KVM: SEV: Require in-GHCB scratch area if GHCB v2+ is in use

KVM: SEV: Require in-GHCB scratch area if GHCB v2+ is in use

As per the GHCB spec, when using GHCB v2+ require the software scratch area
to reside in the GHCB's shared buffer.  Note, things like Page State Change
(PSC) requests _rely_ on this behavior, as the guest can't provide a length
when making the request, i.e. the size of the guest payload is bounded by
the size of the shared buffer.

Failure to force usage of the GHCB, and a slew of other flaws, lets a
malicious SNP guest corrupt host kernel heap memory, and leak host heap
layout information.

setup_vmgexit_scratch() allocates a buffer via kvzalloc(exit_info_2),
where exit_info_2 is guest-controlled. With exit_info_2=24, this yields
a 24-byte allocation in kmalloc-cg-32 (32-byte slab objects). The buffer
holds an 8-byte psc_hdr followed by 8-byte psc_entry structs, so only
entries[0] and entries[1] are in-bounds.

snp_begin_psc() validates end_entry against VMGEXIT_PSC_MAX_COUNT (253)
but NOT against the actual buffer size:

      idx_end = hdr->end_entry;

      if (idx_end >= VMGEXIT_PSC_MAX_COUNT) {   // checks 253, not buffer
          snp_complete_psc(svm, ...);
          return 1;
      }

      for (idx = idx_start; idx <= idx_end; idx++) {
          entry_start = entries[idx];           // OOB when idx >= 2

The guest sets end_entry=10+, causing the host to iterate entries[2+]
which are OOB into adjacent slab objects. For each OOB entry:

  - The host reads 8 bytes (OOB READ / info leak oracle)
  - If the data passes PSC validation, __snp_complete_one_psc() writes
    cur_page = 1 or 512 into the entry (OOB WRITE, sev.c:3806)
  - If validation fails, the error response reveals whether adjacent
    memory is zero vs non-zero (information disclosure to guest)

The guest controls allocation size (exit_info_2), entry range
(cur_entry/end_entry), and can fire unlimited VMGEXITs to repeatedly
hit different slab positions.

By exploiting the variety of bugs, a malicious SEV-SNP guest can:
    - OOB read adjacent kmalloc-cg-32 objects (heap layout disclosure)
    - OOB write cur_page bits into adjacent objects (heap corruption)
    - Trigger use-after-free conditions across VMGEXITs

E.g. with KASAN enabled, a single insmod of the PoC guest module
produces 73 KASAN reports:

    BUG: KASAN: slab-out-of-bounds in snp_begin_psc+0x126/0x890
    Read of size 8 at addr ffff888219ffb5e0 by task qemu-system-x86/2199

    BUG: KASAN: slab-out-of-bounds in snp_begin_psc+0x468/0x890
    Write of size 8 at addr ffff888351566648 by task qemu-system-x86/2199

    The buggy address belongs to the object at ffff888XXXXXXXXX
     which belongs to the cache kmalloc-cg-32 of size 32
    The buggy address is located N bytes to the right of
     allocated 32-byte region [ffff888XXXXXXXXX, ffff888XXXXXXXXX)

  Breakdown:
    62 slab-out-of-bounds (reads + writes past allocation)
     7 slab-use-after-free
     4 use-after-free

All credit to Stan for the wonderful description and reproducer!

Reported-by: Stan Shaw <shawstan96@gmail.com>
Cc: Michael Roth <michael.roth@amd.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Peter Gonda <pgonda@google.com>
Cc: Jacky Li <jackyli@google.com>
Fixes: 4af663c2f64a ("KVM: SEV: Allow per-guest configuration of GHCB protocol version")
Cc: stable@vger.kernel.org
Signed-off-by: Michael Roth <michael.roth@amd.com>
[sean: write changelog]
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-ID: <20260501202250.2115252-2-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>