linux-kernel - Re: [PATCH v2 3/3] selftests/mm: virtual_address

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Message-ID: <9243dd8e-3f03-4ed5-bdcf-95c947c57849@redhat.com>
Date: Fri, 10 Jan 2025 16:41:03 +0100
From: David Hildenbrand <david@...hat.com>
To: Thomas Weißschuh <thomas.weissschuh@...utronix.de>,
 Andrew Morton <akpm@...ux-foundation.org>, Shuah Khan <shuah@...nel.org>,
 Dev Jain <dev.jain@....com>, Thomas Gleixner <tglx@...utronix.de>
Cc: linux-mm@...ck.org, linux-kselftest@...r.kernel.org,
 linux-kernel@...r.kernel.org, kernel test robot <oliver.sang@...el.com>
Subject: Re: [PATCH v2 3/3] selftests/mm: virtual_address_range: Avoid reading
 VVAR mappings

On 10.01.25 14:05, Thomas Weißschuh wrote:
> The virtual_address_range selftest reads from the start of each mapping
> listed in /proc/self/maps.
> However not all mappings are valid to be arbitrarily accessed.
> For example the vvar data used for virtual clocks on x86 [vvar_vclock]
> can only be accessed if 1) the kernel configuration enables virtual
> clocks and 2) the hypervisor provided the data for it.
> Only the VDSO itself has the necessary information to know this.
> Since commit e93d2521b27f ("x86/vdso: Split virtual clock pages into dedicated mapping")
> the virtual clock data was split out into its own mapping, leading
> to EFAULT from read() during the validation.
> 
> Skip the various vvar mappings in virtual_address_range to avoid the issue.
> 
> Fixes: e93d2521b27f ("x86/vdso: Split virtual clock pages into dedicated mapping")
> Fixes: 010409649885 ("selftests/mm: confirm VA exhaustion without reliance on correctness of mmap()")
> Reported-by: kernel test robot <oliver.sang@...el.com>
> Closes: https://lore.kernel.org/oe-lkp/202412271148.2656e485-lkp@intel.com
> Signed-off-by: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
> ---
>   tools/testing/selftests/mm/virtual_address_range.c | 4 ++++
>   1 file changed, 4 insertions(+)
> 
> diff --git a/tools/testing/selftests/mm/virtual_address_range.c b/tools/testing/selftests/mm/virtual_address_range.c
> index 4fc1c21a5e218eaec4d059b75c31a21dd4e8a215..993990aba56fc986c42084ffa91973558aa07e87 100644
> --- a/tools/testing/selftests/mm/virtual_address_range.c
> +++ b/tools/testing/selftests/mm/virtual_address_range.c
> @@ -152,6 +152,10 @@ static int validate_complete_va_space(void)
>   		if (prot[0] != 'r')
>   			continue;
>   
> +		/* Only the VDSO can know if a VVAR mapping is really readable */
> +		if (vma_name && !strncmp(vma_name, "[vvar", 5))
> +			continue;

I'm wondering if there is a more generic way ... but likely not when 
staring at /proc/self/maps.

/proc/self/smaps would indicate this as

VM_IO: "io"
VM_DONTDUMP: "dd"
VM_PFNMAP: "pf"

Especially checking for VM_IO sounds reasonable ...

-- 
Cheers,

David / dhildenb