Re: [PATCH v6 08/13] arm64: expose user PAC bit positions via ptrace

On 12/10/18 6:03 AM, Catalin Marinas wrote:

quoted

However, it won't be too long before someone implements support for
ARMv8.2-LVA, at which point, without changes to mandatory pointer tagging, we
will only have 3 authentication bits: [54:52].  This seems useless and easily
brute-force-able.

Such support is already here (about to be queued):

https://lore.kernel.org/linux-arm-kernel/20181206225042.11548-1-steve.capper@arm.com/ (local)

Thanks for the pointer.

quoted

Unfortunately, there is no obvious path to making this optional that does not
break compatibility with Documentation/arm64/tagged-pointers.txt.

There is also the ARMv8.5 MTE (memory tagging) which relies on tagged
pointers.

So it does.  I hadn't read through that extension completely before.

An alternative would be to allow the opt-in to 52-bit VA, leaving it at
48-bit by default. However, it has the problem of changing the PAC size
and not being able to return.

Perhaps the opt-in should be at exec time, with ELF flags (or equivalent) on
the application.  Because, as you say, changing the shape of the PAC in the
middle of execution is in general not possible.

It isn't perfect, since old kernels won't fail to exec an application setting
flags that can't be supported.  And it requires tooling changes.


r~

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