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

On Tue, Jan 26, 2021 at 11:58:13AM +0000, Vincenzo Frascino wrote:

On 1/25/21 5:56 PM, Catalin Marinas wrote:

quoted

On Mon, Jan 25, 2021 at 04:09:57PM +0000, Vincenzo Frascino wrote:

quoted

On 1/25/21 2:59 PM, Catalin Marinas wrote:

quoted

On Mon, Jan 25, 2021 at 02:36:34PM +0000, Vincenzo Frascino wrote:

quoted

On 1/25/21 1:02 PM, Mark Rutland wrote:

quoted

On Fri, Jan 22, 2021 at 03:56:40PM +0000, Vincenzo Frascino wrote:

quoted

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.

quoted

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

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