[Bug 1859713] Re: ARM v8.3a pauth not working

["Philippe Mathieu-Daudé" <1859713@bugs.launchpad.net>]

Apparently this fixed bug is the official CVE-2020-10702:
https://security-tracker.debian.org/tracker/CVE-2020-10702

** CVE added: https://cve.mitre.org/cgi-bin/cvename.cgi?name=2020-10702

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Title:
  ARM v8.3a pauth not working

Status in QEMU:
  Fix Released

Bug description:
  Host: Ubuntu 19.10 - x86_64 machine
  QEMU version: 3a63b24a1bbf166e6f455fe43a6bbd8dea413d92 (master)

  ARMV8.3 pauth is not working well.

  With a test code containing two pauth instructions:
      - paciasp that sign LR with A key and sp as context;
      - autiasp that verify the signature.

  Test:
      - Run the program and corrupt LR just before autiasp (ex 0x3e00000400660 instead of 0x3e000000400664)

  Expected:
      - autiasp places an invalid pointer in LR

  Result:
      - autiasp successfully auth the pointer and places 0x0400660 in LR.

  Further explanations:
      Adding traces in qemu code shows that "pauth_computepac" is not robust enough against truncating.
      With 0x31000000400664 as input of pauth_auth, we obtain "0x55b1d65b2c138e14" for PAC, "0x30" for bot_bit and "0x38" for top_bit.
      With 0x310040008743ec as input of pauth (with same key), we obtain "0x55b1d65b2c138ef4" for PAC, "0x30" for bot_bit and "0x38" for top_bit.
      Values of top_bit and bottom_bit are strictly the same and it should not.

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