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

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Message-ID: <555e220d-7ff5-58e4-7ca8-282ca88d8392@gmail.com>
Date: Thu, 26 Sep 2024 16:52:53 +0200
From: Uros Bizjak <ubizjak@...il.com>
To: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
 Andrew Cooper <andrew.cooper3@...rix.com>
Cc: 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. 24 02:17, Pawan Gupta wrote:
> On Wed, Sep 25, 2024 at 04:46:23PM -0700, Pawan Gupta wrote:
>> On Thu, Sep 26, 2024 at 12:29:00AM +0100, Andrew Cooper wrote:
>>> On 25/09/2024 11:25 pm, Pawan Gupta wrote:
>>>> 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 | 6 ++++--
>>>>   1 file changed, 4 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
>>>> index ff5f1ecc7d1e..e18a6aaf414c 100644
>>>> --- a/arch/x86/include/asm/nospec-branch.h
>>>> +++ b/arch/x86/include/asm/nospec-branch.h
>>>> @@ -318,12 +318,14 @@
>>>>   /*
>>>>    * 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.
>>>> + * 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.
>>>>    *
>>>>    * 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
>>>> +	ALTERNATIVE "", __stringify(verw %cs:_ASM_RIP(mds_verw_sel)), X86_FEATURE_CLEAR_CPU_BUF
>>>>   .endm
>>>
>>> People ought rightly to double-take at this using %cs and not %ss.
>>> There is a good reason, but it needs describing explicitly.  May I
>>> suggest the following:
>>>
>>> *...
>>> * 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).
>>> *...
>>
>> Thanks for the suggestion. I will include this.
>>
>>>   .macro CLEAR_CPU_BUFFERS
>>> #ifdef __x86_64__
>>>      ALTERNATIVE "", "verw mds_verw_sel(%rip)", X86_FEATURE_CLEAR_CPU_BUF
>>> #else
>>>      ALTERNATIVE "", "verw %cs:mds_verw_sel", X86_FEATURE_CLEAR_CPU_BUF
>>> #endif
>>>   .endm
>>>
>>> This also lets you drop _ASM_RIP().  It's a cute idea, but is more
>>> confusion than it's worth, because there's no such thing in 32bit mode.
>>>
>>> "%cs:_ASM_RIP(mds_verw_sel)" reads as if it does nothing, because it
>>> really doesn't in 64bit mode.
>>
>> Right, will drop _ASM_RIP() in 32-bit mode and %cs in 64-bit mode.
> 
> Its probably too soon for next version, pasting the patch here:
> 
> ---8<---
> 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.

Uros.