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

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Message-ID: <adbf01c2-5ce3-443d-bc6f-b74575ff0d5c@citrix.com>
Date: Thu, 26 Sep 2024 18:01:39 +0100
From: Andrew Cooper <andrew.cooper3@...rix.com>
To: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
Cc: Uros Bizjak <ubizjak@...il.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 26/09/2024 5:56 pm, Pawan Gupta wrote:
> On Thu, Sep 26, 2024 at 05:28:05PM +0100, Andrew Cooper wrote:
>> On 26/09/2024 5:10 pm, Pawan Gupta wrote:
>>> 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
>> You should also delete _ASM_RIP() as you're removing the only user of it.
> Can we? I see that __svm_vcpu_run() and __vmx_vcpu_run() are using _ASM_RIP().

Oh - so it is when I'm on the right branch.  Sorry for the noise.

~Andrew