@@ -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];
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 false-positive due to a GCC bug: 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 Link: https://bugzilla.kernel.org/show_bug.cgi?id=216026 Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105679 Reported-and-tested-by: Robert Dinse <nanook@eskimo.com> Reported-by: Kees Cook <keescook@chromium.org> Reviewed-by: Kees Cook <keescook@chromium.org> Signed-off-by: Sean Christopherson <seanjc@google.com> --- arch/x86/kvm/emulate.c | 6 ++++++ 1 file changed, 6 insertions(+)