On 11. 07. 24, 10:30, Uros Bizjak wrote:
On Thu, Jul 11, 2024 at 1:16 AM Pawan Gupta
<pawan.kumar.gupta@xxxxxxxxxxxxxxx> wrote:
On Wed, Jul 10, 2024 at 11:50:50PM +0200, Uros Bizjak wrote:
On 10. 07. 24 21:06, Pawan Gupta wrote:
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>
---
v4:
- 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 | 18 +++++++++++++++---
1 file changed, 15 insertions(+), 3 deletions(-)
diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index d3a814efbff6..d54f6002e5a0 100644
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -253,6 +253,16 @@
.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 "jmp .Lskip_verw\@", "", X86_FEATURE_CLEAR_CPU_BUF
+ verw %ss:_ASM_RIP(mds_verw_sel)
+.Lskip_verw\@:
+.endm
Why not simply:
.macro CLEAR_CPU_BUFFERS_SAFE
ALTERNATIVE "", __stringify(verw %ss:_ASM_RIP(mds_verw_sel)),
X86_FEATURE_CLEAR_CPU_BUF
.endm
We can do it this way as well. But, there are stable kernels that don't
support relocations in ALTERNATIVEs. The way it is done in current patch
can be backported without worrying about which kernels support relocations.
Then you could use absolute reference in backports to kernels that
don't support relocations in ALTERNATIVEs, "verw %ss:mds_verw_sel"
works as well, but the relocation is one byte larger.
No, the kernels support (and use) relocations. It's only ALTERNATIVEs in
older kernels don't.
--
js
suse labs
