linux-kernel - Re: [PATCH v4 4/4] selftests/mm: virtual_address_range: Avoid reading from VM

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Message-ID: <56a1657c-d60f-4cdc-88a7-826ad977890f@redhat.com>
Date: Tue, 14 Jan 2025 17:53:58 +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>,
 Anshuman Khandual <khandual@...ux.vnet.ibm.com>
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 v4 4/4] selftests/mm: virtual_address_range: Avoid reading
 from VM_IO mappings

On 14.01.25 17:06, 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.
> 
> Check for the VM_IO flag as a proxy.
> It is present for the VVAR mappings and MMIO ranges can be dangerous to
> access arbitrarily.
> 
> Reported-by: kernel test robot <oliver.sang@...el.com>
> Closes: https://lore.kernel.org/oe-lkp/202412271148.2656e485-lkp@intel.com
> Fixes: e93d2521b27f ("x86/vdso: Split virtual clock pages into dedicated mapping")
> Fixes: 010409649885 ("selftests/mm: confirm VA exhaustion without reliance on correctness of mmap()")
> Suggested-by: David Hildenbrand <david@...hat.com>
> Link: https://lore.kernel.org/lkml/e97c2a5d-c815-4936-a767-ac42a3220a90@redhat.com/
> Signed-off-by: Thomas Weißschuh <thomas.weissschuh@...utronix.de>

Acked-by: David Hildenbrand <david@...hat.com>

Unfortunately, vsyscall doesn't seem to set VM_IO, it only has

VmFlags: ex


Which is rather weird.

So we cannot remove that special-casing right now,

-- 
Cheers,

David / dhildenb