Commit 8f828aa4 authored Apr 30, 2024 by Nicolas Bouchinet Committed by Vlastimil Babka May 01, 2024

mm/slub: avoid zeroing outside-object freepointer for single free



Commit 284f17ac ("mm/slub: handle bulk and single object freeing
separately") splits single and bulk object freeing in two functions
slab_free() and slab_free_bulk() which leads slab_free() to call
slab_free_hook() directly instead of slab_free_freelist_hook().

If `init_on_free` is set, slab_free_hook() zeroes the object.
Afterward, if `slub_debug=F` and `CONFIG_SLAB_FREELIST_HARDENED` are
set, the do_slab_free() slowpath executes freelist consistency
checks and try to decode a zeroed freepointer which leads to a
"Freepointer corrupt" detection in check_object().

During bulk free, slab_free_freelist_hook() isn't affected as it always
sets it objects freepointer using set_freepointer() to maintain its
reconstructed freelist after `init_on_free`.

For single free, object's freepointer thus needs to be avoided when
stored outside the object if `init_on_free` is set. The freepointer left
as is, check_object() may later detect an invalid pointer value due to
objects overflow.

To reproduce, set `slub_debug=FU init_on_free=1 log_level=7` on the
command line of a kernel build with `CONFIG_SLAB_FREELIST_HARDENED=y`.

dmesg sample log:
[   10.708715] =============================================================================
[   10.710323] BUG kmalloc-rnd-05-32 (Tainted: G    B           T ): Freepointer corrupt
[   10.712695] -----------------------------------------------------------------------------
[   10.712695]
[   10.712695] Slab 0xffffd8bdc400d580 objects=32 used=4 fp=0xffff9d9a80356f80 flags=0x200000000000a00(workingset|slab|node=0|zone=2)
[   10.716698] Object 0xffff9d9a80356600 @offset=1536 fp=0x7ee4f480ce0ecd7c
[   10.716698]
[   10.716698] Bytes b4 ffff9d9a803565f0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
[   10.720703] Object   ffff9d9a80356600: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
[   10.720703] Object   ffff9d9a80356610: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
[   10.724696] Padding  ffff9d9a8035666c: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
[   10.724696] Padding  ffff9d9a8035667c: 00 00 00 00                                      ....
[   10.724696] FIX kmalloc-rnd-05-32: Object at 0xffff9d9a80356600 not freed

Fixes: 284f17ac ("mm/slub: handle bulk and single object freeing separately")
Cc: <stable@vger.kernel.org>
Co-developed-by: Chengming Zhou <chengming.zhou@linux.dev>
Signed-off-by: Chengming Zhou <chengming.zhou@linux.dev>
Signed-off-by: Nicolas Bouchinet <nicolas.bouchinet@ssi.gouv.fr>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

parent e67572cd

mm/slub.c

+29 −23

Original line number	Diff line number	Diff line
		@@ -557,6 +557,26 @@ static inline void set_freepointer(struct kmem_cache s, void object, void *fp)
		(freeptr_t )freeptr_addr = freelist_ptr_encode(s, fp, freeptr_addr);
		}

		/*
		* See comment in calculate_sizes().
		*/
		static inline bool freeptr_outside_object(struct kmem_cache *s)
		{
		return s->offset >= s->inuse;
		}

		/*
		* Return offset of the end of info block which is inuse + free pointer if
		* not overlapping with object.
		*/
		static inline unsigned int get_info_end(struct kmem_cache *s)
		{
		if (freeptr_outside_object(s))
		return s->inuse + sizeof(void *);
		else
		return s->inuse;
		}

		/* Loop over all objects in a slab */
		#define for_each_object(__p, __s, __addr, __objects) \
		for (__p = fixup_red_left(__s, __addr); \
		@@ -845,26 +865,6 @@ static void print_section(char level, char text, u8 *addr,
		metadata_access_disable();
		}

		/*
		* See comment in calculate_sizes().
		*/
		static inline bool freeptr_outside_object(struct kmem_cache *s)
		{
		return s->offset >= s->inuse;
		}

		/*
		* Return offset of the end of info block which is inuse + free pointer if
		* not overlapping with object.
		*/
		static inline unsigned int get_info_end(struct kmem_cache *s)
		{
		if (freeptr_outside_object(s))
		return s->inuse + sizeof(void *);
		else
		return s->inuse;
		}

		static struct track get_track(struct kmem_cache s, void *object,
		enum track_item alloc)
		{
		@@ -2092,15 +2092,20 @@ bool slab_free_hook(struct kmem_cache s, void x, bool init)
		*
		* The initialization memset's clear the object and the metadata,
		* but don't touch the SLAB redzone.
		*
		* The object's freepointer is also avoided if stored outside the
		* object.
		*/
		if (unlikely(init)) {
		int rsize;
		unsigned int inuse;

		inuse = get_info_end(s);
		if (!kasan_has_integrated_init())
		memset(kasan_reset_tag(x), 0, s->object_size);
		rsize = (s->flags & SLAB_RED_ZONE) ? s->red_left_pad : 0;
		memset((char *)kasan_reset_tag(x) + s->inuse, 0,
		s->size - s->inuse - rsize);
		memset((char *)kasan_reset_tag(x) + inuse, 0,
		s->size - inuse - rsize);
		}
		/* KASAN might put x into memory quarantine, delaying its reuse. */
		return !kasan_slab_free(s, x, init);
		@@ -3722,7 +3727,8 @@ static void __slab_alloc_node(struct kmem_cache s,
		static __always_inline void maybe_wipe_obj_freeptr(struct kmem_cache *s,
		void *obj)
		{
		if (unlikely(slab_want_init_on_free(s)) && obj)
		if (unlikely(slab_want_init_on_free(s)) && obj &&
		!freeptr_outside_object(s))
		memset((void )((char )kasan_reset_tag(obj) + s->offset),
		0, sizeof(void *));
		}