On Thu, Sep 26, 2024 at 04:52:53PM +0200, Uros Bizjak wrote:
> > diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
> > index e18a6aaf414c..4228a1fd2c2e 100644
> > --- a/arch/x86/include/asm/nospec-branch.h
> > +++ b/arch/x86/include/asm/nospec-branch.h
> > @@ -318,14 +318,21 @@
> > /*
> > * Macro to execute VERW instruction that mitigate transient data sampling
> > * attacks such as MDS. On affected systems a microcode update overloaded VERW
> > - * instruction to also clear the CPU buffers. VERW clobbers CFLAGS.ZF. Using %cs
> > - * to reference VERW operand avoids a #GP fault for an arbitrary user %ds in
> > - * 32-bit mode.
> > + * instruction to also clear the CPU buffers. VERW clobbers CFLAGS.ZF.
> > *
> > * Note: Only the memory operand variant of VERW clears the CPU buffers.
> > */
> > .macro CLEAR_CPU_BUFFERS
> > - ALTERNATIVE "", __stringify(verw %cs:_ASM_RIP(mds_verw_sel)), X86_FEATURE_CLEAR_CPU_BUF
> > +#ifdef CONFIG_X86_64
> > + ALTERNATIVE "", __stringify(verw _ASM_RIP(mds_verw_sel)), X86_FEATURE_CLEAR_CPU_BUF
>
> You should drop _ASM_RIP here and direclty use (%rip). This way, you also
> won't need __stringify:
>
> ALTERNATIVE "", "verw mds_verw_sel(%rip)", X86_FEATURE_CLEAR_CPU_BUF
>
> > +#else
> > + /*
> > + * In 32bit mode, the memory operand must be a %cs reference. The data
> > + * segments may not be usable (vm86 mode), and the stack segment may not
> > + * be flat (ESPFIX32).
> > + */
> > + ALTERNATIVE "", __stringify(verw %cs:mds_verw_sel), X86_FEATURE_CLEAR_CPU_BUF
>
> Also here, no need for __stringify:
>
> ALTERNATIVE "", "verw %cs:mds_verw_sel", X86_FEATURE_CLEAR_CPU_BUF
>
> This is in fact what Andrew proposed in his review.
Thanks for pointing out, I completely missed that part. Below is how it
looks like with stringify gone:
--- >8 ---
Subject: [PATCH] x86/bugs: Use code segment selector for VERW operand
Robert Gill reported below #GP in 32-bit mode 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 in 32-bit mode 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. Use %cs selector to reference VERW operand. This ensures VERW will
not #GP for an arbitrary user %ds.
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>
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
---
arch/x86/include/asm/nospec-branch.h | 11 ++++++++++-
1 file changed, 10 insertions(+), 1 deletion(-)
diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index ff5f1ecc7d1e..96b410b1d4e8 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -323,7 +323,16 @@
* Note: Only the memory operand variant of VERW clears the CPU buffers.
*/
.macro CLEAR_CPU_BUFFERS
- ALTERNATIVE "", __stringify(verw _ASM_RIP(mds_verw_sel)), X86_FEATURE_CLEAR_CPU_BUF
+#ifdef CONFIG_X86_64
+ ALTERNATIVE "", "verw mds_verw_sel(%rip)", X86_FEATURE_CLEAR_CPU_BUF
+#else
+ /*
+ * In 32bit mode, the memory operand must be a %cs reference. The data
+ * segments may not be usable (vm86 mode), and the stack segment may not
+ * be flat (ESPFIX32).
+ */
+ ALTERNATIVE "", "verw %cs:mds_verw_sel", X86_FEATURE_CLEAR_CPU_BUF
+#endif
.endm
#ifdef CONFIG_X86_64
--
2.34.1
