On Wed, Jun 18, 2014 at 09:29:35AM +0900, Joonsoo Kim wrote:
> commit 'b1cb098: change the management method of free objects of the slab'
> introduces bug on slab leak detector('/proc/slab_allocators'). This
> detector works like as following decription.
>
> 1. traverse all objects on all the slabs.
> 2. determine whether it is active or not.
> 3. if active, print who allocate this object.
>
> commit 'b1cb098' changes the way how to manage free objects, so logic
> determining whether it is active or not is also changed. In before, we
> regard object in cpu caches as inactive one, but, with this commit, we
> mistakenly regard object in cpu caches as active one.
>
> This intoduces kernel oops if DEBUG_PAGEALLOC is enabled.
> If DEBUG_PAGEALLOC is enabled, kernel_map_pages() is used to detect
> who corrupt free memory in the slab. It unmaps page table mapping if
> object is free and map it if object is active. When slab leak detector
> check object in cpu caches, it mistakenly think this object active so
> try to access object memory to retrieve caller of allocation. At this
> point, page table mapping to this object doesn't exist, so oops occurs.
>
> Following is oops message reported from Dave.
>
> It blew up when something tried to read /proc/slab_allocators
> (Just cat it, and you should see the oops below)
>
> Oops: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
> Modules linked in:
> [snip...]
> CPU: 1 PID: 9386 Comm: trinity-c33 Not tainted 3.14.0-rc5+ #131
> task: ffff8801aa46e890 ti: ffff880076924000 task.ti: ffff880076924000
> RIP: 0010:[<ffffffffaa1a8f4a>] [<ffffffffaa1a8f4a>] handle_slab+0x8a/0x180
> RSP: 0018:ffff880076925de0 EFLAGS: 00010002
> RAX: 0000000000001000 RBX: 0000000000000000 RCX: 000000005ce85ce7
> RDX: ffffea00079be100 RSI: 0000000000001000 RDI: ffff880107458000
> RBP: ffff880076925e18 R08: 0000000000000001 R09: 0000000000000000
> R10: 0000000000000000 R11: 000000000000000f R12: ffff8801e6f84000
> R13: ffffea00079be100 R14: ffff880107458000 R15: ffff88022bb8d2c0
> FS: 00007fb769e45740(0000) GS:ffff88024d040000(0000) knlGS:0000000000000000
> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: ffff8801e6f84ff8 CR3: 00000000a22db000 CR4: 00000000001407e0
> DR0: 0000000002695000 DR1: 0000000002695000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000070602
> Stack:
> ffff8802339dcfc0 ffff88022bb8d2c0 ffff880107458000 ffff88022bb8d2c0
> ffff8802339dd008 ffff8802339dcfc0 ffffea00079be100 ffff880076925e68
> ffffffffaa1ad9be ffff880203fe4f00 ffff88022bb8d318 0000000076925e98
> Call Trace:
> [<ffffffffaa1ad9be>] leaks_show+0xce/0x240
> [<ffffffffaa1e6c0e>] seq_read+0x28e/0x490
> [<ffffffffaa23008d>] proc_reg_read+0x3d/0x80
> [<ffffffffaa1c026b>] vfs_read+0x9b/0x160
> [<ffffffffaa1c0d88>] SyS_read+0x58/0xb0
> [<ffffffffaa7420aa>] tracesys+0xd4/0xd9
> Code: f5 00 00 00 0f 1f 44 00 00 48 63 c8 44 3b 0c 8a 0f 84 e3 00 00 00 83 c0 01 44 39 c0 72 eb 41 f6 47 1a 01 0f 84 e9 00 00 00 89 f0 <4d> 8b 4c 04 f8 4d 85 c9 0f 84 88 00 00 00 49 8b 7e 08 4d 8d 46
> RIP [<ffffffffaa1a8f4a>] handle_slab+0x8a/0x180
> RSP <ffff880076925de0>
> CR2: ffff8801e6f84ff8
>
> To fix the problem, I introduces object status buffer on each slab.
> With this, we can track object status precisely, so slab leak detector
> would not access active object and no kernel oops would occur.
> Memory overhead caused by this fix is only imposed to
> CONFIG_DEBUG_SLAB_LEAK which is mainly used for debugging, so memory
> overhead isn't big problem.
