Re: [PATCH v9] mm,kfence: decouple kfence from page granularity mapping judgement

[Marco Elver <elver@google.com>]

On Thu, Mar 16, 2023 at 04:50PM +0800, Zhenhua Huang wrote:
> Kfence only needs its pool to be mapped as page granularity, if it is
> inited early. Previous judgement was a bit over protected. From [1], Mark
> suggested to "just map the KFENCE region a page granularity". So I
> decouple it from judgement and do page granularity mapping for kfence
> pool only. Need to be noticed that late init of kfence pool still requires
> page granularity mapping.
> 
> Page granularity mapping in theory cost more(2M per 1GB) memory on arm64
> platform. Like what I've tested on QEMU(emulated 1GB RAM) with
> gki_defconfig, also turning off rodata protection:
> Before:
> [root@liebao ]# cat /proc/meminfo
> MemTotal:         999484 kB
> After:
> [root@liebao ]# cat /proc/meminfo
> MemTotal:        1001480 kB
> 
> To implement this, also relocate the kfence pool allocation before the
> linear mapping setting up, arm64_kfence_alloc_pool is to allocate phys
> addr, __kfence_pool is to be set after linear mapping set up.
> 
> LINK: [1] https://lore.kernel.org/linux-arm-kernel/Y+IsdrvDNILA59UN@FVFF77S0Q05N/
> Suggested-by: Mark Rutland <mark.rutland@arm.com>
> Signed-off-by: Zhenhua Huang <quic_zhenhuah@quicinc.com>
> ---
>  arch/arm64/include/asm/kfence.h | 16 +++++++++++
>  arch/arm64/mm/mmu.c             | 59 +++++++++++++++++++++++++++++++++++++++++
>  arch/arm64/mm/pageattr.c        |  9 +++++--
>  include/linux/kfence.h          |  1 +
>  mm/kfence/core.c                |  4 +++
>  5 files changed, 87 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/kfence.h b/arch/arm64/include/asm/kfence.h
> index aa855c6..8143c91 100644
> --- a/arch/arm64/include/asm/kfence.h
> +++ b/arch/arm64/include/asm/kfence.h
> @@ -10,6 +10,22 @@
>  
>  #include <asm/set_memory.h>
>  
> +extern phys_addr_t early_kfence_pool;

This should not be accessible if !CONFIG_KFENCE.

> +#ifdef CONFIG_KFENCE
> +
> +extern char *__kfence_pool;
> +static inline void kfence_set_pool(phys_addr_t addr)
> +{
> +	__kfence_pool = phys_to_virt(addr);
> +}

kfence_set_pool() is redundant if it's for arm64 only, because we know
where it's needed, and there you could just access __kfence_pool
directly. So let's just remove this function. (Initially I thought you
want to provide it generally, also for other architectures.)

> +#else
> +
> +static inline void kfence_set_pool(phys_addr_t addr) { }
> +
> +#endif
> +
>  static inline bool arch_kfence_init_pool(void) { return true; }
[...]
> +#endif
> +
> +phys_addr_t early_kfence_pool;

This variable now exists in non-KFENCE builds, which is wrong.

>  static void __init map_mem(pgd_t *pgdp)
>  {
>  	static const u64 direct_map_end = _PAGE_END(VA_BITS_MIN);
> @@ -543,6 +587,10 @@ static void __init map_mem(pgd_t *pgdp)
>  	 */
>  	BUILD_BUG_ON(pgd_index(direct_map_end - 1) == pgd_index(direct_map_end));
>  
> +	early_kfence_pool = arm64_kfence_alloc_pool();
> +	if (early_kfence_pool)
> +		memblock_mark_nomap(early_kfence_pool, KFENCE_POOL_SIZE);
> +
>  	if (can_set_direct_map())
>  		flags |= NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;
>  
> @@ -608,6 +656,17 @@ static void __init map_mem(pgd_t *pgdp)
>  		}
>  	}
>  #endif
> +
> +	/* Kfence pool needs page-level mapping */
> +	if (early_kfence_pool) {
> +		__map_memblock(pgdp, early_kfence_pool,
> +			early_kfence_pool + KFENCE_POOL_SIZE,
> +			pgprot_tagged(PAGE_KERNEL),
> +			NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS);
> +		memblock_clear_nomap(early_kfence_pool, KFENCE_POOL_SIZE);
> +		/* kfence_pool really mapped now */
> +		kfence_set_pool(early_kfence_pool);
> +	}

This whole piece of code could also be wrapped in another function,
which becomes a no-op if !CONFIG_KFENCE. Then you also don't need to
provide the KFENCE_POOL_SIZE define for 0 if !CONFIG_KFENCE.

