[PATCH] x86/bugs: Use fixed addressing for VERW operand

[PATCH] x86/bugs: Use fixed addressing for VERW operand

[Pawan Gupta]

Macro used for MDS mitigation executes VERW with relative addressing for
the operand. This is unnecessary and creates a problem for backports on
older kernels that don't support relocations in alternatives. Relocation
support was added by commit 270a69c4485d ("x86/alternative: Support
relocations in alternatives"). Also asm for fixed addressing is much
more cleaner than relative RIP addressing.

Simplify the asm by using fixed addressing for VERW operand.

Fixes: baf8361e5455 ("x86/bugs: Add asm helpers for executing VERW")
Reported-by: Nikolay Borisov <nik.borisov@suse.com>
Closes: https://lore.kernel.org/lkml/20558f89-299b-472e-9a96-171403a83bd6@suse.com/
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
---
 arch/x86/include/asm/nospec-branch.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 2aa52cab1e46..ab19c7f1167b 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -323,7 +323,7 @@
  * 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 mds_verw_sel), X86_FEATURE_CLEAR_CPU_BUF
 .endm
 
 #else /* __ASSEMBLY__ */

---
base-commit: d206a76d7d2726f3b096037f2079ce0bd3ba329b
change-id: 20240226-verw-arg-fix-796a63088c4d

Re: [PATCH] x86/bugs: Use fixed addressing for VERW operand

[Nikolay Borisov]

On 27.02.24 г. 1:52 ч., Pawan Gupta wrote:
> Macro used for MDS mitigation executes VERW with relative addressing for
> the operand. This is unnecessary and creates a problem for backports on
> older kernels that don't support relocations in alternatives. Relocation
> support was added by commit 270a69c4485d ("x86/alternative: Support
> relocations in alternatives"). Also asm for fixed addressing is much
> more cleaner than relative RIP addressing.
> 
> Simplify the asm by using fixed addressing for VERW operand.
> 
> Fixes: baf8361e5455 ("x86/bugs: Add asm helpers for executing VERW")
> Reported-by: Nikolay Borisov <nik.borisov@suse.com>
> Closes: https://lore.kernel.org/lkml/20558f89-299b-472e-9a96-171403a83bd6@suse.com/
> Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
> ---
>   arch/x86/include/asm/nospec-branch.h | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
> index 2aa52cab1e46..ab19c7f1167b 100644
> --- a/arch/x86/include/asm/nospec-branch.h
> +++ b/arch/x86/include/asm/nospec-branch.h
> @@ -323,7 +323,7 @@
>    * 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 mds_verw_sel), X86_FEATURE_CLEAR_CPU_BUF

Actually thinking about it more and discussing with Jiri (cc'ed), will 
this work with KASLR enabled?

>   .endm
>   
>   #else /* __ASSEMBLY__ */
> 
> ---
> base-commit: d206a76d7d2726f3b096037f2079ce0bd3ba329b
> change-id: 20240226-verw-arg-fix-796a63088c4d
> 

Re: [PATCH] x86/bugs: Use fixed addressing for VERW operand

[Jiri Slaby]

On 27. 02. 24, 9:47, Nikolay Borisov wrote:
> 
> 
> On 27.02.24 г. 1:52 ч., Pawan Gupta wrote:
>> Macro used for MDS mitigation executes VERW with relative addressing for
>> the operand. This is unnecessary and creates a problem for backports on
>> older kernels that don't support relocations in alternatives. Relocation
>> support was added by commit 270a69c4485d ("x86/alternative: Support
>> relocations in alternatives"). Also asm for fixed addressing is much
>> more cleaner than relative RIP addressing.
>>
>> Simplify the asm by using fixed addressing for VERW operand.
>>
>> Fixes: baf8361e5455 ("x86/bugs: Add asm helpers for executing VERW")
>> Reported-by: Nikolay Borisov <nik.borisov@suse.com>
>> Closes: 
>> https://lore.kernel.org/lkml/20558f89-299b-472e-9a96-171403a83bd6@suse.com/
>> Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
>> ---
>>   arch/x86/include/asm/nospec-branch.h | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/include/asm/nospec-branch.h 
>> b/arch/x86/include/asm/nospec-branch.h
>> index 2aa52cab1e46..ab19c7f1167b 100644
>> --- a/arch/x86/include/asm/nospec-branch.h
>> +++ b/arch/x86/include/asm/nospec-branch.h
>> @@ -323,7 +323,7 @@
>>    * 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 mds_verw_sel), 
>> X86_FEATURE_CLEAR_CPU_BUF
> 
> Actually thinking about it more and discussing with Jiri (cc'ed), will 
> this work with KASLR enabled?

