Thanks Mark!
On 2023/2/7 18:48, Mark Rutland wrote:
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@xxxxxxxxxxx> 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@xxxxxxxxxxx>
---
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?
Yes,that's expected saving which I would like to address here.
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?).
Yes, by default we're setting rodata_full. While for low-ram target, we
also can lower the security requirement. This is why I am pursuing..
BTW, rodata_full can be easily runtime configured through bootargs.
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.
It sounds a good solution. Let me also dig into this. Many Thanks!
Thanks,
Mark.
