[PATCH v2 1/8] KVM: x86: Grab regs_dirty in local 'unsigned long'

Sean Christopherson
2022-05-26T21:24:21Z

Capture ctxt->regs_dirty in a local 'unsigned long' instead of casting it
to an 'unsigned long *' for use in for_each_set_bit(). The bitops helpers
really do read the entire 'unsigned long', even though the walking of the
read value is capped at the specified size. I.e. 64-bit KVM is reading
memory beyond ctxt->regs_dirty, which is a u32 and thus 4 bytes, whereas
an unsigned long is 8 bytes. Functionally it's not an issue because
regs_dirty is in the middle of x86_emulate_ctxt, i.e. KVM is just reading
its own memory, but relying on that coincidence is gross and unsafe.

Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/emulate.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 89b11e7dca8a..7226a127ccb4 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -269,9 +269,10 @@ static ulong *reg_rmw(struct x86_emulate_ctxt *ctxt, unsigned nr)

static void writeback_registers(struct x86_emulate_ctxt *ctxt)
{
+ unsigned long dirty = ctxt->regs_dirty;
unsigned reg;

- for_each_set_bit(reg, (ulong *)&ctxt->regs_dirty, 16)
+ for_each_set_bit(reg, &dirty, 16)
ctxt->ops->write_gpr(ctxt, reg, ctxt->_regs[reg]);
}

--
2.36.1.255.ge46751e96f-goog