I might of course be wrong. We appear to rely on the asm+linker here. 
Please see also:
https://lore.kernel.org/r/fd8f2df0-563e-4f5c-aca4-bc92a14e9426@kernel.org

thanks,
-- 
js
suse labs

Re: [PATCH] x86/bugs: Use fixed addressing for VERW operand

[Pawan Gupta]

On Tue, Feb 27, 2024 at 10:43:53AM +0100, Jiri Slaby wrote:
> On 27. 02. 24, 9:47, Nikolay Borisov wrote:
> > 
> > 
> > On 27.02.24 г. 1:52 ч., Pawan Gupta wrote:
> > > Macro used for MDS mitigation executes VERW with relative addressing for
> > > the operand. This is unnecessary and creates a problem for backports on
> > > older kernels that don't support relocations in alternatives. Relocation
> > > support was added by commit 270a69c4485d ("x86/alternative: Support
> > > relocations in alternatives"). Also asm for fixed addressing is much
> > > more cleaner than relative RIP addressing.
> > > 
> > > Simplify the asm by using fixed addressing for VERW operand.
> > > 
> > > Fixes: baf8361e5455 ("x86/bugs: Add asm helpers for executing VERW")
> > > Reported-by: Nikolay Borisov <nik.borisov@suse.com>
> > > Closes: https://lore.kernel.org/lkml/20558f89-299b-472e-9a96-171403a83bd6@suse.com/
> > > Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
> > > ---
> > >   arch/x86/include/asm/nospec-branch.h | 2 +-
> > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/arch/x86/include/asm/nospec-branch.h
> > > b/arch/x86/include/asm/nospec-branch.h
> > > index 2aa52cab1e46..ab19c7f1167b 100644
> > > --- a/arch/x86/include/asm/nospec-branch.h
> > > +++ b/arch/x86/include/asm/nospec-branch.h
> > > @@ -323,7 +323,7 @@
> > >    * 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 mds_verw_sel),
> > > X86_FEATURE_CLEAR_CPU_BUF
> > 
> > Actually thinking about it more and discussing with Jiri (cc'ed), will
> > this work with KASLR enabled?
> 
> I might of course be wrong. We appear to rely on the asm+linker here.

You were right, with KASLR enabled, instructions with fixed addressing
in alternatives don't get relocated. I guess we will have to keep
rip-relative as is. Thanks for catching that.

Re: [PATCH] x86/bugs: Use fixed addressing for VERW operand

[Pawan Gupta]

On Wed, Feb 28, 2024 at 05:39:27PM -0800, Pawan Gupta wrote:
> On Tue, Feb 27, 2024 at 10:43:53AM +0100, Jiri Slaby wrote:
> > On 27. 02. 24, 9:47, Nikolay Borisov wrote:
> > > 
> > > 
> > > On 27.02.24 г. 1:52 ч., Pawan Gupta wrote:
> > > > Macro used for MDS mitigation executes VERW with relative addressing for
> > > > the operand. This is unnecessary and creates a problem for backports on
> > > > older kernels that don't support relocations in alternatives. Relocation
> > > > support was added by commit 270a69c4485d ("x86/alternative: Support
> > > > relocations in alternatives"). Also asm for fixed addressing is much
> > > > more cleaner than relative RIP addressing.
> > > > 
> > > > Simplify the asm by using fixed addressing for VERW operand.
> > > > 
> > > > Fixes: baf8361e5455 ("x86/bugs: Add asm helpers for executing VERW")
> > > > Reported-by: Nikolay Borisov <nik.borisov@suse.com>
> > > > Closes: https://lore.kernel.org/lkml/20558f89-299b-472e-9a96-171403a83bd6@suse.com/
> > > > Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
> > > > ---
> > > >   arch/x86/include/asm/nospec-branch.h | 2 +-
> > > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > > > 
> > > > diff --git a/arch/x86/include/asm/nospec-branch.h
> > > > b/arch/x86/include/asm/nospec-branch.h
> > > > index 2aa52cab1e46..ab19c7f1167b 100644
> > > > --- a/arch/x86/include/asm/nospec-branch.h
> > > > +++ b/arch/x86/include/asm/nospec-branch.h
> > > > @@ -323,7 +323,7 @@
> > > >    * 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 mds_verw_sel),
> > > > X86_FEATURE_CLEAR_CPU_BUF
> > > 
> > > Actually thinking about it more and discussing with Jiri (cc'ed), will
> > > this work with KASLR enabled?
> > 
> > I might of course be wrong. We appear to rely on the asm+linker here.
> 
> You were right, with KASLR enabled, instructions with fixed addressing
> in alternatives don't get relocated. I guess we will have to keep
> rip-relative as is. Thanks for catching that.

