On Fri, Aug 20, 2021, Brijesh Singh wrote:
> When SEV-SNP is enabled, the guest private pages are added in the RMP
> table; while adding the pages, the rmp_make_private() unmaps the pages
> from the direct map. If KSM attempts to access those unmapped pages then
> it will trigger #PF (page-not-present).
>
> Encrypted guest pages cannot be shared between the process, so an
> userspace should not mark the region mergeable but to be safe, mark the
> process vma unmerable before adding the pages in the RMP table.
To be safe from what? Does the !PRESENT #PF crash the kernel?
> Signed-off-by: Brijesh Singh <brijesh.singh@xxxxxxx>
> ---
> arch/x86/kvm/svm/sev.c | 32 ++++++++++++++++++++++++++++++++
> 1 file changed, 32 insertions(+)
>
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index 4b126598b7aa..dcef0ae5f8e4 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -18,11 +18,13 @@
> #include <linux/processor.h>
> #include <linux/trace_events.h>
> #include <linux/sev.h>
> +#include <linux/ksm.h>
> #include <asm/fpu/internal.h>
>
> #include <asm/pkru.h>
> #include <asm/trapnr.h>
> #include <asm/sev.h>
> +#include <asm/mman.h>
>
> #include "x86.h"
> #include "svm.h"
> @@ -1683,6 +1685,30 @@ static bool is_hva_registered(struct kvm *kvm, hva_t hva, size_t len)
> return false;
> }
>
> +static int snp_mark_unmergable(struct kvm *kvm, u64 start, u64 size)
> +{
> + struct vm_area_struct *vma;
> + u64 end = start + size;
> + int ret;
> +
> + do {
> + vma = find_vma_intersection(kvm->mm, start, end);
> + if (!vma) {
> + ret = -EINVAL;
> + break;
> + }
> +
> + ret = ksm_madvise(vma, vma->vm_start, vma->vm_end,
> + MADV_UNMERGEABLE, &vma->vm_flags);
> + if (ret)
> + break;
> +
> + start = vma->vm_end;
> + } while (end > vma->vm_end);
> +
> + return ret;
> +}
> +
> static int snp_launch_update(struct kvm *kvm, struct kvm_sev_cmd *argp)
> {
> struct kvm_sev_info *sev = &to_kvm_svm(kvm)->sev_info;
> @@ -1707,6 +1733,12 @@ static int snp_launch_update(struct kvm *kvm, struct kvm_sev_cmd *argp)
> if (!is_hva_registered(kvm, params.uaddr, params.len))
> return -EINVAL;
>
> + mmap_write_lock(kvm->mm);
> + ret = snp_mark_unmergable(kvm, params.uaddr, params.len);
> + mmap_write_unlock(kvm->mm);
This does not, and practically speaking cannot, work. There are multiple TOCTOU
bugs, here and in __snp_handle_page_state_change(). Userspace can madvise() the
range at any later point, munmap()/mmap() the entire range, mess with the memslots
in the PSC case, and so on and so forth. Relying on MADV_UNMERGEABLE for functional
correctness simply cannot work in KVM, barring mmu_notifier and a big pile of code.
> + if (ret)
> + return -EFAULT;
> +
> /*
> * The userspace memory is already locked so technically we don't
> * need to lock it again. Later part of the function needs to know
> --
> 2.17.1
>
