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

[Andrew Cooper]

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@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

You should also delete _ASM_RIP() as you're removing the only user of it.

But yes, with that, Reviewed-by: Andrew Cooper
<andrew.cooper3@xxxxxxxxxx> FWIW.