[Qemu-devel] [Bug 1503031] Re: 32-to-64-bit call gate unsupported in IA32e mode

[Andrew Oates]

I also suspect that qemu will incorrectly allow calls through 32-bit
call gates in compatibility mode (rather than raising a GP fault --- see
Intel manuals volume 3A 5-21).  And I doubt 64-to-64-bit call gates work
either.  I haven't actually tested either of those scenarios, though,
this is just from reading the code.

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https://bugs.launchpad.net/bugs/1503031

Title:
  32-to-64-bit call gate unsupported in IA32e mode

Status in QEMU:
  New

Bug description:
  In particular, the lcall implementation doesn't support the 64-bit
  TSS.

  helper_lcall_protected (target-i386/seg_helper.c:1884) calls
  get_ss_esp_from_tss() on a call gate to a lower privilege level, which
  tries to extract a 32-bit ESP and 16-bit SS from the TSS.  In IA32e
  mode (64-bit or compatibility mode), this instead grabs the lower
  32-bits of the target RSP, and 16 of the upper bits as the SS.
  Additionally, several of the subsequent checks are incorrect (even if
  the correct stack pointer were extracted).

  This isn't a problem for interrupts since the interrupts are given
  their own implementation entirely, that uses get_rsp_from_tss() rather
  than get_ss_esp_from_tss().

  I believe the missing logic is from the branch starting "ELSE (*
  current TSS is 64-bit *)" in the CALL pseudocode in the Intel manual
  (page 3-124 of the PDF I have).

  Reproduced at master (c0b520dfb8890294a9f8879f4759172900585995), and
  also as of a qemu built a year ago.

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