david942j / kvm-kernel-example Goto Github PK

Many thanks for making this code available. I have one doubt on the working of the code.

In the function setup_regs in hypervisor.c, shouldn't the value of RSP be set to MAX_KERNEL_SIZE + 0x5000 + KERNEL_STACK_SIZE instead of MAX_KERNEL_SIZE +KERNEL_STACK_SIZE? My understanding of the memory layout of the guest is that you place the kernel, then the page table, and then the kernel stack. With the current setting of RSP, the stack is writing to the same page that is used for the page tables. Of course, the code works correctly even now, because the page table and the kernel stack are both not using the full page. But, if they did, it would cause issues.

Please let me know if my understanding is correct.

I tried to add exception/interruput handler based on this but fail. The VM exit with "shutdown" reason.

I realize there is not gdt setup in the orignal code. But even after I add it, it still doesn't work.

Could you please let me know what I need to do beside setup gdt and idt?

The functions that do the hypercall for read and write does check whether if the beginning of the buffer is valid memory, but the buffer could extend into memory that is not valid.

If the user is able to gain control of the kernel, they can at the very least leak memory that is mmap'd right before the the guest memory segment. If, however, the mmap'd segment is also writable, the user can also write to that memory segment, and compromise the hypervisor, by passing data as stdin, and invoking a hp read call.

Any idea? :3

alon @ ~/kvm-test/kvm-kernel-example - [master] $ sudo make
make -C hypervisor
make[1]: Entering directory '/home/alon/kvm-test/kvm-kernel-example/hypervisor'
make[1]: Nothing to be done for 'all'.
make[1]: Leaving directory '/home/alon/kvm-test/kvm-kernel-example/hypervisor'
make -C kernel
make[1]: Entering directory '/home/alon/kvm-test/kvm-kernel-example/kernel'
make -C syscalls/ syscalls.a
make[2]: Entering directory '/home/alon/kvm-test/kvm-kernel-example/kernel/syscalls'
make[2]: 'syscalls.a' is up to date.
make[2]: Leaving directory '/home/alon/kvm-test/kvm-kernel-example/kernel/syscalls'
make -C hypercalls/ hypercalls.a
make[2]: Entering directory '/home/alon/kvm-test/kvm-kernel-example/kernel/hypercalls'
make[2]: 'hypercalls.a' is up to date.
make[2]: Leaving directory '/home/alon/kvm-test/kvm-kernel-example/kernel/hypercalls'
make -C mm/ mm.a
make[2]: Entering directory '/home/alon/kvm-test/kvm-kernel-example/kernel/mm'
make[2]: 'mm.a' is up to date.
make[2]: Leaving directory '/home/alon/kvm-test/kvm-kernel-example/kernel/mm'
make -C elf/ elf.a
make[2]: Entering directory '/home/alon/kvm-test/kvm-kernel-example/kernel/elf'
make[2]: 'elf.a' is up to date.
make[2]: Leaving directory '/home/alon/kvm-test/kvm-kernel-example/kernel/elf'
make -C utils/ utils.a
make[2]: Entering directory '/home/alon/kvm-test/kvm-kernel-example/kernel/utils'
cc -nostdlib -Os -Wall -Werror -fPIE -pie -masm=intel -I.. -c -MMD -MP string.c
string.c: In function ‘strlen’:
string.c:6:10: error: ‘strnlen’ specified bound 18446744073709551615 exceeds maximum object size 9223372036854775807 [-Werror=stringop-overflow=]
6 | return strnlen(s, ~0ll);
| ^~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors
make[2]: *** [../template.mk:16: string.o] Error 1
make[2]: Leaving directory '/home/alon/kvm-test/kvm-kernel-example/kernel/utils'
make[1]: *** [Makefile:24: utils/utils.a] Error 2
make[1]: Leaving directory '/home/alon/kvm-test/kvm-kernel-example/kernel'
make: *** [Makefile:6: kernel] Error 2

I was seeing the following errors when compiling on Ubuntu 22.04:

as syscall_entry.s -o syscall_entry.o
syscall_entry.s: Assembler messages:
syscall_entry.s:36: Error: ambiguous operand size for `sysret'

cc -nostdlib -Os -Wall -Werror -fPIE -pie -masm=intel -I.. -c -MMD -MP sys_execve.c
syscall_entry.s: Assembler messages:
syscall_entry.s:36: Error: ambiguous operand size for `sysret'
make[2]: *** [../template.mk:19: syscall_entry.o] Error 1
make[2]: *** Waiting for unfinished jobs....
sys_execve.c: Assembler messages:
sys_execve.c:123: Error: ambiguous operand size for `sysret'

I changed sysret to sysretq in both files and it worked.