On 11/21/22 22:35, Kees Cook wrote:
> On November 21, 2022 9:11:51 AM PST, Vlastimil Babka <vbabka@xxxxxxx> wrote:
>>With CONFIG_HARDENED_USERCOPY not enabled, there are no
>>__check_heap_object() checks happening that would use the kmem_cache
>>useroffset and usersize fields. Yet the fields are still initialized,
>>preventing merging of otherwise compatible caches. Thus ignore the
>>values passed to cache creation and leave them zero when
>>CONFIG_HARDENED_USERCOPY is disabled.
>>
>>In a quick virtme boot test, this has reduced the number of caches in
>>/proc/slabinfo from 131 to 111.
>>
>>Cc: Kees Cook <keescook@xxxxxxxxxxxx>
>>Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
>>---
>> mm/slab_common.c | 6 +++++-
>> 1 file changed, 5 insertions(+), 1 deletion(-)
>>
>>diff --git a/mm/slab_common.c b/mm/slab_common.c
>>index 0042fb2730d1..a8cb5de255fc 100644
>>--- a/mm/slab_common.c
>>+++ b/mm/slab_common.c
>>@@ -317,7 +317,8 @@ kmem_cache_create_usercopy(const char *name,
>> flags &= CACHE_CREATE_MASK;
>>
>> /* Fail closed on bad usersize of useroffset values. */
>>- if (WARN_ON(!usersize && useroffset) ||
>>+ if (!IS_ENABLED(CONFIG_HARDENED_USERCOPY) ||
>>+ WARN_ON(!usersize && useroffset) ||
>> WARN_ON(size < usersize || size - usersize < useroffset))
>> usersize = useroffset = 0;
>>
>>@@ -640,6 +641,9 @@ void __init create_boot_cache(struct kmem_cache *s, const char *name,
>> align = max(align, size);
>> s->align = calculate_alignment(flags, align, size);
>>
>>+ if (!IS_ENABLED(CONFIG_HARDENED_USERCOPY))
>>+ useroffset = usersize = 0;
>>+
>> s->useroffset = useroffset;
>> s->usersize = usersize;
>>
>
> "Always non-mergeable" is intentional here, but I do see the argument
> for not doing it under hardened-usercopy.
>
> That said, if you keep this part, maybe go the full step and ifdef away
> useroffset/usersize's struct member definition and other logic, especially
> for SLUB_TINY benefits, so 2 ulongs are dropped from the cache struct?
Okay, probably won't make much difference in practice, but for consistency...
----8<----
>From 3cdb7b6ad16a9d95603b482969fa870f996ac9dc Mon Sep 17 00:00:00 2001
From: Vlastimil Babka <vbabka@xxxxxxx>
Date: Wed, 16 Nov 2022 15:56:32 +0100
Subject: [PATCH] mm, slab: ignore hardened usercopy parameters when disabled
With CONFIG_HARDENED_USERCOPY not enabled, there are no
__check_heap_object() checks happening that would use the struct
kmem_cache useroffset and usersize fields. Yet the fields are still
initialized, preventing merging of otherwise compatible caches.
Also the fields contribute to struct kmem_cache size unnecessarily when
unused. Thus #ifdef them out completely when CONFIG_HARDENED_USERCOPY is
disabled.
In a quick virtme boot test, this has reduced the number of caches in
/proc/slabinfo from 131 to 111.
