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

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 26/09/2024 1:17 am, 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@vger.kernel.org # 5.10+
>>>> Reported-by: Robert Gill <rtgill82@gmail.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@linux.intel.com>
>>>> Suggested-by: Brian Gerst <brgerst@gmail.com>
>>>> Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.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
> +#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).
> +	 */

I was intending for this to replace the "Using %cs" sentence, as a new
paragraph in that main comment block.

Otherwise, yes, this is half of what I had in mind.

~Andrew