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: a0e2dab44d22 ("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

diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index d3a814e..9ad6cd8 100644 (file)
--- a/arch/x86/entry/entry_32.S
+++ b/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.