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

[Catalin Marinas]

On Tue, Jan 26, 2021 at 11:58:13AM +0000, Vincenzo Frascino wrote:
> On 1/25/21 5:56 PM, Catalin Marinas wrote:
> > 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@xxxxxxx>
> >>>>>> Cc: Will Deacon <will@xxxxxxxxxx>
> >>>>>> Suggested-by: Catalin Marinas <catalin.marinas@xxxxxxx>
> >>>>>> Reviewed-by: Catalin Marinas <catalin.marinas@xxxxxxx>
> >>>>>> Signed-off-by: Vincenzo Frascino <vincenzo.frascino@xxxxxxx>
> >>>>>
> >>>>> 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.
> 
> Then call it preparatory patch ;)

It's preparatory only for the stable backports, not for current
mainline. But I'd rather change the upstream patch when backporting to
apply cleanly, no need for a preparatory stable patch.

-- 
Catalin