Re: [PATCH] mm: kfence: export kfence_enabled as global variables

[Mark Rutland <mark.rutland@arm.com>]

On Tue, Feb 07, 2023 at 03:46:53PM +0800, Zhenhua Huang wrote:
> Thanks Marco!
> 
> On 2023/2/7 15:19, Marco Elver wrote:
> > On Tue, 7 Feb 2023 at 07:15, Zhenhua Huang <quic_zhenhuah@quicinc.com> wrote:
> > > 
> > > Export the variable to ease the judgement of whether kfence enabled
> > > at runtime. It should be more precise than through kernel config
> > > "CONFIG_KFENCE".
> > > 
> > > For example We can disable kfence at runtime using bootargs
> > > "kfence.sample_interval=0" but CONFIG_KFENCE enabled.
> > > It was false positive.
> > > 
> > > Signed-off-by: Zhenhua Huang <quic_zhenhuah@quicinc.com>
> > > ---
> > >   arch/arm64/mm/pageattr.c | 3 ++-
> > >   include/linux/kfence.h   | 2 ++
> > >   mm/kfence/core.c         | 2 +-
> > >   3 files changed, 5 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c
> > > index 79dd201..208d780 100644
> > > --- a/arch/arm64/mm/pageattr.c
> > > +++ b/arch/arm64/mm/pageattr.c
> > > @@ -7,6 +7,7 @@
> > >   #include <linux/module.h>
> > >   #include <linux/sched.h>
> > >   #include <linux/vmalloc.h>
> > > +#include <linux/kfence.h>
> > > 
> > >   #include <asm/cacheflush.h>
> > >   #include <asm/set_memory.h>
> > > @@ -27,7 +28,7 @@ bool can_set_direct_map(void)
> > >           * protect/unprotect single pages.
> > >           */
> > >          return (rodata_enabled && rodata_full) || debug_pagealloc_enabled() ||
> > > -               IS_ENABLED(CONFIG_KFENCE);
> > > +               kfence_enabled;
> > 
> > Unfortunately this won't work, because it's possible to enable KFENCE
> > after the kernel has booted with e.g.: echo 100 >
> > /sys/module/kfence/parameters/sample_interval
> Yeah, got it. Thanks for catching it.
> > 
> > What is the problem you have encountered? Is the page-granular direct
> > map causing issues?
> We're working on a low memory target, page-granular mapping costed more (2M
> per 1GB) memory. Due to GKI constraints, it is not easy to disable
> CONFIG_KFENCE. So my intention was to move the judgement to runtime
> configurable w/ CONFIG_KFENCE on...
> 
> Do you have any further suggestion/proposal on this? Many Thanks!

Just to check, the cost is because we're mapping *all* of memory at page
granulatrity, right? If we were to just map the KFENCE region a page
granularity, would that be a sufficient saving? 

We didn't do that so far because it was simpler to just map everything at page
granularity (and that's also required by rodata_full, which I though android
used?).

If it's really important (and rodata_full isn't being used), we could try to do
that.

To do that we'd need to choose the KFENCE region *before* arm64 creates the
fine-grain translation tables, which probable needs an arch_ hook.

Thanks,
Mark.

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