[Bug 1911351] Re: x86-64 MTTCG Does not update page table entries atomically

[Peter Maydell]

Yeah, it's a long standing API deficiency inside QEMU that we don't have
a way to do atomic modifications in things like page-table-walk code:
mostly you don't notice unless you go looking for it, but we really
ought to fix this. Thanks for the unit test.


** Changed in: qemu
       Status: New => Confirmed

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https://bugs.launchpad.net/bugs/1911351

Title:
  x86-64 MTTCG Does not update page table entries atomically

Status in QEMU:
  Confirmed

Bug description:
  It seems like the qemu tcg code for x86-64 doesn't write the access
  and dirty flags of the page table entries atomically. Instead, they
  first read the entry, see if they need to set the page table entry,
  and then overwrite the entry. So if you have two threads running at
  the same time, one accessing the virtual address over and over again,
  and the other modifying the page table entry, it is possible that
  after the second thread modifies the page table entry, qemu overwrites
  the value with the old page table entry value, with the access/dirty
  flags set.

  Here's a unit test that reproduces this behavior:

  https://github.com/mvanotti/kvm-unit-
  tests/commit/09f9722807271226a714b04f25174776454b19cd

  You can run it with:

  ```
  /usr/bin/qemu-system-x86_64 --no-reboot -nodefaults \
  -device pc-testdev -device isa-debug-exit,iobase=0xf4,iosize=0x4 \
  -vnc none -serial stdio -device pci-testdev \
  -smp 4 -machine q35 --accel tcg,thread=multi \
  -kernel x86/mmu-race.flat # -initrd /tmp/tmp.avvPpezMFf
  ```

  Expected output (failure):

  ```
  kvm-unit-tests$ make && /usr/bin/qemu-system-x86_64 --no-reboot -nodefaults 
-device pc-testdev -device isa-debug-exit,iobase=0xf4,iosize=0x4 -vnc none 
-serial stdio -device pci-testdev -smp 4 -machine q35 --accel tcg,thread=multi  
-kernel x86/mmu-race.flat # -initrd /tmp/tmp.avvPpezMFf
  enabling apic
  enabling apic
  enabling apic
  enabling apic
  paging enabled
  cr0 = 80010011
  cr3 = 627000
  cr4 = 20
  found 4 cpus
  PASS: Need more than 1 CPU
  Detected overwritten PTE:
          want: 0x000000000062e007
          got:  0x000000000062d027
  FAIL: PTE not overwritten
  PASS: All Reads were zero
  SUMMARY: 3 tests, 1 unexpected failures
  ```

  This bug has allows user-to-root privilege escalation inside the guest
  VM: if the user is able overwrite an entry that belongs to a second-
  to-last level page table, and is able to allocate the referenced page,
  then the user would be in control of a last-level page table, being
  able to map any memory they want. This is not uncommon in situations
  where memory is being decomitted.

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