Re: [PATCH v2 3/3] selftests/mm: virtual_address_range: Avoid reading VVAR mappings

[Thomas Weißschuh]

On Fri, Jan 10, 2025 at 04:41:03PM +0100, David Hildenbrand wrote:
> 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@xxxxxxxxx>
> > Closes: https://lore.kernel.org/oe-lkp/202412271148.2656e485-lkp@xxxxxxxxx
> > Signed-off-by: Thomas Weißschuh <thomas.weissschuh@xxxxxxxxxxxxx>
> > ---
> >   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 ...

Agreed.

Can we instead rely on madvise(MADV_DOFORK) returning EINVAL iff VM_IO?
That would remove the need for custom parsing and the dependency on
CONFIG_PROC_PAGE_MONITOR.


Thomas