[PATCH 2/4] KVM: x86: Harden _regs accesses to guard against buggy input

[Sean Christopherson <seanjc@google.com>]

WARN and truncate the incoming GPR number/index when reading/writing GPRs
in the emulator to guard against KVM bugs, e.g. to avoid out-of-bounds
accesses to ctxt->_regs[] if KVM generates a bogus index.  Truncate the
index instead of returning e.g. zero, as reg_write() returns a pointer
to the register, i.e. returning zero would result in a NULL pointer
dereference.  KVM could also force the index to any arbitrary GPR, but
that's no better or worse, just different.

Open code the restriction to 16 registers; RIP is handled via _eip and
should never be accessed through reg_read() or reg_write().  See the
comments above the declarations of reg_read() and reg_write(), and the
behavior of writeback_registers().  The horrific open coded mess will be
cleaned up in a future commit.

There are no such bugs known to exist in the emulator, but determining
that KVM is bug-free is not at all simple and requires a deep dive into
the emulator.  The code is so convoluted that GCC-12 with the recently
enable -Warray-bounds spits out a (suspected) false-positive:

  arch/x86/kvm/emulate.c:254:27: warning: array subscript 32 is above array
                                 bounds of 'long unsigned int[17]' [-Warray-bounds]
    254 |         return ctxt->_regs[nr];
        |                ~~~~~~~~~~~^~~~
  In file included from arch/x86/kvm/emulate.c:23:
  arch/x86/kvm/kvm_emulate.h: In function 'reg_rmw':
  arch/x86/kvm/kvm_emulate.h:366:23: note: while referencing '_regs'
    366 |         unsigned long _regs[NR_VCPU_REGS];
        |                       ^~~~~

Link: https://lore.kernel.org/all/YofQlBrlx18J7h9Y@google.com
Cc: Robert Dinse <nanook@eskimo.com>
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/emulate.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 7226a127ccb4..c58366ae4da2 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -247,6 +247,9 @@ enum x86_transfer_type {
 
 static ulong reg_read(struct x86_emulate_ctxt *ctxt, unsigned nr)
 {
+	if (WARN_ON_ONCE(nr >= 16))
+		nr &= 16 - 1;
+
 	if (!(ctxt->regs_valid & (1 << nr))) {
 		ctxt->regs_valid |= 1 << nr;
 		ctxt->_regs[nr] = ctxt->ops->read_gpr(ctxt, nr);
@@ -256,6 +259,9 @@ static ulong reg_read(struct x86_emulate_ctxt *ctxt, unsigned nr)
 
 static ulong *reg_write(struct x86_emulate_ctxt *ctxt, unsigned nr)
 {
+	if (WARN_ON_ONCE(nr >= 16))
+		nr &= 16 - 1;
+
 	ctxt->regs_valid |= 1 << nr;
 	ctxt->regs_dirty |= 1 << nr;
 	return &ctxt->_regs[nr];
-- 
2.36.1.124.g0e6072fb45-goog