On Fri, Feb 5, 2021 at 6:19 PM Will Deacon [off-list ref] wrote:

On Fri, Feb 05, 2021 at 12:37:21AM +0800, Lecopzer Chen wrote:

quoted

quoted

On Thu, Feb 04, 2021 at 10:46:12PM +0800, Lecopzer Chen wrote:

quoted

quoted

On Sat, Jan 09, 2021 at 06:32:49PM +0800, Lecopzer Chen wrote:

quoted

Linux support KAsan for VMALLOC since commit 3c5c3cfb9ef4da9
("kasan: support backing vmalloc space with real shadow memory")

Like how the MODULES_VADDR does now, just not to early populate
the VMALLOC_START between VMALLOC_END.
similarly, the kernel code mapping is now in the VMALLOC area and
should keep these area populated.

Signed-off-by: Lecopzer Chen <redacted>
---
 arch/arm64/mm/kasan_init.c | 23 ++++++++++++++++++-----
 1 file changed, 18 insertions(+), 5 deletions(-)

diff --git a/arch/arm64/mm/kasan_init.c b/arch/arm64/mm/kasan_init.c
index d8e66c78440e..39b218a64279 100644
--- a/arch/arm64/mm/kasan_init.c
+++ b/arch/arm64/mm/kasan_init.c

@@ -214,6 +214,7 @@ static void __init kasan_init_shadow(void)
 {
  u64 kimg_shadow_start, kimg_shadow_end;
  u64 mod_shadow_start, mod_shadow_end;
+ u64 vmalloc_shadow_start, vmalloc_shadow_end;
  phys_addr_t pa_start, pa_end;
  u64 i;

@@ -223,6 +224,9 @@ static void __init kasan_init_shadow(void)
  mod_shadow_start = (u64)kasan_mem_to_shadow((void *)MODULES_VADDR);
  mod_shadow_end = (u64)kasan_mem_to_shadow((void *)MODULES_END);

+ vmalloc_shadow_start = (u64)kasan_mem_to_shadow((void *)VMALLOC_START);
+ vmalloc_shadow_end = (u64)kasan_mem_to_shadow((void *)VMALLOC_END);
+
  /*
   * We are going to perform proper setup of shadow memory.
   * At first we should unmap early shadow (clear_pgds() call below).

@@ -241,12 +245,21 @@ static void __init kasan_init_shadow(void)

  kasan_populate_early_shadow(kasan_mem_to_shadow((void *)PAGE_END),
                             (void *)mod_shadow_start);
- kasan_populate_early_shadow((void *)kimg_shadow_end,
-                            (void *)KASAN_SHADOW_END);
+ if (IS_ENABLED(CONFIG_KASAN_VMALLOC)) {

Do we really need yet another CONFIG option for KASAN? What's the use-case
for *not* enabling this if you're already enabling one of the KASAN
backends?

As I know, KASAN_VMALLOC now only supports KASAN_GENERIC and also
KASAN_VMALLOC uses more memory to map real shadow memory (1/8 of vmalloc va).

The shadow is allocated dynamically though, isn't it?

Yes, but It's still a cost.

quoted

quoted

There should be someone can enable KASAN_GENERIC but can't use VMALLOC
due to memory issue.

That doesn't sound particularly realistic to me. The reason I'm pushing here
is because I would _really_ like to move to VMAP stack unconditionally, and
that would effectively force KASAN_VMALLOC to be set if KASAN is in use.

So unless there's a really good reason not to do that, please can we make
this unconditional for arm64? Pretty please?

I think it's fine since we have a good reason.
Also if someone have memory issue in KASAN_VMALLOC,
they can use SW_TAG, right?

However the SW_TAG/HW_TAG is not supported VMALLOC yet.
So the code would be like

      if (IS_ENABLED(CONFIG_KASAN_GENERIC))

Just make this CONFIG_KASAN_VMALLOC, since that depends on KASAN_GENERIC.

quoted

              /* explain the relationship between
               * KASAN_GENERIC and KASAN_VMALLOC in arm64
               * XXX: because we want VMAP stack....
               */

I don't understand the relation with SW_TAGS. The VMAP_STACK dependency is:

        depends on !KASAN || KASAN_HW_TAGS || KASAN_VMALLOC

This means that VMAP_STACK can be only enabled if KASAN_HW_TAGS=y or
if KASAN_VMALLOC=y for other modes.

which doesn't mention SW_TAGS at all. So that seems to imply that SW_TAGS
and VMAP_STACK are mutually exclusive :(

SW_TAGS doesn't yet have vmalloc support, so it's not compatible with
VMAP_STACK. Once vmalloc support is added to SW_TAGS, KASAN_VMALLOC
should be allowed to be enabled with SW_TAGS. This series is a step
towards having that support, but doesn't implement it. That will be a
separate effort.

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