Re: [PATCH 35/35] kasan: add documentation for hardware tag-based mode

[Andrey Konovalov]

On Fri, Aug 28, 2020 at 1:12 PM Marco Elver <elver@xxxxxxxxxx> wrote:
>
> On Fri, Aug 14, 2020 at 07:27PM +0200, Andrey Konovalov wrote:
> > Add documentation for hardware tag-based KASAN mode and also add some
> > clarifications for software tag-based mode.
> >
> > Signed-off-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
> > ---
> >  Documentation/dev-tools/kasan.rst | 73 +++++++++++++++++++++----------
> >  1 file changed, 51 insertions(+), 22 deletions(-)
> >
> > diff --git a/Documentation/dev-tools/kasan.rst b/Documentation/dev-tools/kasan.rst
> > index a3030fc6afe5..aeed89d6eaf5 100644
> [...]
> > -Tag-based KASAN uses the Top Byte Ignore (TBI) feature of modern arm64 CPUs to
> > -store a pointer tag in the top byte of kernel pointers. Like generic KASAN it
> > -uses shadow memory to store memory tags associated with each 16-byte memory
> > -cell (therefore it dedicates 1/16th of the kernel memory for shadow memory).
> > +Software tag-based KASAN uses the Top Byte Ignore (TBI) feature of modern arm64
> > +CPUs to store a pointer tag in the top byte of kernel pointers. Like generic
> > +KASAN it uses shadow memory to store memory tags associated with each 16-byte
> > +memory cell (therefore it dedicates 1/16th of the kernel memory for shadow
> > +memory).
>
> It might be helpful to be more specific vs. saying "modern arm64 CPUs".
> Does the "modern" qualifier suggest not all arm64 CPUs support the
> feature?  (HW tag-based KASAN below is specific, and mentions ARMv8.5.)

Will clarify this in v2.

> > +On each memory allocation software tag-based KASAN generates a random tag, tags
> > +the allocated memory with this tag, and embeds this tag into the returned
> > +pointer.
> >
> > -On each memory allocation tag-based KASAN generates a random tag, tags the
> > -allocated memory with this tag, and embeds this tag into the returned pointer.
> >  Software tag-based KASAN uses compile-time instrumentation to insert checks
> >  before each memory access. These checks make sure that tag of the memory that
> >  is being accessed is equal to tag of the pointer that is used to access this
> > -memory. In case of a tag mismatch tag-based KASAN prints a bug report.
> > +memory. In case of a tag mismatch software tag-based KASAN prints a bug report.
> >
> >  Software tag-based KASAN also has two instrumentation modes (outline, that
> >  emits callbacks to check memory accesses; and inline, that performs the shadow
> > @@ -215,9 +222,31 @@ simply printed from the function that performs the access check. With inline
> >  instrumentation a brk instruction is emitted by the compiler, and a dedicated
> >  brk handler is used to print bug reports.
> >
> > -A potential expansion of this mode is a hardware tag-based mode, which would
> > -use hardware memory tagging support instead of compiler instrumentation and
> > -manual shadow memory manipulation.
> > +Software tag-based KASAN uses 0xFF as a match-all pointer tag (accesses aren't
> > +checked).
> > +
> > +Software tag-based KASAN currently only supports tagging of slab memory.
> > +
> > +Hardware tag-based KASAN
> > +~~~~~~~~~~~~~~~~~~~~~~~~
> > +
> > +Hardware tag-based KASAN is similar to the software mode in concept, but uses
> > +hardware memory tagging support instead of compiler instrumentation and
> > +shadow memory.
> > +
> > +Hardware tag-based KASAN is based on both arm64 Memory Tagging Extension (MTE)
> > +introduced in ARMv8.5 Instruction Set Architecture, and Top Byte Ignore (TBI).
>
> Is there anything inherently tying tag-based KASAN to arm64?

Not really, the approach is generic and can be used by any arch that
supports memory tagging.

> I guess if
> some other architecture supports MTE, they just have to touch arch/,
> right?

For the most part - yes, but maybe adjustments to the generic code
will be required. No way to know before one tries to integrate another
arch.

> You could reword to say that "Hardware tag-based KASAN is currently only
> supported on the ARM64 architecture.
>
> On the ARM64 architecture, tag-based KASAN is based on both ..."

Will do in v2, thanks!