On Fri, Sep 25, 2020 at 12:28:24PM +0100, Vincenzo Frascino wrote:

On 9/25/20 11:15 AM, Catalin Marinas wrote:

quoted

On Fri, Sep 25, 2020 at 12:50:31AM +0200, Andrey Konovalov wrote:

quoted

+u8 mte_get_mem_tag(void *addr);
+u8 mte_get_random_tag(void);
+void *mte_set_mem_tag_range(void *addr, size_t size, u8 tag);
+
+#else /* CONFIG_ARM64_MTE */
+
+static inline u8 mte_get_ptr_tag(void *ptr)
+{
+	return 0xFF;
+}
+
+static inline u8 mte_get_mem_tag(void *addr)
+{
+	return 0xFF;
+}
+static inline u8 mte_get_random_tag(void)
+{
+	return 0xFF;
+}
+static inline void *mte_set_mem_tag_range(void *addr, size_t size, u8 tag)
+{
+	return addr;
+}

Maybe these can stay in mte-kasan.h, although they are not a direct
interface for KASAN AFAICT (the arch_* equivalent are defined in
asm/memory.h. If there's no good reason, we could move them to mte.h.

This is here because it is not a direct interface as you noticed. I tried to
keep the separation (even if it I have something to fix based on your comment
below ;)).

The other kasan implementation define the arch_* indirection in asm/memory.h in
every architecture. I think maintaining the design is the best way to non create
confusion.

I'm ok with asm/memory.h for kasan, no need to change that. You can also
keep these functions in asm/mte-kasan.h but add a comment that they are
only for the kasan interface defined in asm/memory.h.

quoted

quoted

diff --git a/arch/arm64/include/asm/mte.h b/arch/arm64/include/asm/mte.h
index 1c99fcadb58c..3a2bf3ccb26c 100644
--- a/arch/arm64/include/asm/mte.h
+++ b/arch/arm64/include/asm/mte.h

@@ -5,14 +5,13 @@
 #ifndef __ASM_MTE_H
 #define __ASM_MTE_H
 
-#define MTE_GRANULE_SIZE	UL(16)
-#define MTE_GRANULE_MASK	(~(MTE_GRANULE_SIZE - 1))
-#define MTE_TAG_SHIFT		56
-#define MTE_TAG_SIZE		4
+#include <asm/mte-kasan.h>

And this include should be replaced by asm/mte-hwdef.h.

quoted

quoted

 #ifndef __ASSEMBLY__
 
+#include <linux/bitfield.h>
 #include <linux/page-flags.h>
+#include <linux/types.h>
 
 #include <asm/pgtable-types.h>
 

@@ -45,7 +44,9 @@ long get_mte_ctrl(struct task_struct *task);
 int mte_ptrace_copy_tags(struct task_struct *child, long request,
 			 unsigned long addr, unsigned long data);
 
-#else
+void mte_assign_mem_tag_range(void *addr, size_t size);

So mte_set_mem_tag_range() is KASAN specific but
mte_assign_mem_tag_range() is not. Slightly confusing.

mte_assign_mem_tag_range() is the internal function implemented in assembler
which is not used directly by KASAN. Is it the name that you find confusing? Do
you have a better proposal?

I don't mind the name, just trying to find some consistency in the
headers.

quoted

quoted

diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
index 52a0638ed967..833b63fdd5e2 100644
--- a/arch/arm64/kernel/mte.c
+++ b/arch/arm64/kernel/mte.c

@@ -13,8 +13,10 @@
 #include <linux/swap.h>
 #include <linux/swapops.h>
 #include <linux/thread_info.h>
+#include <linux/types.h>
 #include <linux/uio.h>
 
+#include <asm/barrier.h>
 #include <asm/cpufeature.h>
 #include <asm/mte.h>
 #include <asm/ptrace.h>

@@ -72,6 +74,48 @@ int memcmp_pages(struct page *page1, struct page *page2)
 	return ret;
 }
 
+u8 mte_get_mem_tag(void *addr)
+{
+	if (!system_supports_mte())
+		return 0xFF;
+
+	asm volatile(__MTE_PREAMBLE "ldg %0, [%0]"
+		    : "+r" (addr));

[...]

quoted

I wonder whether we'd need the "memory" clobber. I don't see how this
would fail though, maybe later on with stack tagging if the compiler
writes tags behind our back.

As you said, I do not see how this can fail either. We can be overcautious
though here and add a comment that the clobber has been added in prevision of
stack tagging.

I don't think we should bother, it may not even matter.

quoted

quoted

+ */
+SYM_FUNC_START(mte_assign_mem_tag_range)
+	/* if (src == NULL) return; */
+	cbz	x0, 2f
+	/* if (size == 0) return; */
+	cbz	x1, 2f

I find these checks unnecessary, as I said a couple of times before,
just document the function pre-conditions. They are also incomplete
(i.e. you check for NULL but not alignment).

I thought we agreed to harden the code further, based on [1]. Maybe I
misunderstood. I am going to remove them and extend the comment in the next version.

[1]
https://lore.kernel.org/linux-arm-kernel/921c4ed0-b5b5-bc01-5418-c52d80f1af59@arm.com/ (local)

Well, you concluded that but I haven't confirmed ;). Since it's called
from a single place which does the checks already, I don't see the point
in duplicating them. Documenting should be sufficient.

-- 
Catalin

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