[...]
> +	 *
> +	 * Kfence pool requires page granularity mapping also if we init it
> +	 * late.
>  	 */
>  	return (rodata_enabled && rodata_full) || debug_pagealloc_enabled() ||
> -		IS_ENABLED(CONFIG_KFENCE);
> +	    (IS_ENABLED(CONFIG_KFENCE) && !early_kfence_pool);

Accessing a non-existent variable if !CONFIG_KFENCE works because the
compiler optimizes out the access, but is generally bad style.


I think the only issue that I have is that the separation between KFENCE
and non-KFENCE builds is not great.

At the end of the email are is a diff against your patch which would be
my suggested changes (while at it, I fixed up a bunch of other issues).
Untested, so if you decide to adopt these changes, please test.

Thanks,
-- Marco

------ >8 ------


diff --git a/arch/arm64/include/asm/kfence.h b/arch/arm64/include/asm/kfence.h
index 8143c91854e1..a81937fae9f6 100644
--- a/arch/arm64/include/asm/kfence.h
+++ b/arch/arm64/include/asm/kfence.h
@@ -10,22 +10,6 @@
 
 #include <asm/set_memory.h>
 
-extern phys_addr_t early_kfence_pool;
-
-#ifdef CONFIG_KFENCE
-
-extern char *__kfence_pool;
-static inline void kfence_set_pool(phys_addr_t addr)
-{
-	__kfence_pool = phys_to_virt(addr);
-}
-
-#else
-
-static inline void kfence_set_pool(phys_addr_t addr) { }
-
-#endif
-
 static inline bool arch_kfence_init_pool(void) { return true; }
 
 static inline bool kfence_protect_page(unsigned long addr, bool protect)
@@ -35,4 +19,14 @@ static inline bool kfence_protect_page(unsigned long addr, bool protect)
 	return true;
 }
 
+#ifdef CONFIG_KFENCE
+extern bool kfence_early_init;
+static inline bool arm64_kfence_can_set_direct_map(void)
+{
+	return !kfence_early_init;
+}
+#else /* CONFIG_KFENCE */
+static inline bool arm64_kfence_can_set_direct_map(void) { return false; }
+#endif /* CONFIG_KFENCE */
+
 #endif /* __ASM_KFENCE_H */
diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index 61944c7091f0..683958616ac1 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -528,17 +528,14 @@ static int __init enable_crash_mem_map(char *arg)
 early_param("crashkernel", enable_crash_mem_map);
 
 #ifdef CONFIG_KFENCE
+bool kfence_early_init = !!CONFIG_KFENCE_SAMPLE_INTERVAL;
 
-static bool kfence_early_init __initdata = !!CONFIG_KFENCE_SAMPLE_INTERVAL;
-/*
- * early_param can be parsed before linear mapping
- * set up
- */
-static int __init parse_kfence_early_init(char *p)
+/* early_param() will be parsed before map_mem() below. */
+static int __init parse_kfence_early_init(char *arg)
 {
 	int val;
 
-	if (get_option(&p, &val))
+	if (get_option(&arg, &val))
 		kfence_early_init = !!val;
 	return 0;
 }
@@ -552,22 +549,34 @@ static phys_addr_t arm64_kfence_alloc_pool(void)
 		return 0;
 
 	kfence_pool = memblock_phys_alloc(KFENCE_POOL_SIZE, PAGE_SIZE);
-	if (!kfence_pool)
+	if (!kfence_pool) {
 		pr_err("failed to allocate kfence pool\n");
+		kfence_early_init = false;
+		return 0;
+	}
+
+	/* Temporarily mark as NOMAP. */
+	memblock_mark_nomap(kfence_pool, KFENCE_POOL_SIZE);
 
 	return kfence_pool;
 }
 
-#else
-
-static phys_addr_t arm64_kfence_alloc_pool(void)
+static void arm64_kfence_map_pool(phys_addr_t kfence_pool, pgd_t *pgdp)
 {
-	return 0;
-}
-
-#endif
+	if (!kfence_pool)
+		return;
 
