Re: [PATCH v4 0/7] arm64: untag user pointers passed to the kernel

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

On Mon, Jul 16, 2018 at 01:25:59PM +0200, Andrey Konovalov wrote:
> On Thu, Jun 28, 2018 at 9:30 PM, Andrey Konovalov <andreyknvl@google.com> wrote:
> > On Wed, Jun 27, 2018 at 5:05 PM, Andrey Konovalov <andreyknvl@google.com> wrote:
> >> On Tue, Jun 26, 2018 at 7:29 PM, Catalin Marinas
> >> <catalin.marinas@arm.com> wrote:
> >>> While I support this work, as a maintainer I'd like to understand
> >>> whether we'd be in a continuous chase of ABI breaks with every kernel
> >>> release or we have a better way to identify potential issues. Is there
> >>> any way to statically analyse conversions from __user ptr to long for
> >>> example? Or, could we get the compiler to do this for us?
> >>
> >> OK, got it, I'll try to figure out a way to find these conversions.
> >
> > I've prototyped a checker on top of clang static analyzer (initially
> > looked at sparse, but couldn't find any documentation or examples).
> > The results are here [1], search for "warning: user pointer cast".
> > Sharing in case anybody wants to take a look, will look at them myself
> > tomorrow.
> >
> > [1] https://gist.github.com/xairy/433edd5c86456a64026247cb2fef2115
> 
> So the checker reports ~100 different places where a __user pointer
> being casted. I've looked through them and found 3 places where we
> need to add untagging. Source code lines below come from 4.18-rc2+
> (6f0d349d).
[...]
> I'll add the 3 patches with fixes to v5 of this patchset.

Thanks for investigating. You can fix those three places in your code
but I was rather looking for a way to check such casting in the future
for newly added code. While for the khwasan we can assume it's a debug
option, the tagged user pointers are ABI and we need to keep it stable.

We could we actually add some macros for explicit conversion between
__user ptr and long and silence the warning there (I guess this would
work better for sparse). We can then detect new ptr to long casts as
they appear. I just hope that's not too intrusive.

(I haven't tried the sparse patch yet, hopefully sometime this week)

-- 
Catalin