entries are not inert: + * a bare reservation is an XA_ZERO_ENTRY, which the !attrs fast path of + * kvm_range_has_memory_attributes() counts as present (it calls + * xas_find() directly) even though kvm_get_memory_attributes() reports + * it as absent, so a straddling hugepage over such an entry gets marked + * mixed and KVM stops using a hugepage for a range whose attributes are + * uniform. xa_release() erases an entry only while it is still a + * reservation, so value entries that predate this call are untouched. + */ + while (i-- > start) { + xa_release(&kvm->mem_attr_array, i); + cond_resched(); + } + mutex_unlock(&kvm->slots_lock); + return r; } static int kvm_vm_ioctl_set_mem_attributes(struct kvm *kvm,