Robert Gill reported below #GP when dosemu software was executing vm86()
system call:
general protection fault: 0000 [#1] PREEMPT SMP
CPU: 4 PID: 4610 Comm: dosemu.bin Not tainted 6.6.21-gentoo-x86 #1
Hardware name: Dell Inc. PowerEdge 1950/0H723K, BIOS 2.7.0 10/30/2010
EIP: restore_all_switch_stack+0xbe/0xcf
EAX: 00000000 EBX: 00000000 ECX: 00000000 EDX: 00000000
ESI: 00000000 EDI: 00000000 EBP: 00000000 ESP: ff8affdc
DS: 0000 ES: 0000 FS: 0000 GS: 0033 SS: 0068 EFLAGS: 00010046
CR0: 80050033 CR2: 00c2101c CR3: 04b6d000 CR4: 000406d0
Call Trace:
show_regs+0x70/0x78
die_addr+0x29/0x70
exc_general_protection+0x13c/0x348
exc_bounds+0x98/0x98
handle_exception+0x14d/0x14d
exc_bounds+0x98/0x98
restore_all_switch_stack+0xbe/0xcf
exc_bounds+0x98/0x98
restore_all_switch_stack+0xbe/0xcf
This only happens when VERW based mitigations like MDS/RFDS are enabled.
This is because segment registers with an arbitrary user value can result
in #GP when executing VERW. Intel SDM vol. 2C documents the following
behavior for VERW instruction:
#GP(0) - If a memory operand effective address is outside the CS, DS, ES,
FS, or GS segment limit.
CLEAR_CPU_BUFFERS macro executes VERW instruction before returning to user
space. Replace CLEAR_CPU_BUFFERS with a safer version that uses %ss to
refer VERW operand mds_verw_sel. This ensures VERW will not #GP for an
arbitrary user %ds. Also, in NMI return path, move VERW to after
RESTORE_ALL_NMI that touches GPRs.
For clarity, below are the locations where the new CLEAR_CPU_BUFFERS_SAFE
version is being used:
* entry_INT80_32(), entry_SYSENTER_32() and interrupts (via
handle_exception_return) do:
restore_all_switch_stack:
[...]
mov %esi,%esi
verw %ss:0xc0fc92c0 <-------------
iret
* Opportunistic SYSEXIT:
[...]
verw %ss:0xc0fc92c0 <-------------
btrl $0x9,(%esp)
popf
pop %eax
sti
sysexit
* nmi_return and nmi_from_espfix:
mov %esi,%esi
verw %ss:0xc0fc92c0 <-------------
jmp .Lirq_return
Fixes: a0e2dab44d22 ("x86/entry_32: Add VERW just before userspace transition")
Cc: stable@xxxxxxxxxxxxxxx # 5.10+
Reported-by: Robert Gill <rtgill82@xxxxxxxxx>
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=218707
Closes: https://lore.kernel.org/all/8c77ccfd-d561-45a1-8ed5-6b75212c7a58@xxxxxxxxxxxxx/
Suggested-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Suggested-by: Brian Gerst <brgerst@xxxxxxxxx> # Use %ss
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
---
v5:
- Simplify the use of ALTERNATIVE construct (Uros/Jiri/Peter).
v4: https://lore.kernel.org/r/20240710-fix-dosemu-vm86-v4-1-aa6464e1de6f@xxxxxxxxxxxxxxx
- Further simplify the patch by using %ss for all VERW calls in 32-bit mode (Brian).
- In NMI exit path move VERW after RESTORE_ALL_NMI that touches GPRs (Dave).
v3: https://lore.kernel.org/r/20240701-fix-dosemu-vm86-v3-1-b1969532c75a@xxxxxxxxxxxxxxx
- Simplify CLEAR_CPU_BUFFERS_SAFE by using %ss instead of %ds (Brian).
- Do verw before popf in SYSEXIT path (Jari).
v2: https://lore.kernel.org/r/20240627-fix-dosemu-vm86-v2-1-d5579f698e77@xxxxxxxxxxxxxxx
- Safe guard against any other system calls like vm86() that might change %ds (Dave).
v1: https://lore.kernel.org/r/20240426-fix-dosemu-vm86-v1-1-88c826a3f378@xxxxxxxxxxxxxxx
---
---
arch/x86/entry/entry_32.S | 16 +++++++++++++---
1 file changed, 13 insertions(+), 3 deletions(-)
diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index d3a814efbff6..25c942149fb5 100644
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -253,6 +253,14 @@
.Lend_\@:
.endm
+/*
+ * Safer version of CLEAR_CPU_BUFFERS that uses %ss to reference VERW operand
+ * mds_verw_sel. This ensures VERW will not #GP for an arbitrary user %ds.
+ */
+.macro CLEAR_CPU_BUFFERS_SAFE
+ ALTERNATIVE "", __stringify(verw %ss:_ASM_RIP(mds_verw_sel)), X86_FEATURE_CLEAR_CPU_BUF
+.endm
+
.macro RESTORE_INT_REGS
popl %ebx
popl %ecx
@@ -871,6 +879,8 @@ SYM_FUNC_START(entry_SYSENTER_32)
/* Now ready to switch the cr3 */
SWITCH_TO_USER_CR3 scratch_reg=%eax
+ /* Clobbers ZF */
+ CLEAR_CPU_BUFFERS_SAFE
/*
* Restore all flags except IF. (We restore IF separately because
@@ -881,7 +891,6 @@ 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.
@@ -951,7 +960,7 @@ restore_all_switch_stack:
/* Restore user state */
RESTORE_REGS pop=4 # skip orig_eax/error_code
- CLEAR_CPU_BUFFERS
+ CLEAR_CPU_BUFFERS_SAFE
.Lirq_return:
/*
* ARCH_HAS_MEMBARRIER_SYNC_CORE rely on IRET core serialization
@@ -1144,7 +1153,6 @@ SYM_CODE_START(asm_exc_nmi)
/* Not on SYSENTER stack. */
call exc_nmi
- CLEAR_CPU_BUFFERS
jmp .Lnmi_return
.Lnmi_from_sysenter_stack:
@@ -1165,6 +1173,7 @@ SYM_CODE_START(asm_exc_nmi)
CHECK_AND_APPLY_ESPFIX
RESTORE_ALL_NMI cr3_reg=%edi pop=4
+ CLEAR_CPU_BUFFERS_SAFE
jmp .Lirq_return
#ifdef CONFIG_X86_ESPFIX32
@@ -1206,6 +1215,7 @@ SYM_CODE_START(asm_exc_nmi)
* 1 - orig_ax
*/
lss (1+5+6)*4(%esp), %esp # back to espfix stack
+ CLEAR_CPU_BUFFERS_SAFE
jmp .Lirq_return
#endif
SYM_CODE_END(asm_exc_nmi)
---
base-commit: f2661062f16b2de5d7b6a5c42a9a5c96326b8454
change-id: 20240426-fix-dosemu-vm86-dd111a01737e