>
> v2: edit one more function, calculate_slab_order(), that wants to know
> how much space per object is spent for freelist management.
>
> Cc: <stable@xxxxxxxxxxxxxxx>
> Reported-by: Dave Jones <davej@xxxxxxxxxx>
> Reported-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
Reviewed-by: Vladimir Davydov <vdavydov@xxxxxxxxxxxxx>
>
> diff --git a/mm/slab.c b/mm/slab.c
> index 9ca3b87..3070b92 100644
> --- a/mm/slab.c
> +++ b/mm/slab.c
> @@ -386,6 +386,39 @@ static void **dbg_userword(struct kmem_cache *cachep, void *objp)
>
> #endif
>
> +#define OBJECT_FREE (0)
> +#define OBJECT_ACTIVE (1)
> +
> +#ifdef CONFIG_DEBUG_SLAB_LEAK
> +
> +static void set_obj_status(struct page *page, int idx, int val)
> +{
> + int freelist_size;
> + char *status;
> + struct kmem_cache *cachep = page->slab_cache;
> +
> + freelist_size = cachep->num * sizeof(freelist_idx_t);
> + status = (char *)page->freelist + freelist_size;
> + status[idx] = val;
> +}
> +
> +static inline unsigned int get_obj_status(struct page *page, int idx)
> +{
> + int freelist_size;
> + char *status;
> + struct kmem_cache *cachep = page->slab_cache;
> +
> + freelist_size = cachep->num * sizeof(freelist_idx_t);
> + status = (char *)page->freelist + freelist_size;
> +
> + return status[idx];
> +}
> +
> +#else
> +static inline void set_obj_status(struct page *page, int idx, int val) {}
> +
> +#endif
> +
> /*
> * Do not go above this order unless 0 objects fit into the slab or
> * overridden on the command line.
> @@ -576,12 +609,30 @@ static inline struct array_cache *cpu_cache_get(struct kmem_cache *cachep)
> return cachep->array[smp_processor_id()];
> }
>
> +static size_t calculate_freelist_size(int nr_objs, size_t align)
> +{
> + size_t freelist_size;
> +
> + freelist_size = nr_objs * sizeof(freelist_idx_t);
> + if (IS_ENABLED(CONFIG_DEBUG_SLAB_LEAK))
> + freelist_size += nr_objs * sizeof(char);
> +
> + if (align)
> + freelist_size = ALIGN(freelist_size, align);
> +
> + return freelist_size;
> +}
> +
> static int calculate_nr_objs(size_t slab_size, size_t buffer_size,
> size_t idx_size, size_t align)
> {
> int nr_objs;
> + size_t remained_size;
> size_t freelist_size;
> + int extra_space = 0;
>
> + if (IS_ENABLED(CONFIG_DEBUG_SLAB_LEAK))
> + extra_space = sizeof(char);
> /*
> * Ignore padding for the initial guess. The padding
> * is at most @align-1 bytes, and @buffer_size is at
> @@ -590,14 +641,15 @@ static int calculate_nr_objs(size_t slab_size, size_t buffer_size,
> * into the memory allocation when taking the padding
> * into account.
> */
> - nr_objs = slab_size / (buffer_size + idx_size);
> + nr_objs = slab_size / (buffer_size + idx_size + extra_space);
>
> /*
> * This calculated number will be either the right
> * amount, or one greater than what we want.
> */
> - freelist_size = slab_size - nr_objs * buffer_size;
> - if (freelist_size < ALIGN(nr_objs * idx_size, align))
> + remained_size = slab_size - nr_objs * buffer_size;
> + freelist_size = calculate_freelist_size(nr_objs, align);
> + if (remained_size < freelist_size)
> nr_objs--;
>
> return nr_objs;
> @@ -635,7 +687,7 @@ static void cache_estimate(unsigned long gfporder, size_t buffer_size,
> } else {
> nr_objs = calculate_nr_objs(slab_size, buffer_size,
> sizeof(freelist_idx_t), align);
> - mgmt_size = ALIGN(nr_objs * sizeof(freelist_idx_t), align);
> + mgmt_size = calculate_freelist_size(nr_objs, align);
> }
> *num = nr_objs;
> *left_over = slab_size - nr_objs*buffer_size - mgmt_size;
> @@ -2041,13 +2093,16 @@ static size_t calculate_slab_order(struct kmem_cache *cachep,
> break;
>
> if (flags & CFLGS_OFF_SLAB) {
> + size_t freelist_size_per_obj = sizeof(freelist_idx_t);
> /*
> * Max number of objs-per-slab for caches which
> * use off-slab slabs. Needed to avoid a possible
> * looping condition in cache_grow().
