On Wed, Nov 23, 2022 at 03:23:15PM +0100, Vlastimil Babka wrote:
>
> 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;
I think this change is no longer needed as slab_unmergeable()
now does not check usersize when CONFIG_HARDENED_USERCOPY=n?
> @@ -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
>
>
--
Thanks,
Hyeonggon
