linux-kernel - Re: [PATCH v4 32/35] s390/uaccess: Add KMSAN support to put_user() and get

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Date: Tue, 18 Jun 2024 11:52:03 +0200
From: Alexander Potapenko <glider@...gle.com>
To: Ilya Leoshkevich <iii@...ux.ibm.com>
Cc: Alexander Gordeev <agordeev@...ux.ibm.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	Christoph Lameter <cl@...ux.com>, David Rientjes <rientjes@...gle.com>, Heiko Carstens <hca@...ux.ibm.com>, 
	Joonsoo Kim <iamjoonsoo.kim@....com>, Marco Elver <elver@...gle.com>, 
	Masami Hiramatsu <mhiramat@...nel.org>, Pekka Enberg <penberg@...nel.org>, 
	Steven Rostedt <rostedt@...dmis.org>, Vasily Gorbik <gor@...ux.ibm.com>, 
	Vlastimil Babka <vbabka@...e.cz>, Christian Borntraeger <borntraeger@...ux.ibm.com>, 
	Dmitry Vyukov <dvyukov@...gle.com>, Hyeonggon Yoo <42.hyeyoo@...il.com>, kasan-dev@...glegroups.com, 
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, linux-s390@...r.kernel.org, 
	linux-trace-kernel@...r.kernel.org, Mark Rutland <mark.rutland@....com>, 
	Roman Gushchin <roman.gushchin@...ux.dev>, Sven Schnelle <svens@...ux.ibm.com>
Subject: Re: [PATCH v4 32/35] s390/uaccess: Add KMSAN support to put_user()
 and get_user()

On Tue, Jun 18, 2024 at 11:40 AM Ilya Leoshkevich <iii@...ux.ibm.com> wrote:
>
> On Tue, 2024-06-18 at 11:24 +0200, Alexander Potapenko wrote:
> > On Thu, Jun 13, 2024 at 5:39 PM Ilya Leoshkevich <iii@...ux.ibm.com>
> > wrote:
> > >
> > > put_user() uses inline assembly with precise constraints, so Clang
> > > is
> > > in principle capable of instrumenting it automatically.
> > > Unfortunately,
> > > one of the constraints contains a dereferenced user pointer, and
> > > Clang
> > > does not currently distinguish user and kernel pointers. Therefore
> > > KMSAN attempts to access shadow for user pointers, which is not a
> > > right
> > > thing to do.
> > >
> > > An obvious fix to add __no_sanitize_memory to __put_user_fn() does
> > > not
> > > work, since it's __always_inline. And __always_inline cannot be
> > > removed
> > > due to the __put_user_bad() trick.
> > >
> > > A different obvious fix of using the "a" instead of the "+Q"
> > > constraint
> > > degrades the code quality, which is very important here, since it's
> > > a
> > > hot path.
> > >
> > > Instead, repurpose the __put_user_asm() macro to define
> > > __put_user_{char,short,int,long}_noinstr() functions and mark them
> > > with
> > > __no_sanitize_memory. For the non-KMSAN builds make them
> > > __always_inline in order to keep the generated code quality. Also
> > > define __put_user_{char,short,int,long}() functions, which call the
> > > aforementioned ones and which *are* instrumented, because they call
> > > KMSAN hooks, which may be implemented as macros.
> >
> > I am not really familiar with s390 assembly, but I think you still
> > need to call kmsan_copy_to_user() and kmsan_copy_from_user() to
> > properly initialize the copied data and report infoleaks.
> > Would it be possible to insert calls to linux/instrumented.h hooks
> > into uaccess functions?
>
> Aren't the existing instrument_get_user() / instrument_put_user() calls
> sufficient?

Oh, sorry, I overlooked them. Yes, those should be sufficient.
But you don't include linux/instrumented.h, do you?