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

From: Thomas Huth
Subject: [Bug 1503031] Re: 32-to-64-bit call gate unsupported in IA32e mode
Date: Fri, 07 Aug 2020 18:31:26 -0000

Looking through old bug tickets... is this still an issue with the
latest version of QEMU? Or could we close this ticket nowadays?

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

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

Status in QEMU:

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

  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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