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kvm: x86: fix emulator buffer overflow (CVE-2014-0049)
The problem occurs when the guest performs a pusha with the stack address pointing to an mmio address (or an invalid guest physical address) to start with, but then extending into an ordinary guest physical address. When doing repeated emulated pushes emulator_read_write sets mmio_needed to 1 on the first one. On a later push when the stack points to regular memory, mmio_nr_fragments is set to 0, but mmio_is_needed is not set to 0. As a result, KVM exits to userspace, and then returns to complete_emulated_mmio. In complete_emulated_mmio vcpu->mmio_cur_fragment is incremented. The termination condition of vcpu->mmio_cur_fragment == vcpu->mmio_nr_fragments is never achieved. The code bounces back and fourth to userspace incrementing mmio_cur_fragment past it's buffer. If the guest does nothing else it eventually leads to a a crash on a memcpy from invalid memory address. However if a guest code can cause the vm to be destroyed in another vcpu with excellent timing, then kvm_clear_async_pf_completion_queue can be used by the guest to control the data that's pointed to by the call to cancel_work_item, which can be used to gain execution. Fixes: f78146b Signed-off-by: Andrew Honig <email@example.com> Cc: firstname.lastname@example.org (3.5+) Signed-off-by: Paolo Bonzini <email@example.com>
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