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
---
include/linux/slab_def.h | 2 ++
include/linux/slub_def.h | 2 ++
mm/slab.h | 2 --
mm/slab_common.c | 9 ++++++++-
mm/slub.c | 4 ++++
5 files changed, 16 insertions(+), 3 deletions(-)
diff --git a/include/linux/slab_def.h b/include/linux/slab_def.h
index f0ffad6a3365..5834bad8ad78 100644
--- a/include/linux/slab_def.h
+++ b/include/linux/slab_def.h
@@ -80,8 +80,10 @@ struct kmem_cache {
unsigned int *random_seq;
#endif
+#ifdef CONFIG_HARDENED_USERCOPY
unsigned int useroffset; /* Usercopy region offset */
unsigned int usersize; /* Usercopy region size */
+#endif
struct kmem_cache_node *node[MAX_NUMNODES];
};
diff --git a/include/linux/slub_def.h b/include/linux/slub_def.h
index f9c68a9dac04..7ed5e455cbf4 100644
--- a/include/linux/slub_def.h
+++ b/include/linux/slub_def.h
@@ -136,8 +136,10 @@ struct kmem_cache {
struct kasan_cache kasan_info;
#endif
+#ifdef CONFIG_HARDENED_USERCOPY
unsigned int useroffset; /* Usercopy region offset */
unsigned int usersize; /* Usercopy region size */
+#endif
struct kmem_cache_node *node[MAX_NUMNODES];
};
diff --git a/mm/slab.h b/mm/slab.h
index 0202a8c2f0d2..db9a7984e22e 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -207,8 +207,6 @@ struct kmem_cache {
unsigned int size; /* The aligned/padded/added on size */
unsigned int align; /* Alignment as calculated */
slab_flags_t flags; /* Active flags on the slab */
- unsigned int useroffset;/* Usercopy region offset */
- unsigned int usersize; /* Usercopy region size */
const char *name; /* Slab name for sysfs */
int refcount; /* Use counter */
void (*ctor)(void *); /* Called on object slot creation */
diff --git a/mm/slab_common.c b/mm/slab_common.c
index 0042fb2730d1..4339c839a452 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -143,8 +143,10 @@ int slab_unmergeable(struct kmem_cache *s)
if (s->ctor)
return 1;
+#ifdef CONFIG_HARDENED_USERCOPY
if (s->usersize)
return 1;
+#endif
/*
* We may have set a slab to be unmergeable during bootstrap.
@@ -223,8 +225,10 @@ static struct kmem_cache *create_cache(const char *name,
s->size = s->object_size = object_size;
s->align = align;
s->ctor = ctor;
+#ifdef CONFIG_HARDENED_USERCOPY
s->useroffset = useroffset;
s->usersize = usersize;
+#endif
err = __kmem_cache_create(s, flags);
if (err)
@@ -317,7 +321,8 @@ kmem_cache_create_usercopy(const char *name,
flags &= CACHE_CREATE_MASK;
/* Fail closed on bad usersize of useroffset values. */
- if (WARN_ON(!usersize && useroffset) ||
+ if (!IS_ENABLED(CONFIG_HARDENED_USERCOPY) ||
+ WARN_ON(!usersize && useroffset) ||
WARN_ON(size < usersize || size - usersize < useroffset))
usersize = useroffset = 0;
@@ -640,8 +645,10 @@ void __init create_boot_cache(struct kmem_cache *s, const char *name,
align = max(align, size);
s->align = calculate_alignment(flags, align, size);
+#ifdef CONFIG_HARDENED_USERCOPY
s->useroffset = useroffset;
s->usersize = usersize;
+#endif
err = __kmem_cache_create(s, flags);
diff --git a/mm/slub.c b/mm/slub.c
index 157527d7101b..e32db8540767 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -5502,11 +5502,13 @@ static ssize_t cache_dma_show(struct kmem_cache *s, char *buf)
SLAB_ATTR_RO(cache_dma);
#endif
+#ifdef CONFIG_HARDENED_USERCOPY
static ssize_t usersize_show(struct kmem_cache *s, char *buf)
{
return sysfs_emit(buf, "%u\n", s->usersize);
}
SLAB_ATTR_RO(usersize);
+#endif
static ssize_t destroy_by_rcu_show(struct kmem_cache *s, char *buf)
{
@@ -5803,7 +5805,9 @@ static struct attribute *slab_attrs[] = {
#ifdef CONFIG_FAILSLAB
&failslab_attr.attr,
#endif
+#ifdef CONFIG_HARDENED_USERCOPY
&usersize_attr.attr,
+#endif
#ifdef CONFIG_KFENCE
&skip_kfence_attr.attr,
#endif
--
2.38.1