-phys_addr_t early_kfence_pool;
+	/* KFENCE pool needs page-level mapping. */
+	__map_memblock(pgdp, kfence_pool, kfence_pool + KFENCE_POOL_SIZE,
+		       pgprot_tagged(PAGE_KERNEL),
+		       NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS);
+	memblock_clear_nomap(kfence_pool, KFENCE_POOL_SIZE);
+	__kfence_pool = phys_to_virt(kfence_pool);
+}
+#else /* CONFIG_KFENCE */
+static inline phys_addr_t arm64_kfence_alloc_pool(void) { return 0; }
+static inline void arm64_kfence_map_pool(phys_addr_t kfence_pool, pgd_t *pgdp) { }
+#endif /* CONFIG_KFENCE */
 
 static void __init map_mem(pgd_t *pgdp)
 {
@@ -575,6 +584,7 @@ static void __init map_mem(pgd_t *pgdp)
 	phys_addr_t kernel_start = __pa_symbol(_stext);
 	phys_addr_t kernel_end = __pa_symbol(__init_begin);
 	phys_addr_t start, end;
+	phys_addr_t early_kfence_pool;
 	int flags = NO_EXEC_MAPPINGS;
 	u64 i;
 
@@ -588,8 +598,6 @@ static void __init map_mem(pgd_t *pgdp)
 	BUILD_BUG_ON(pgd_index(direct_map_end - 1) == pgd_index(direct_map_end));
 
 	early_kfence_pool = arm64_kfence_alloc_pool();
-	if (early_kfence_pool)
-		memblock_mark_nomap(early_kfence_pool, KFENCE_POOL_SIZE);
 
 	if (can_set_direct_map())
 		flags |= NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;
@@ -656,17 +664,7 @@ static void __init map_mem(pgd_t *pgdp)
 		}
 	}
 #endif
-
-	/* Kfence pool needs page-level mapping */
-	if (early_kfence_pool) {
-		__map_memblock(pgdp, early_kfence_pool,
-			early_kfence_pool + KFENCE_POOL_SIZE,
-			pgprot_tagged(PAGE_KERNEL),
-			NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS);
-		memblock_clear_nomap(early_kfence_pool, KFENCE_POOL_SIZE);
-		/* kfence_pool really mapped now */
-		kfence_set_pool(early_kfence_pool);
-	}
+	arm64_kfence_map_pool(early_kfence_pool, pgdp);
 }
 
 void mark_rodata_ro(void)
diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c
index 7ce5295cc6fb..aa8fd12cc96f 100644
--- a/arch/arm64/mm/pageattr.c
+++ b/arch/arm64/mm/pageattr.c
@@ -7,7 +7,6 @@
 #include <linux/module.h>
 #include <linux/sched.h>
 #include <linux/vmalloc.h>
-#include <linux/kfence.h>
 
 #include <asm/cacheflush.h>
 #include <asm/set_memory.h>
@@ -28,11 +27,10 @@ bool can_set_direct_map(void)
 	 * mapped at page granularity, so that it is possible to
 	 * protect/unprotect single pages.
 	 *
-	 * Kfence pool requires page granularity mapping also if we init it
-	 * late.
+	 * KFENCE pool requires page-granular mapping if initialized late.
 	 */
 	return (rodata_enabled && rodata_full) || debug_pagealloc_enabled() ||
-	    (IS_ENABLED(CONFIG_KFENCE) && !early_kfence_pool);
+	       arm64_kfence_can_set_direct_map();
 }
 
 static int change_page_range(pte_t *ptep, unsigned long addr, void *data)
diff --git a/include/linux/kfence.h b/include/linux/kfence.h
index 91cbcc98e293..726857a4b680 100644
--- a/include/linux/kfence.h
+++ b/include/linux/kfence.h
@@ -222,7 +222,6 @@ bool __kfence_obj_info(struct kmem_obj_info *kpp, void *object, struct slab *sla
 
 #else /* CONFIG_KFENCE */
 
-#define KFENCE_POOL_SIZE 0
 static inline bool is_kfence_address(const void *addr) { return false; }
 static inline void kfence_alloc_pool(void) { }
 static inline void kfence_init(void) { }
diff --git a/mm/kfence/core.c b/mm/kfence/core.c
index fab087d39633..e7f22af5e710 100644
--- a/mm/kfence/core.c
+++ b/mm/kfence/core.c
@@ -818,7 +818,7 @@ void __init kfence_alloc_pool(void)
 	if (!kfence_sample_interval)
 		return;
 
-	/* if the pool has already been initialized by arch, skip the below */
+	/* If the pool has already been initialized by arch, skip the below. */
 	if (__kfence_pool)
 		return;
 

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