linux-kernel - Re: [PATCH v7 3/3] x86/bugs: Use code segment selector for VERW operand

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Message-ID: <20240926161031.dcohgbkbqs4bk32n@desk>
Date: Thu, 26 Sep 2024 09:10:31 -0700
From: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To: Uros Bizjak <ubizjak@...il.com>
Cc: Andrew Cooper <andrew.cooper3@...rix.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	linux-kernel@...r.kernel.org, x86@...nel.org,
	Robert Gill <rtgill82@...il.com>,
	Jari Ruusu <jariruusu@...tonmail.com>,
	Brian Gerst <brgerst@...il.com>,
	"Linux regression tracking (Thorsten Leemhuis)" <regressions@...mhuis.info>,
	antonio.gomez.iglesias@...ux.intel.com,
	daniel.sneddon@...ux.intel.com, stable@...r.kernel.org
Subject: Re: [PATCH v7 3/3] x86/bugs: Use code segment selector for VERW
 operand

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@...r.kernel.org # 5.10+
Reported-by: Robert Gill <rtgill82@...il.com>
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=218707
Closes: https://lore.kernel.org/all/8c77ccfd-d561-45a1-8ed5-6b75212c7a58@leemhuis.info/
Suggested-by: Dave Hansen <dave.hansen@...ux.intel.com>
Suggested-by: Brian Gerst <brgerst@...il.com>
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
---
 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