Re: [PATCH v2 3/3] riscv: Fix crash when flushing executable ioremap regions

[Alex Ghiti <alex@ghiti.fr>]

Hi Jan,

On 2/16/20 2:56 PM, Alex Ghiti wrote:
> On 2/16/20 11:05 AM, Jan Kiszka wrote:
>> On 16.02.20 15:41, Alex Ghiti wrote:
>>> Hi Jan,
>>>
>>> On 2/15/20 6:49 AM, Jan Kiszka wrote:
>>>> From: Jan Kiszka <jan.kiszka@siemens.com>
>>>>
>>>> Those are not backed by page structs, and pte_page is returning an
>>>> invalid pointer.
>>>>
>>>> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
>>>> =2D--
>>>>   arch/riscv/mm/cacheflush.c | 3 ++-
>>>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/arch/riscv/mm/cacheflush.c b/arch/riscv/mm/cacheflush.c
>>>> index 8930ab7278e6..9ee2c1a387cc 100644
>>>> =2D-- a/arch/riscv/mm/cacheflush.c
>>>> +++ b/arch/riscv/mm/cacheflush.c
>>>> @@ -84,7 +84,8 @@ void flush_icache_pte(pte_t pte)
>>>>   {
>>>>       struct page *page =3D pte_page(pte);
>>>>
>>>> -    if (!test_and_set_bit(PG_dcache_clean, &page->flags))
>>>> +    if (!pfn_valid(pte_pfn(pte)) ||
>>>> +        !test_and_set_bit(PG_dcache_clean, &page->flags))
>>>>           flush_icache_all();
>>>>   }
>>>>   #endif /* CONFIG_MMU */
>>>> =2D-
>>>> 2.16.4
>>>>
>>>>
>>>
>>> When did you encounter such a situation ? i.e. executable code that is
>>> not backed by struct page ?
>>>
>>> Riscv uses the generic implementation of ioremap and the way
>>> _PAGE_IOREMAP is defined does not allow to map executable memory region
>>> using ioremap, so I'm interested to understand how we end up in
>>> flush_icache_pte for an executable region not backed by any struct page.
>>
>> You can create executable mappings of memory that Linux does not
>> initially consider as RAM via ioremap_prot or ioremap_page_range. We are
>> using that in Jailhouse to load the hypervisor code into reserved memory
>> that is ioremapped for the purpose. Works fine on x86, arm and arm64.
>>
>> Jan
> 
> Ok thanks, I had missed this API.
> 
> Regarding your patch, I find it weird to do anything if the pfn is 
> invalid, we could have garbage in pte pointing to an invalid region for 
> example (I admit that the effect of flushing the icache would not be 
> catastrophic in that situation).
> 
> I'm not saying I will come with a better solution but I'll take a deeper 
> look tomorrow.
> 
> Alex
> 

I took a look at the Jailhouse driver. After loading the hypervisor into 
the ioremapped region, it explicitly ensures icache/dcache consistency 
by calling flush_icache_range here:

https://github.com/siemens/jailhouse/blob/master/driver/main.c#L505

There seems to be an implicit (?) rule that states that in-kernel code 
modification must handle icache/dcache consistency:

In arm64 set_pte_at definition, they do not sync icache/dcache when the 
pte is kernel:

https://elixir.bootlin.com/linux/latest/source/arch/arm64/include/asm/pgtable.h#L271

In mips, they do the same:

https://elixir.bootlin.com/linux/latest/source/arch/mips/mm/cache.c#L137

So funnily, I'd do the contrary of what you have done, the mips way:

diff --git a/arch/riscv/mm/cacheflush.c b/arch/riscv/mm/cacheflush.c
index 8930ab7278e6..c90c8bb49109 100644
--- a/arch/riscv/mm/cacheflush.c
+++ b/arch/riscv/mm/cacheflush.c
@@ -84,6 +84,9 @@ void flush_icache_pte(pte_t pte)
  {
         struct page *page = pte_page(pte);

+       if (unlikely(!pfn_valid(pte_pfn(pte))))
+               return;
+
         if (!test_and_set_bit(PG_dcache_clean, &page->flags))
                 flush_icache_all();
  }

What do you think ?

Alex