> */
> + if (IS_ENABLED(CONFIG_DEBUG_SLAB_LEAK))
> + freelist_size_per_obj += sizeof(char);
> offslab_limit = size;
> - offslab_limit /= sizeof(freelist_idx_t);
> + offslab_limit /= freelist_size_per_obj;
>
> if (num > offslab_limit)
> break;
> @@ -2294,8 +2349,7 @@ __kmem_cache_create (struct kmem_cache *cachep, unsigned long flags)
> if (!cachep->num)
> return -E2BIG;
>
> - freelist_size =
> - ALIGN(cachep->num * sizeof(freelist_idx_t), cachep->align);
> + freelist_size = calculate_freelist_size(cachep->num, cachep->align);
>
> /*
> * If the slab has been placed off-slab, and we have enough space then
> @@ -2308,7 +2362,7 @@ __kmem_cache_create (struct kmem_cache *cachep, unsigned long flags)
>
> if (flags & CFLGS_OFF_SLAB) {
> /* really off slab. No need for manual alignment */
> - freelist_size = cachep->num * sizeof(freelist_idx_t);
> + freelist_size = calculate_freelist_size(cachep->num, 0);
>
> #ifdef CONFIG_PAGE_POISONING
> /* If we're going to use the generic kernel_map_pages()
> @@ -2612,6 +2666,7 @@ static void cache_init_objs(struct kmem_cache *cachep,
> if (cachep->ctor)
> cachep->ctor(objp);
> #endif
> + set_obj_status(page, i, OBJECT_FREE);
> set_free_obj(page, i, i);
> }
> }
> @@ -2820,6 +2875,7 @@ static void *cache_free_debugcheck(struct kmem_cache *cachep, void *objp,
> BUG_ON(objnr >= cachep->num);
> BUG_ON(objp != index_to_obj(cachep, page, objnr));
>
> + set_obj_status(page, objnr, OBJECT_FREE);
> if (cachep->flags & SLAB_POISON) {
> #ifdef CONFIG_DEBUG_PAGEALLOC
> if ((cachep->size % PAGE_SIZE)==0 && OFF_SLAB(cachep)) {
> @@ -2953,6 +3009,8 @@ static inline void cache_alloc_debugcheck_before(struct kmem_cache *cachep,
> static void *cache_alloc_debugcheck_after(struct kmem_cache *cachep,
> gfp_t flags, void *objp, unsigned long caller)
> {
> + struct page *page;
> +
> if (!objp)
> return objp;
> if (cachep->flags & SLAB_POISON) {
> @@ -2983,6 +3041,9 @@ static void *cache_alloc_debugcheck_after(struct kmem_cache *cachep,
> *dbg_redzone1(cachep, objp) = RED_ACTIVE;
> *dbg_redzone2(cachep, objp) = RED_ACTIVE;
> }
> +
> + page = virt_to_head_page(objp);
> + set_obj_status(page, obj_to_index(cachep, page, objp), OBJECT_ACTIVE);
> objp += obj_offset(cachep);
> if (cachep->ctor && cachep->flags & SLAB_POISON)
> cachep->ctor(objp);
> @@ -4219,21 +4280,12 @@ static void handle_slab(unsigned long *n, struct kmem_cache *c,
> struct page *page)
> {
> void *p;
> - int i, j;
> + int i;
>
> if (n[0] == n[1])
> return;
> for (i = 0, p = page->s_mem; i < c->num; i++, p += c->size) {
> - bool active = true;
> -
> - for (j = page->active; j < c->num; j++) {
> - /* Skip freed item */
> - if (get_free_obj(page, j) == i) {
> - active = false;
> - break;
> - }
> - }
> - if (!active)
> + if (get_obj_status(page, i) != OBJECT_ACTIVE)
> continue;
>
> if (!add_caller(n, (unsigned long)*dbg_userword(c, p)))
> --
> 1.7.9.5
>
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