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

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Message-ID: <b63e5945-0e31-940f-5ff7-6754ef5c034f@web.de>
Date:   Sun, 16 Feb 2020 17:05:55 +0100
From:   Jan Kiszka <jan.kiszka@....de>
To:     Alex Ghiti <alex@...ti.fr>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        linux-riscv@...ts.infradead.org
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/3] riscv: Fix crash when flushing executable ioremap
 regions

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@...mens.com>
>>
>> Those are not backed by page structs, and pte_page is returning an
>> invalid pointer.
>>
>> Signed-off-by: Jan Kiszka <jan.kiszka@...mens.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