[PATCH v2 2/8] KVM: x86: Harden _regs accesses to guard against buggy input

Sean Christopherson
2022-05-26T21:24:27Z

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(+)

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];
--
2.36.1.255.ge46751e96f-goog