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x86/entry_32: Do not clobber user EFLAGS.ZF

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Opportunistic SYSEXIT executes VERW to clear CPU buffers after user EFLAGS
are restored. This can clobber user EFLAGS.ZF.

Move CLEAR_CPU_BUFFERS before the user EFLAGS are restored. This ensures
that the user EFLAGS.ZF is not clobbered.

Closes: https://lore.kernel.org/lkml/yVXwe8gvgmPADpRB6lXlicS2fcHoV5OHHxyuFbB_MEleRPD7-KhGe5VtORejtPe-KCkT8Uhcg5d7-IBw4Ojb4H7z5LQxoZylSmJ8KNL3A8o=@protonmail.com/
Fixes: a0e2dab44d ("x86/entry_32: Add VERW just before userspace transition")
Reported-by: Jari Ruusu <jariruusu@protonmail.com>
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc:stable@vger.kernel.org
Link: https://lore.kernel.org/all/20240925-fix-dosemu-vm86-v7-1-1de0daca2d42%40linux.intel.com
This commit is contained in:
Pawan Gupta 2024-09-25 15:25:38 -07:00 committed by Dave Hansen
parent d5fd042bf4
commit 2e2e5143d4
1 changed files with 2 additions and 1 deletions
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3
arch/x86/entry/entry_32.S
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@ -871,6 +871,8 @@ SYM_FUNC_START(entry_SYSENTER_32)
/* Now ready to switch the cr3 */ /* Now ready to switch the cr3 */
SWITCH_TO_USER_CR3 scratch_reg=%eax SWITCH_TO_USER_CR3 scratch_reg=%eax
/* Clobbers ZF */
CLEAR_CPU_BUFFERS
/* /*
* Restore all flags except IF. (We restore IF separately because * Restore all flags except IF. (We restore IF separately because
@ -881,7 +883,6 @@ SYM_FUNC_START(entry_SYSENTER_32)
BUG_IF_WRONG_CR3 no_user_check=1 BUG_IF_WRONG_CR3 no_user_check=1
popfl popfl
popl %eax popl %eax
CLEAR_CPU_BUFFERS
/* /*
* Return back to the vDSO, which will pop ecx and edx. * Return back to the vDSO, which will pop ecx and edx.