Looks like this is not settled yet, it was naive of me to trust gdb on
/proc/kcore earlier with KASLR enabled.

With the below debug patch it appears the relocation with fixed
addresses is working as expected with KASLR enabled.

diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 6a1fe302c1fd..ae4168db453d 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -323,7 +323,7 @@
  * Note: Only the memory operand variant of VERW clears the CPU buffers.
  */
 .macro CLEAR_CPU_BUFFERS
-	ALTERNATIVE "", __stringify(verw mds_verw_sel), X86_FEATURE_CLEAR_CPU_BUF
+	ALTERNATIVE "", __stringify(verw mds_verw_sel; ud2), X86_FEATURE_CLEAR_CPU_BUF
 .endm
 
 #else /* __ASSEMBLY__ */
---

With the patch adding #UD after verw I get below panic:

[    1.402347] traps: PANIC: double fault, error_code: 0x0
[    1.402350] double fault: 0000 [#1] PREEMPT SMP PTI
[    1.402353] CPU: 0 PID: 1 Comm: init Not tainted 6.7.6-00007-ga040ab9fe827-dirty #27
[    1.402355] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014
[    1.402356] RIP: 0010:entry_SYSRETQ_unsafe_stack+0xb/0x10
[    1.402365] Code: c7 eb 08 48 89 c7 48 0f ba ef 3f 48 81 cf 00 08 00 00 48 81 cf 00 10 00 00 0f 22 df 58 5f 5c 0f 01 f8 0f 00 2c 25 00 00 60 ab <0f> 0b 48 0f 07 cc 0f 1f 44 00 00 56 48 8b 74 24 08 48 89 7c 24 08
[    1.402366] RSP: 0018:00007ffe253461f8 EFLAGS: 000100c6
[    1.402369] RAX: 000055e5c94f7000 RBX: 00007f3d8d9b4e80 RCX: 00007f3d8d9b8f9b
[    1.402370] RDX: 0000000000000000 RSI: 00007ffe253461ec RDI: 0000000000000000
[    1.402371] RBP: 00007ffe25346290 R08: 0000000000000000 R09: 0000000000000022
[    1.402372] R10: 0000000000000000 R11: 0000000000000246 R12: 00007f3d8d993000
[    1.402373] R13: 0000002300000007 R14: 0000000000000007 R15: 00007ffe253462a0
[    1.402378] FS:  0000000000000000(0000) GS:ffff906d3bc00000(0000) knlGS:ffff906d3bc00000
[    1.402380] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[    1.402381] CR2: 00007ffe253461e8 CR3: 00000001002a8003 CR4: 00000000007706f0
[    1.402382] PKRU: 55555554
[    1.402383] Call Trace:
[    1.402384]  <#DF>
[    1.402386]  ? die+0x37/0x90
[    1.402390]  ? exc_double_fault+0xcf/0x180
[    1.402394]  ? asm_exc_double_fault+0x23/0x30
[    1.402397]  ? entry_SYSRETQ_unsafe_stack+0xb/0x10
[    1.402400]  </#DF>
[    1.402401] Modules linked in:
[    1.402403] Dumping ftrace buffer:
[    1.402406]    (ftrace buffer empty)
[    1.402407] ---[ end trace 0000000000000000 ]---
[    1.402408] RIP: 0010:entry_SYSRETQ_unsafe_stack+0xb/0x10
[    1.402411] Code: c7 eb 08 48 89 c7 48 0f ba ef 3f 48 81 cf 00 08 00 00 48 81 cf 00 10 00 00 0f 22 df 58 5f 5c 0f 01 f8 0f 00
 2c 25 00 00 60 ab <0f> 0b 48 0f 07 cc 0f 1f 44 00 00 56 48 8b 74 24 08 48 89 7c 24 08
[    1.402412] RSP: 0018:00007ffe253461f8 EFLAGS: 000100c6
[    1.402413] RAX: 000055e5c94f7000 RBX: 00007f3d8d9b4e80 RCX: 00007f3d8d9b8f9b
[    1.402414] RDX: 0000000000000000 RSI: 00007ffe253461ec RDI: 0000000000000000
[    1.402415] RBP: 00007ffe25346290 R08: 0000000000000000 R09: 0000000000000022
[    1.402416] R10: 0000000000000000 R11: 0000000000000246 R12: 00007f3d8d993000
[    1.402417] R13: 0000002300000007 R14: 0000000000000007 R15: 00007ffe253462a0
[    1.402419] FS:  0000000000000000(0000) GS:ffff906d3bc00000(0000) knlGS:ffff906d3bc00000
[    1.402420] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[    1.402421] CR2: 00007ffe253461e8 CR3: 00000001002a8003 CR4: 00000000007706f0
[    1.402422] PKRU: 55555554
[    1.402423] Kernel panic - not syncing: Fatal exception in interrupt
[    1.402551] Dumping ftrace buffer:
[    1.402552]    (ftrace buffer empty)
[    1.402552] Kernel Offset: 0x29400000 from 0xffffffff81000000 (relocation range: 0xffffffff80000000-0xffffffffbfffffff)
                              ^
			      Note the KASLR offset
			      ~~~~~~~~~~~~~~~~~~~~~

