[PATCH] x86/entry_32: Move CLEAR_CPU_BUFFERS before CR3 switch

[Pawan Gupta]

As the mitigation for MDS and RFDS, CLEAR_CPU_BUFFERS macro executes VERW
instruction that is used to clear the CPU buffers before returning to user
space. Currently, VERW is executed after the user CR3 is restored. This
leads to vm86() to fault because VERW takes a memory operand that is not
mapped in user page tables when vm86() syscall returns. This is an issue
with 32-bit kernels only, as 64-bit kernels do not support vm86().

Move the VERW before the CR3 switch for 32-bit kernels as a workaround.
This is slightly less secure because there is a possibility that the data
in the registers may be sensitive, and doesn't get cleared from CPU
buffers. As 32-bit kernels haven't received some of the other transient
execution mitigations, this is a reasonable trade-off to ensure that
vm86() syscall works.

Fixes: a0e2dab44d22 ("x86/entry_32: Add VERW just before userspace transition")
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=218707
Closes: https://lore.kernel.org/all/8c77ccfd-d561-45a1-8ed5-6b75212c7a58@xxxxxxxxxxxxx/
Reported-by: Robert Gill <rtgill82@xxxxxxxxx>
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
---
 arch/x86/entry/entry_32.S | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index d3a814efbff6..1b9c1587f06e 100644
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -837,6 +837,7 @@ SYM_FUNC_START(entry_SYSENTER_32)
 	jz	.Lsyscall_32_done
 
 	STACKLEAK_ERASE
+	CLEAR_CPU_BUFFERS
 
 	/* Opportunistic SYSEXIT */
 
@@ -881,7 +882,6 @@ 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.
@@ -941,6 +941,7 @@ SYM_FUNC_START(entry_INT80_32)
 	STACKLEAK_ERASE
 
 restore_all_switch_stack:
+	CLEAR_CPU_BUFFERS
 	SWITCH_TO_ENTRY_STACK
 	CHECK_AND_APPLY_ESPFIX
 
@@ -951,7 +952,6 @@ restore_all_switch_stack:
 
 	/* Restore user state */
 	RESTORE_REGS pop=4			# skip orig_eax/error_code
-	CLEAR_CPU_BUFFERS
 .Lirq_return:
 	/*
 	 * ARCH_HAS_MEMBARRIER_SYNC_CORE rely on IRET core serialization

---
base-commit: 0bbac3facb5d6cc0171c45c9873a2dc96bea9680
change-id: 20240426-fix-dosemu-vm86-dd111a01737e