On Thu, Aug 27, 2020 at 11:38 AM Catalin Marinas
[off-list ref] wrote:

On Fri, Aug 14, 2020 at 07:27:02PM +0200, Andrey Konovalov wrote:

quoted

diff --git a/arch/arm64/include/asm/mte.h b/arch/arm64/include/asm/mte.h
index 1c99fcadb58c..733be1cb5c95 100644
--- a/arch/arm64/include/asm/mte.h
+++ b/arch/arm64/include/asm/mte.h

@@ -5,14 +5,19 @@
 #ifndef __ASM_MTE_H
 #define __ASM_MTE_H

-#define MTE_GRANULE_SIZE     UL(16)
+#include <asm/mte_asm.h>

So the reason for this move is to include it in asm/cache.h. Fine by
me but...

quoted

 #define MTE_GRANULE_MASK     (~(MTE_GRANULE_SIZE - 1))
 #define MTE_TAG_SHIFT                56
 #define MTE_TAG_SIZE         4
+#define MTE_TAG_MASK         GENMASK((MTE_TAG_SHIFT + (MTE_TAG_SIZE - 1)), MTE_TAG_SHIFT)
+#define MTE_TAG_MAX          (MTE_TAG_MASK >> MTE_TAG_SHIFT)

... I'd rather move all these definitions in a file with a more
meaningful name like mte-def.h. The _asm implies being meant for .S
files inclusion which isn't the case.

Sounds good, I'll leave fixing this and other arm64-specific comments
to Vincenzo. I'll change KASAN code to use mte-def.h once I have
patches where this file is renamed.

quoted

diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
index eb39504e390a..e2d708b4583d 100644
--- a/arch/arm64/kernel/mte.c
+++ b/arch/arm64/kernel/mte.c

@@ -72,6 +74,47 @@ int memcmp_pages(struct page *page1, struct page *page2)
      return ret;
 }

+u8 mte_get_mem_tag(void *addr)
+{
+     if (system_supports_mte())
+             addr = mte_assign_valid_ptr_tag(addr);

The mte_assign_valid_ptr_tag() is slightly misleading. All it does is
read the allocation tag from memory.

I also think this should be inline asm, possibly using alternatives.
It's just an LDG instruction (and it saves us from having to invent a
better function name).

quoted

+
+     return 0xF0 | mte_get_ptr_tag(addr);
+}
+
+u8 mte_get_random_tag(void)
+{
+     u8 tag = 0xF;
+
+     if (system_supports_mte())
+             tag = mte_get_ptr_tag(mte_assign_random_ptr_tag(NULL));

Another alternative inline asm with an IRG instruction.

quoted

+
+     return 0xF0 | tag;
+}
+
+void * __must_check mte_set_mem_tag_range(void *addr, size_t size, u8 tag)
+{
+     void *ptr = addr;
+
+     if ((!system_supports_mte()) || (size == 0))
+             return addr;
+
+     tag = 0xF0 | (tag & 0xF);
+     ptr = (void *)__tag_set(ptr, tag);
+     size = ALIGN(size, MTE_GRANULE_SIZE);

I think aligning the size is dangerous. Can we instead turn it into a
WARN_ON if not already aligned? At a quick look, the callers of
kasan_{un,}poison_memory() already align the size.

quoted

+
+     mte_assign_mem_tag_range(ptr, size);
+
+     /*
+      * mte_assign_mem_tag_range() can be invoked in a multi-threaded
+      * context, ensure that tags are written in memory before the
+      * reference is used.
+      */
+     smp_wmb();
+
+     return ptr;

I'm not sure I understand the barrier here. It ensures the relative
ordering of memory (or tag) accesses on a CPU as observed by other CPUs.
While the first access here is setting the tag, I can't see what other
access on _this_ CPU it is ordered with.

quoted

+}
+
 static void update_sctlr_el1_tcf0(u64 tcf0)
 {
      /* ISB required for the kernel uaccess routines */

diff --git a/arch/arm64/lib/mte.S b/arch/arm64/lib/mte.S
index 03ca6d8b8670..8c743540e32c 100644
--- a/arch/arm64/lib/mte.S
+++ b/arch/arm64/lib/mte.S

@@ -149,3 +149,44 @@ SYM_FUNC_START(mte_restore_page_tags)

      ret
 SYM_FUNC_END(mte_restore_page_tags)
+
+/*
+ * Assign pointer tag based on the allocation tag
+ *   x0 - source pointer
+ * Returns:
+ *   x0 - pointer with the correct tag to access memory
+ */
+SYM_FUNC_START(mte_assign_valid_ptr_tag)
+     ldg     x0, [x0]
+     ret
+SYM_FUNC_END(mte_assign_valid_ptr_tag)
+
+/*
+ * Assign random pointer tag
+ *   x0 - source pointer
+ * Returns:
+ *   x0 - pointer with a random tag
+ */
+SYM_FUNC_START(mte_assign_random_ptr_tag)
+     irg     x0, x0
+     ret
+SYM_FUNC_END(mte_assign_random_ptr_tag)

As I said above, these two can be inline asm.

quoted

+
+/*
+ * Assign allocation tags for a region of memory based on the pointer tag
+ *   x0 - source pointer
+ *   x1 - size
+ *
+ * Note: size is expected to be MTE_GRANULE_SIZE aligned
+ */
+SYM_FUNC_START(mte_assign_mem_tag_range)
+     /* if (src == NULL) return; */
+     cbz     x0, 2f
+     /* if (size == 0) return; */

You could skip the cbz here and just document that the size should be
non-zero and aligned. The caller already takes care of this check.

quoted

+     cbz     x1, 2f
+1:   stg     x0, [x0]
+     add     x0, x0, #MTE_GRANULE_SIZE
+     sub     x1, x1, #MTE_GRANULE_SIZE
+     cbnz    x1, 1b
+2:   ret
+SYM_FUNC_END(mte_assign_mem_tag_range)

--
Catalin

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