Disassembling the code stream in the trace ... :

eb 08 48 89 c7 48 0f ba ef 3f 48 81 cf 00 08 00 00 48 81 cf 00 10 00 00 0f 22 df 58 5f 5c 0f 01 f8 0f 00 2c 25 00 00 60 ab 0f 0b 48 0f 07 cc 0f 1f 44 00 00 56 48 8b 74 24 08 48 89 7c 24 08

... gives:

  jmp 0xa
  mov rdi, rax
  bts rdi, 0x3f
  or rdi, 0x800
  or rdi, 0x1000
  mov cr3, rdi
  pop rax
  pop rdi
  pop rsp
  swapgs
  verw word ptr [0xffffffffab600000]       <----- mds_verw_sel
  ud2
  sysretq
  int3
  nop dword ptr [rax + rax]
  push rsi
  mov rsi, qword ptr [rsp + 8]
  mov qword ptr [rsp + 8], rdi

$ objdump -S --disassemble=mds_verw_sel vmlinux

ffffffff82200000 <mds_verw_sel>:
ffffffff82200000:       18 00                   sbb    %al,(%rax)

Adding KASLR offset to mds_verw_sel:

  0xffffffff82200000 + 0x29400000 = 0xffffffffab600000

which is the address of mds_verw_sel after KASLR that we saw in the code
stream disassembly. To me it looks like fixed addresses for VERW is
working as intended. Please let me know if I am missing something. It
will be great if someone can independently verify this.

Even with fixed addressing working with KASLR it may still be a good
idea to stick with RIP relative addresses as they are less likely to
be broken with relocation.

And sorry for the noise.

Re: [PATCH] x86/bugs: Use fixed addressing for VERW operand

[Jiri Slaby]

