Re: [PATCH 02/10] x86/ucode: Delete the microcode_init() initcall

[Jan Beulich <jbeulich@xxxxxxxx>]

On 28.10.2024 18:12, Andrew Cooper wrote:
> On 28/10/2024 1:38 pm, Jan Beulich wrote:
>> On 28.10.2024 10:18, Andrew Cooper wrote:
>>> The comment highlights just how bogus this really is.  Being an initcall, 
>>> the
>>> boot allocator is long gone, and bootstrap_unmap() is a no-op.
>> How's the boot allocator coming into the picture here? This is all about
>> (un)mapping, not allocating.
>>
>>> The fact there is nothing to do should be a giant red flag about the 
>>> validity
>>> of the mappings "being freed".  Indeed, they both constitute 
>>> use-after-frees.
>> I can't spot any use-after-free; the pointers in question ...
>>
>>> --- a/xen/arch/x86/cpu/microcode/core.c
>>> +++ b/xen/arch/x86/cpu/microcode/core.c
>>> @@ -758,28 +758,6 @@ int microcode_update(XEN_GUEST_HANDLE(const_void) buf,
>>>      return continue_hypercall_on_cpu(0, microcode_update_helper, buffer);
>>>  }
>>>  
>>> -static int __init cf_check microcode_init(void)
>>> -{
>>> -    /*
>>> -     * At this point, all CPUs should have updated their microcode
>>> -     * via the early_microcode_* paths so free the microcode blob.
>>> -     */
>>> -    if ( ucode_blob.size )
>>> -    {
>>> -        bootstrap_unmap();
>>> -        ucode_blob.size = 0;
>>> -        ucode_blob.data = NULL;
>>> -    }
>>> -    else if ( ucode_mod.mod_end )
>>> -    {
>>> -        bootstrap_unmap();
>>> -        ucode_mod.mod_end = 0;
>>> -    }
>>> -
>>> -    return 0;
>>> -}
>>> -__initcall(microcode_init);
>> ... aren't used anywhere. bootstrap_unmap() is "just in case" (perhaps indeed
>> a no-op at least nowadays), and the rest is field clobbering. I'm okay with 
>> the
>> code change, so
>> Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
>> yet I'd like to ask for the description to be "softened" some.
> 
> As I said, this could be folded into patch 9, given this particular
> arrangement of the series.

I certainly don't mind the folding, but then I'd still like to see the
description of the resulting patch before giving a (hopefully) unconditional
ack.

> The UAFs are apparent *because* the comment demonstrates a false line of
> reasoning.
> 
> ucode_mod literally is used after free.  ucode=$n is genuinely buggy
> today, because its a stash of a physical pointer across move_xen().

Maybe my problem is that the UAF is elsewhere, not in the code you delete?
If so, it might help to simply point out where the actual bad use is. As it
stands, microcode_init() doesn't have any uses (reads), only writes. What I
agree with is that the comment there is at best misleading.

Jan

> ucode_blob stashes a virtual pointer.  This was even noticed in
> dc380df12acf ("x86/ucode: load microcode earlier on boot CPU")
> 
> ---
> It needs to rescan the modules in order to find the new virtual address
> of the ucode blob because it changes during the boot process, e.g.
> from 0x00000000010802fc to 0xffff83204dac52fc.
> ---
> 
> which highlighted the problem but duct-taped over it.
> 
> ~Andrew