Re: [PATCH v5 01/19] KVM: x86: Allocate new rmap and large page tracking when moving memslot

Peter Xu
2020-02-05T21:50:01Z

On Tue, Jan 21, 2020 at 02:31:39PM -0800, Sean Christopherson wrote:
> Reallocate a rmap array and recalcuate large page compatibility when
> moving an existing memslot to correctly handle the alignment properties
> of the new memslot. The number of rmap entries required at each level
> is dependent on the alignment of the memslot's base gfn with respect to
> that level, e.g. moving a large-page aligned memslot so that it becomes
> unaligned will increase the number of rmap entries needed at the now
> unaligned level.
>
> Not updating the rmap array is the most obvious bug, as KVM accesses
> garbage data beyond the end of the rmap. KVM interprets the bad data as
> pointers, leading to non-canonical #GPs, unexpected #PFs, etc...
>
> general protection fault: 0000 [#1] SMP
> CPU: 0 PID: 1909 Comm: move_memory_reg Not tainted 5.4.0-rc7+ #139
> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
> RIP: 0010:rmap_get_first+0x37/0x50 [kvm]
> Code: <48> 8b 3b 48 85 ff 74 ec e8 6c f4 ff ff 85 c0 74 e3 48 89 d8 5b c3
> RSP: 0018:ffffc9000021bbc8 EFLAGS: 00010246
> RAX: ffff00617461642e RBX: ffff00617461642e RCX: 0000000000000012
> RDX: ffff88827400f568 RSI: ffffc9000021bbe0 RDI: ffff88827400f570
> RBP: 0010000000000000 R08: ffffc9000021bd00 R09: ffffc9000021bda8
> R10: ffffc9000021bc48 R11: 0000000000000000 R12: 0030000000000000
> R13: 0000000000000000 R14: ffff88827427d700 R15: ffffc9000021bce8
> FS: 00007f7eda014700(0000) GS:ffff888277a00000(0000) knlGS:0000000000000000
> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007f7ed9216ff8 CR3: 0000000274391003 CR4: 0000000000162eb0
> Call Trace:
> kvm_mmu_slot_set_dirty+0xa1/0x150 [kvm]
> __kvm_set_memory_region.part.64+0x559/0x960 [kvm]
> kvm_set_memory_region+0x45/0x60 [kvm]
> kvm_vm_ioctl+0x30f/0x920 [kvm]
> do_vfs_ioctl+0xa1/0x620
> ksys_ioctl+0x66/0x70
> __x64_sys_ioctl+0x16/0x20
> do_syscall_64+0x4c/0x170
> entry_SYSCALL_64_after_hwframe+0x44/0xa9
> RIP: 0033:0x7f7ed9911f47
> Code: <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 21 6f 2c 00 f7 d8 64 89 01 48
> RSP: 002b:00007ffc00937498 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
> RAX: ffffffffffffffda RBX: 0000000001ab0010 RCX: 00007f7ed9911f47
> RDX: 0000000001ab1350 RSI: 000000004020ae46 RDI: 0000000000000004
> RBP: 000000000000000a R08: 0000000000000000 R09: 00007f7ed9214700
> R10: 00007f7ed92149d0 R11: 0000000000000246 R12: 00000000bffff000
> R13: 0000000000000003 R14: 00007f7ed9215000 R15: 0000000000000000
> Modules linked in: kvm_intel kvm irqbypass
> ---[ end trace 0c5f570b3358ca89 ]---
>
> The disallow_lpage tracking is more subtle. Failure to update results
> in KVM creating large pages when it shouldn't, either due to stale data
> or again due to indexing beyond the end of the metadata arrays, which
> can lead to memory corruption and/or leaking data to guest/userspace.
>
> Note, the arrays for the old memslot are freed by the unconditional call
> to kvm_free_memslot() in __kvm_set_memory_region().

If __kvm_set_memory_region() failed, I think the old memslot will be
kept and the new memslot will be freed instead?

>
> Fixes: 05da45583de9b ("KVM: MMU: large page support")
> Cc: stable@vger.kernel.org
> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> ---
> arch/x86/kvm/x86.c | 11 +++++++++++
> 1 file changed, 11 insertions(+)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 4c30ebe74e5d..1953c71c52f2 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -9793,6 +9793,13 @@ int kvm_arch_create_memslot(struct kvm *kvm, struct kvm_memory_slot *slot,
> {
> int i;
>
> + /*
> + * Clear out the previous array pointers for the KVM_MR_MOVE case. The
> + * old arrays will be freed by __kvm_set_memory_region() if installing
> + * the new memslot is successful.
> + */
> + memset(&slot->arch, 0, sizeof(slot->arch));

I actually gave r-b on this patch but it was lost... And then when I
read it again I start to confuse on why we need to set these to zeros.
Even if they're not zeros, iiuc kvm_free_memslot() will compare each
of the array pointer and it will only free the changed pointers, then
it looks fine even without zeroing?

> +
> for (i = 0; i < KVM_NR_PAGE_SIZES; ++i) {
> struct kvm_lpage_info *linfo;
> unsigned long ugfn;
> @@ -9867,6 +9874,10 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
> const struct kvm_userspace_memory_region *mem,
> enum kvm_mr_change change)
> {
> + if (change == KVM_MR_MOVE)
> + return kvm_arch_create_memslot(kvm, memslot,
> + mem->memory_size >> PAGE_SHIFT);
> +

Instead of calling kvm_arch_create_memslot() explicitly again here,
can it be replaced by below?

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 72b45f491692..85a7b02fd752 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1144,7 +1144,7 @@ int __kvm_set_memory_region(struct kvm *kvm,
new.dirty_bitmap = NULL;

r = -ENOMEM;
- if (change == KVM_MR_CREATE) {
+ if (change == KVM_MR_CREATE || change == KVM_MR_MOVE) {
new.userspace_addr = mem->userspace_addr;

if (kvm_arch_create_memslot(kvm, &new, npages))

> return 0;
> }
>
> --
> 2.24.1
>

--
Peter Xu