On 29. 02. 24, 10:14, Pawan Gupta wrote:
> On Wed, Feb 28, 2024 at 05:39:27PM -0800, Pawan Gupta wrote:
>> On Tue, Feb 27, 2024 at 10:43:53AM +0100, Jiri Slaby wrote:
>>> On 27. 02. 24, 9:47, Nikolay Borisov wrote:
>>>>
>>>>
>>>> On 27.02.24 г. 1:52 ч., Pawan Gupta wrote:
>>>>> Macro used for MDS mitigation executes VERW with relative addressing for
>>>>> the operand. This is unnecessary and creates a problem for backports on
>>>>> older kernels that don't support relocations in alternatives. Relocation
>>>>> support was added by commit 270a69c4485d ("x86/alternative: Support
>>>>> relocations in alternatives"). Also asm for fixed addressing is much
>>>>> more cleaner than relative RIP addressing.
>>>>>
>>>>> Simplify the asm by using fixed addressing for VERW operand.
>>>>>
>>>>> Fixes: baf8361e5455 ("x86/bugs: Add asm helpers for executing VERW")
>>>>> Reported-by: Nikolay Borisov <nik.borisov@suse.com>
>>>>> Closes: https://lore.kernel.org/lkml/20558f89-299b-472e-9a96-171403a83bd6@suse.com/
>>>>> Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
>>>>> ---
>>>>>    arch/x86/include/asm/nospec-branch.h | 2 +-
>>>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/arch/x86/include/asm/nospec-branch.h
>>>>> b/arch/x86/include/asm/nospec-branch.h
>>>>> index 2aa52cab1e46..ab19c7f1167b 100644
>>>>> --- a/arch/x86/include/asm/nospec-branch.h
>>>>> +++ b/arch/x86/include/asm/nospec-branch.h
>>>>> @@ -323,7 +323,7 @@
>>>>>     * 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 mds_verw_sel),
>>>>> X86_FEATURE_CLEAR_CPU_BUF
>>>>
>>>> Actually thinking about it more and discussing with Jiri (cc'ed), will
>>>> this work with KASLR enabled?
>>>
>>> I might of course be wrong. We appear to rely on the asm+linker here.
>>
>> You were right, with KASLR enabled, instructions with fixed addressing
>> in alternatives don't get relocated. I guess we will have to keep
>> rip-relative as is. Thanks for catching that.
> 
> Looks like this is not settled yet, it was naive of me to trust gdb on
> /proc/kcore earlier with KASLR enabled.
> 
> With the below debug patch it appears the relocation with fixed
> addresses is working as expected with KASLR enabled.

As I wrote already, asm+linker converts the fixed address to rip-rela 
anyway:

https://lore.kernel.org/all/fd8f2df0-563e-4f5c-aca4-bc92a14e9426@kernel.org/

I also raised questions in there:
====
The assembler generates a relocation for the fixed address anyway. And
the linker resolves it as rip-relative. At least the pair from my
binutils-2.42.

But if it generates a rip-relative address, is < 6.5 with no support of
rip-rel in alternatives still fine?

Another question: can we rely on the assembler to generate a relocation
and on the linker to resolve it as rip-relative?
====

thanks,
-- 
js
suse labs

Re: [PATCH] x86/bugs: Use fixed addressing for VERW operand

[Pawan Gupta]

