In the Linux kernel, the following vulnerability has been resolved: KVM: SEV: Require in-GHCB...

In the Linux kernel, the following vulnerability has been resolved:

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!

[sean: write changelog]