linux-kernel - [PATCH v2 1/8] KVM: x86: Grab regs

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Message-Id: <20220526210817.3428868-2-seanjc@google.com>
Date:   Thu, 26 May 2022 21:08:10 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Kees Cook <keescook@...omium.org>,
        Robert Dinse <nanook@...imo.com>
Subject: [PATCH v2 1/8] KVM: x86: Grab regs_dirty in local 'unsigned long'

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@...hat.com>
Reviewed-by: Kees Cook <keescook@...omium.org>
Signed-off-by: Sean Christopherson <seanjc@...gle.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