On Thu, Feb 29, 2024 at 10:19:09AM +0100, Jiri Slaby wrote:
> On 29. 02. 24, 10:14, Pawan Gupta wrote:
> > On Wed, Feb 28, 2024 at 05:39:27PM -0800, Pawan Gupta wrote:
> > > On Tue, Feb 27, 2024 at 10:43:53AM +0100, Jiri Slaby wrote:
> > > > On 27. 02. 24, 9:47, Nikolay Borisov wrote:
> > > > > 
> > > > > 
> > > > > On 27.02.24 г. 1:52 ч., Pawan Gupta wrote:
> > > > > > Macro used for MDS mitigation executes VERW with relative addressing for
> > > > > > the operand. This is unnecessary and creates a problem for backports on
> > > > > > older kernels that don't support relocations in alternatives. Relocation
> > > > > > support was added by commit 270a69c4485d ("x86/alternative: Support
> > > > > > relocations in alternatives"). Also asm for fixed addressing is much
> > > > > > more cleaner than relative RIP addressing.
> > > > > > 
> > > > > > Simplify the asm by using fixed addressing for VERW operand.
> > > > > > 
> > > > > > Fixes: baf8361e5455 ("x86/bugs: Add asm helpers for executing VERW")
> > > > > > Reported-by: Nikolay Borisov <nik.borisov@suse.com>
> > > > > > Closes: https://lore.kernel.org/lkml/20558f89-299b-472e-9a96-171403a83bd6@suse.com/
> > > > > > Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
> > > > > > ---
> > > > > >    arch/x86/include/asm/nospec-branch.h | 2 +-
> > > > > >    1 file changed, 1 insertion(+), 1 deletion(-)
> > > > > > 
> > > > > > diff --git a/arch/x86/include/asm/nospec-branch.h
> > > > > > b/arch/x86/include/asm/nospec-branch.h
> > > > > > index 2aa52cab1e46..ab19c7f1167b 100644
> > > > > > --- a/arch/x86/include/asm/nospec-branch.h
> > > > > > +++ b/arch/x86/include/asm/nospec-branch.h
> > > > > > @@ -323,7 +323,7 @@
> > > > > >     * 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 mds_verw_sel),
> > > > > > X86_FEATURE_CLEAR_CPU_BUF
> > > > > 
> > > > > Actually thinking about it more and discussing with Jiri (cc'ed), will
> > > > > this work with KASLR enabled?
> > > > 
> > > > I might of course be wrong. We appear to rely on the asm+linker here.
> > > 
> > > You were right, with KASLR enabled, instructions with fixed addressing
> > > in alternatives don't get relocated. I guess we will have to keep
> > > rip-relative as is. Thanks for catching that.
> > 
> > Looks like this is not settled yet, it was naive of me to trust gdb on
> > /proc/kcore earlier with KASLR enabled.
> > 
> > With the below debug patch it appears the relocation with fixed
> > addresses is working as expected with KASLR enabled.
> 
> As I wrote already, asm+linker converts the fixed address to rip-rela
> anyway:
> 
> https://lore.kernel.org/all/fd8f2df0-563e-4f5c-aca4-bc92a14e9426@kernel.org/
> 
> I also raised questions in there:
> ====
> The assembler generates a relocation for the fixed address anyway. And
> the linker resolves it as rip-relative. At least the pair from my
> binutils-2.42.
> 
> But if it generates a rip-relative address, is < 6.5 with no support of
> rip-rel in alternatives still fine?

In that case backports < 6.5 can do:

+.macro CLEAR_CPU_BUFFERS
+	ALTERNATIVE "jmp .Lskip_verw_\@", "", X86_FEATURE_CLEAR_CPU_BUF
+	verw _ASM_RIP(mds_verw_sel)
+.Lskip_verw_\@:
+.endm

As done in Nikolay's 5.4 backport:

https://lore.kernel.org/stable/20240226122237.198921-3-nik.borisov@suse.com/

> Another question: can we rely on the assembler to generate a relocation
> and on the linker to resolve it as rip-relative?

This is definitely not my area of expertise, but with the above approach
VERW should be inlined always, and rip-relative should be resolved as
with any other instruction.

Re: [PATCH] x86/bugs: Use fixed addressing for VERW operand

[Pawan Gupta]

On Mon, Feb 26, 2024 at 03:52:33PM -0800, Pawan Gupta wrote:
> Macro used for MDS mitigation executes VERW with relative addressing for
> the operand. This is unnecessary and creates a problem for backports on
> older kernels that don't support relocations in alternatives. Relocation
> support was added by commit 270a69c4485d ("x86/alternative: Support
> relocations in alternatives"). Also asm for fixed addressing is much
> more cleaner than relative RIP addressing.
> 
> Simplify the asm by using fixed addressing for VERW operand.
> 
> Fixes: baf8361e5455 ("x86/bugs: Add asm helpers for executing VERW")
> Reported-by: Nikolay Borisov <nik.borisov@suse.com>
> Closes: https://lore.kernel.org/lkml/20558f89-299b-472e-9a96-171403a83bd6@suse.com/
> Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
> ---
>  arch/x86/include/asm/nospec-branch.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
> index 2aa52cab1e46..ab19c7f1167b 100644
> --- a/arch/x86/include/asm/nospec-branch.h
> +++ b/arch/x86/include/asm/nospec-branch.h
> @@ -323,7 +323,7 @@
>   * 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 mds_verw_sel), X86_FEATURE_CLEAR_CPU_BUF

Extremely sorry this change will not work with KASLR. The patch was
rushed to facilitate backports, and was only tested on qemu that had
KASLR disabled. :( Lesson learnt.

Please drop this patch.