[PATCH mm v4 31/39] kasan, vmalloc: only tag normal vmalloc allocations

andrey.konovalov
2021-12-20T22:02:31Z

The kernel can use to allocate executable memory. The only supported way
to do that is via __vmalloc_node_range() with the executable bit set in
the prot argument. (vmap() resets the bit via pgprot_nx()).

Once tag-based KASAN modes start tagging vmalloc allocations, executing
code from such allocations will lead to the PC register getting a tag,
which is not tolerated by the kernel.

Only tag the allocations for normal kernel pages.

Signed-off-by: Andrey Konovalov <andreyknvl@google.com>

---

Changes v3->v4:
- Rename KASAN_VMALLOC_NOEXEC to KASAN_VMALLOC_PROT_NORMAL.
- Compare with PAGE_KERNEL instead of using pgprot_nx().
- Update patch description.

Changes v2->v3:
- Add this patch.
---
include/linux/kasan.h | 7 ++++---
mm/kasan/hw_tags.c | 7 +++++++
mm/kasan/shadow.c | 7 +++++++
mm/vmalloc.c | 49 +++++++++++++++++++++++++------------------
4 files changed, 47 insertions(+), 23 deletions(-)

diff --git a/include/linux/kasan.h b/include/linux/kasan.h
index 499f1573dba4..3593c95d1fa5 100644
--- a/include/linux/kasan.h
+++ b/include/linux/kasan.h
@@ -27,9 +27,10 @@ struct kunit_kasan_expectation {

typedef unsigned int __bitwise kasan_vmalloc_flags_t;

-#define KASAN_VMALLOC_NONE 0x00u
-#define KASAN_VMALLOC_INIT 0x01u
-#define KASAN_VMALLOC_VM_ALLOC 0x02u
+#define KASAN_VMALLOC_NONE 0x00u
+#define KASAN_VMALLOC_INIT 0x01u
+#define KASAN_VMALLOC_VM_ALLOC 0x02u
+#define KASAN_VMALLOC_PROT_NORMAL 0x04u

#if defined(CONFIG_KASAN_GENERIC) || defined(CONFIG_KASAN_SW_TAGS)

diff --git a/mm/kasan/hw_tags.c b/mm/kasan/hw_tags.c
index 21104fd51872..2e9378a4f07f 100644
--- a/mm/kasan/hw_tags.c
+++ b/mm/kasan/hw_tags.c
@@ -247,6 +247,13 @@ void *__kasan_unpoison_vmalloc(const void *start, unsigned long size,
if (!(flags & KASAN_VMALLOC_VM_ALLOC))
return (void *)start;

+ /*
+ * Don't tag executable memory.
+ * The kernel doesn't tolerate having the PC register tagged.
+ */
+ if (!(flags & KASAN_VMALLOC_PROT_NORMAL))
+ return (void *)start;
+
tag = kasan_random_tag();
start = set_tag(start, tag);

diff --git a/mm/kasan/shadow.c b/mm/kasan/shadow.c
index b958babc8fed..7272e248db87 100644
--- a/mm/kasan/shadow.c
+++ b/mm/kasan/shadow.c
@@ -488,6 +488,13 @@ void *__kasan_unpoison_vmalloc(const void *start, unsigned long size,
if (!is_vmalloc_or_module_addr(start))
return (void *)start;

+ /*
+ * Don't tag executable memory.
+ * The kernel doesn't tolerate having the PC register tagged.
+ */
+ if (!(flags & KASAN_VMALLOC_PROT_NORMAL))
+ return (void *)start;
+
start = set_tag(start, kasan_random_tag());
kasan_unpoison(start, size, false);
return (void *)start;
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 47f3de7a3396..01ec2ef447af 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -2220,7 +2220,7 @@ void *vm_map_ram(struct page **pages, unsigned int count, int node)
* With hardware tag-based KASAN, marking is skipped for
* non-VM_ALLOC mappings, see __kasan_unpoison_vmalloc().
*/
- mem = kasan_unpoison_vmalloc(mem, size, KASAN_VMALLOC_NONE);
+ mem = kasan_unpoison_vmalloc(mem, size, KASAN_VMALLOC_PROT_NORMAL);

