x86/entry_32: Do not clobber user EFLAGS.ZF

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: a0e2dab ("x86/entry_32: Add VERW just before userspace transition")
Reported-by: Jari Ruusu <[email protected]>
Signed-off-by: Pawan Gupta <[email protected]>
Signed-off-by: Dave Hansen <[email protected]>
Cc:[email protected]
Link: https://lore.kernel.org/all/20240925-fix-dosemu-vm86-v7-1-1de0daca2d42%40linux.intel.com

Author: pa1gupta

arch/x86/entry/entry_32.S

@@ -871,6 +871,8 @@ SYM_FUNC_START(entry_SYSENTER_32)
 
 	/* Now ready to switch the cr3 */
 	SWITCH_TO_USER_CR3 scratch_reg=%eax
+	/* Clobbers ZF */
+	CLEAR_CPU_BUFFERS
 
 	/*
 	 * 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
 	popfl
 	popl	%eax
-	CLEAR_CPU_BUFFERS
 
 	/*
 	 * Return back to the vDSO, which will pop ecx and edx.