Re: [PATCH v4 1/3] arm64: Improve kernel address detection of __is_lm_address()

[Catalin Marinas <catalin.marinas@arm.com>]

On Mon, Jan 25, 2021 at 04:09:57PM +0000, Vincenzo Frascino wrote:
> On 1/25/21 2:59 PM, Catalin Marinas wrote:
> > On Mon, Jan 25, 2021 at 02:36:34PM +0000, Vincenzo Frascino wrote:
> >> On 1/25/21 1:02 PM, Mark Rutland wrote:
> >>> On Fri, Jan 22, 2021 at 03:56:40PM +0000, Vincenzo Frascino wrote:
> >>>> Currently, the __is_lm_address() check just masks out the top 12 bits
> >>>> of the address, but if they are 0, it still yields a true result.
> >>>> This has as a side effect that virt_addr_valid() returns true even for
> >>>> invalid virtual addresses (e.g. 0x0).
> >>>>
> >>>> Improve the detection checking that it's actually a kernel address
> >>>> starting at PAGE_OFFSET.
> >>>>
> >>>> Cc: Catalin Marinas <catalin.marinas@arm.com>
> >>>> Cc: Will Deacon <will@kernel.org>
> >>>> Suggested-by: Catalin Marinas <catalin.marinas@arm.com>
> >>>> Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
> >>>> Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
> >>>
> >>> Looking around, it seems that there are some existing uses of
> >>> virt_addr_valid() that expect it to reject addresses outside of the
> >>> TTBR1 range. For example, check_mem_type() in drivers/tee/optee/call.c.
> >>>
> >>> Given that, I think we need something that's easy to backport to stable.
> >>>
> >>
> >> I agree, I started looking at it this morning and I found cases even in the main
> >> allocators (slub and page_alloc) either then the one you mentioned.
> >>
> >>> This patch itself looks fine, but it's not going to backport very far,
> >>> so I suspect we might need to write a preparatory patch that adds an
> >>> explicit range check to virt_addr_valid() which can be trivially
> >>> backported.
> >>>
> >>
> >> I checked the old releases and I agree this is not back-portable as it stands.
> >> I propose therefore to add a preparatory patch with the check below:
> >>
> >> #define __is_ttrb1_address(addr)	((u64)(addr) >= PAGE_OFFSET && \
> >> 					(u64)(addr) < PAGE_END)
> >>
> >> If it works for you I am happy to take care of it and post a new version of my
> >> patches.
> > 
> > I'm not entirely sure we need a preparatory patch. IIUC (it needs
> > checking), virt_addr_valid() was fine until 5.4, broken by commit
> > 14c127c957c1 ("arm64: mm: Flip kernel VA space"). Will addressed the
> > flip case in 68dd8ef32162 ("arm64: memory: Fix virt_addr_valid() using
> > __is_lm_address()") but this broke the <PAGE_OFFSET case. So in 5.4 a
> > NULL address is considered valid.
> > 
> > Ard's commit f4693c2716b3 ("arm64: mm: extend linear region for 52-bit
> > VA configurations") changed the test to no longer rely on va_bits but
> > did not change the broken semantics.
> > 
> > If Ard's change plus the fix proposed in this test works on 5.4, I'd say
> > we just merge this patch with the corresponding Cc stable and Fixes tags
> > and tweak it slightly when doing the backports as it wouldn't apply
> > cleanly. IOW, I wouldn't add another check to virt_addr_valid() as we
> > did not need one prior to 5.4.
> 
> Thank you for the detailed analysis. I checked on 5.4 and it seems that Ard
> patch (not a clean backport) plus my proposed fix works correctly and solves the
> issue.

I didn't mean the backport of the whole commit f4693c2716b3 as it
probably has other dependencies, just the __is_lm_address() change in
that patch.

> Tomorrow I will post a new version of the series that includes what you are
> suggesting.

Please post the __is_lm_address() fix separately from the kasan patches.
I'll pick it up as a fix via the arm64 tree. The kasan change can go in
5.12 since it's not currently broken but I'll leave the decision with
Andrey.

-- 
Catalin

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