[PATCH v3 00/12] mm/kasan: make kasan=on|off work for all three modes

Baoquan He
2025-08-20T05:35:18Z

Currently only hw_tags mode of kasan can be enabled or disabled with
kernel parameter kasan=on|off for built kernel. For kasan generic and
sw_tags mode, there's no way to disable them once kernel is built.
This is not convenient sometime, e.g in system kdump is configured.
When the 1st kernel has KASAN enabled and crash triggered to switch to
kdump kernel, the generic or sw_tags mode will cost much extra memory
for kasan shadow while in fact it's meaningless to have kasan in kdump
kernel.

So this patchset moves the kasan=on|off out of hw_tags scope and into
common code to make it visible in generic and sw_tags mode too. Then we
can add kasan=off in kdump kernel to reduce the unneeded meomry cost for
kasan.

Changelog:
====
v2->v3:
- Fix a building error on UML ARCH when CONFIG_KASAN is not set. The
change of fixing is appended into patch patch 11. This is reported
by LKP, thanks to them.

v1->v2:
- Add __ro_after_init for kasan_arg_disabled, and remove redundant blank
lines in mm/kasan/common.c. Thanks to Marco.
- Fix a code bug in <linux/kasan-enabled.h> when CONFIG_KASAN is unset,
this is found out by SeongJae and Lorenzo, and also reported by LKP
report, thanks to them.
- Add a missing kasan_enabled() checking in kasan_report(). This will
cause below KASAN report info even though kasan=off is set:
==================================================================
BUG: KASAN: stack-out-of-bounds in tick_program_event+0x130/0x150
Read of size 4 at addr ffff00005f747778 by task swapper/0/1

CPU: 0 UID: 0 PID: 1 Comm: swapper/0 Not tainted 6.16.0+ #8 PREEMPT(voluntary)
Hardware name: GIGABYTE R272-P30-JG/MP32-AR0-JG, BIOS F31n (SCP: 2.10.20220810) 09/30/2022
Call trace:
show_stack+0x30/0x90 (C)
dump_stack_lvl+0x7c/0xa0
print_address_description.constprop.0+0x90/0x310
print_report+0x104/0x1f0
kasan_report+0xc8/0x110
__asan_report_load4_noabort+0x20/0x30
tick_program_event+0x130/0x150
......snip...
==================================================================

- Add jump_label_init() calling before kasan_init() in setup_arch() in these
architectures: xtensa, arm. Because they currenly rely on
jump_label_init() in main() which is a little late. Then the early static
key kasan_flag_enabled in kasan_init() won't work.

- In UML architecture, change to enable kasan_flag_enabled in arch_mm_preinit()
because kasan_init() is enabled before main(), there's no chance to operate
on static key in kasan_init().

Test:
=====
In v1, I took test on x86_64 for generic mode, and on arm64 for
generic, sw_tags and hw_tags mode. All of them works well.

In v2, I only tested on arm64 for generic, sw_tags and hw_tags mode, it
works. For powerpc, I got a BOOK3S/64 machine, while it says
'KASAN not enabled as it requires radix' and KASAN is disabled. Will
look for other POWER machine to test this.

In v3, I only built UML kernel successfully w and w/o CONFIG_KASAN
setting with LKP's testing steps.
====

Baoquan He (12):
mm/kasan: add conditional checks in functions to return directly if
kasan is disabled
mm/kasan: move kasan= code to common place
mm/kasan/sw_tags: don't initialize kasan if it's disabled
arch/arm: don't initialize kasan if it's disabled
arch/arm64: don't initialize kasan if it's disabled
arch/loongarch: don't initialize kasan if it's disabled
arch/powerpc: don't initialize kasan if it's disabled
arch/riscv: don't initialize kasan if it's disabled
arch/x86: don't initialize kasan if it's disabled
arch/xtensa: don't initialize kasan if it's disabled
arch/um: don't initialize kasan if it's disabled
mm/kasan: make kasan=on|off take effect for all three modes

arch/arm/kernel/setup.c | 6 +++++
arch/arm/mm/kasan_init.c | 6 +++++
arch/arm64/mm/kasan_init.c | 7 ++++++
arch/loongarch/mm/kasan_init.c | 5 ++++
arch/powerpc/mm/kasan/init_32.c | 8 +++++-
arch/powerpc/mm/kasan/init_book3e_64.c | 6 +++++
arch/powerpc/mm/kasan/init_book3s_64.c | 6 +++++
arch/riscv/mm/kasan_init.c | 6 +++++
arch/um/kernel/mem.c | 7 ++++++
arch/x86/mm/kasan_init_64.c | 6 +++++
arch/xtensa/kernel/setup.c | 1 +
arch/xtensa/mm/kasan_init.c | 6 +++++
include/linux/kasan-enabled.h | 18 ++++++-------
mm/kasan/common.c | 25 ++++++++++++++++++
mm/kasan/generic.c | 20 +++++++++++++--
mm/kasan/hw_tags.c | 35 ++------------------------
mm/kasan/init.c | 6 +++++
mm/kasan/quarantine.c | 3 +++
mm/kasan/report.c | 4 ++-
mm/kasan/shadow.c | 23 ++++++++++++++++-
mm/kasan/sw_tags.c | 9 +++++++
21 files changed, 166 insertions(+), 47 deletions(-)

--
2.41.0