return mem;
}
@@ -2459,7 +2459,7 @@ static struct vm_struct *__get_vm_area_node(unsigned long size,
*/
if (!(flags & VM_ALLOC))
area->addr = kasan_unpoison_vmalloc(area->addr, requested_size,
- KASAN_VMALLOC_NONE);
+ KASAN_VMALLOC_PROT_NORMAL);

return area;
}
@@ -3065,7 +3065,7 @@ void *__vmalloc_node_range(unsigned long size, unsigned long align,
{
struct vm_struct *area;
void *ret;
- kasan_vmalloc_flags_t kasan_flags;
+ kasan_vmalloc_flags_t kasan_flags = KASAN_VMALLOC_NONE;
unsigned long real_size = size;
unsigned long real_align = align;
unsigned int shift = PAGE_SHIFT;
@@ -3118,21 +3118,28 @@ void *__vmalloc_node_range(unsigned long size, unsigned long align,
goto fail;
}

- /* Prepare arguments for __vmalloc_area_node(). */
- if (kasan_hw_tags_enabled() &&
- pgprot_val(prot) == pgprot_val(PAGE_KERNEL)) {
- /*
- * Modify protection bits to allow tagging.
- * This must be done before mapping in __vmalloc_area_node().
- */
- prot = arch_vmap_pgprot_tagged(prot);
+ /*
+ * Prepare arguments for __vmalloc_area_node() and
+ * kasan_unpoison_vmalloc().
+ */
+ if (pgprot_val(prot) == pgprot_val(PAGE_KERNEL)) {
+ if (kasan_hw_tags_enabled()) {
+ /*
+ * Modify protection bits to allow tagging.
+ * This must be done before mapping.
+ */
+ prot = arch_vmap_pgprot_tagged(prot);

- /*
- * Skip page_alloc poisoning and zeroing for physical pages
- * backing VM_ALLOC mapping. Memory is instead poisoned and
- * zeroed by kasan_unpoison_vmalloc().
- */
- gfp_mask |= __GFP_SKIP_KASAN_UNPOISON | __GFP_SKIP_ZERO;
+ /*
+ * Skip page_alloc poisoning and zeroing for physical
+ * pages backing VM_ALLOC mapping. Memory is instead
+ * poisoned and zeroed by kasan_unpoison_vmalloc().
+ */
+ gfp_mask |= __GFP_SKIP_KASAN_UNPOISON | __GFP_SKIP_ZERO;
+ }
+
+ /* Take note that the mapping is PAGE_KERNEL. */
+ kasan_flags |= KASAN_VMALLOC_PROT_NORMAL;
}

/* Allocate physical pages and map them into vmalloc space. */
@@ -3146,10 +3153,13 @@ void *__vmalloc_node_range(unsigned long size, unsigned long align,
* (except for the should_skip_init() check) to make sure that memory
* is initialized under the same conditions regardless of the enabled
* KASAN mode.
+ * Tag-based KASAN modes only assign tags to normal non-executable
+ * allocations, see __kasan_unpoison_vmalloc().
*/
- kasan_flags = KASAN_VMALLOC_VM_ALLOC;
+ kasan_flags |= KASAN_VMALLOC_VM_ALLOC;
if (!want_init_on_free() && want_init_on_alloc(gfp_mask))
kasan_flags |= KASAN_VMALLOC_INIT;
+ /* KASAN_VMALLOC_PROT_NORMAL already set if required. */
area->addr = kasan_unpoison_vmalloc(area->addr, real_size, kasan_flags);

/*
@@ -3863,8 +3873,7 @@ struct vm_struct **pcpu_get_vm_areas(const unsigned long *offsets,
*/
for (area = 0; area < nr_vms; area++)
vms[area]->addr = kasan_unpoison_vmalloc(vms[area]->addr,
- vms[area]->size,
- KASAN_VMALLOC_NONE);
+ vms[area]->size, KASAN_VMALLOC_PROT_NORMAL);

kfree(vas);
return vms;
--
2.25.1