Re: [PATCH V1] mm, slub: avoid zeroing kmalloc redzone

[Feng Tang <feng.tang@intel.com>]

On Fri, Aug 23, 2024 at 02:24:15PM +0800, Peng Fan (OSS) wrote:
> From: Peng Fan <peng.fan@nxp.com>
> 
> With commit 946fa0dbf2d8
> ("mm/slub: extend redzone check to extra allocated kmalloc space than requested"),
> setting orig_size treats the wasted space (object_size - orig_size) as
> redzones. But (in check_object()) when orig_size is set to zero, the entire
> object is perceived as a redzone. To a valid allocated kmalloc space,
> when init_on_free=1, the wasted space and the orig_size should
> not be cleared to 0, otherwise there will be kernel dump:
> 
> [    0.000000] =============================================================================
> [    0.000000] BUG kmalloc-8 (Not tainted): kmalloc Redzone overwritten
> [    0.000000] -----------------------------------------------------------------------------
> [    0.000000]
> [    0.000000] 0xffff000010032858-0xffff00001003285f @offset=2136. First byte 0x0 instead of 0xcc
> [    0.000000] FIX kmalloc-8: Restoring kmalloc Redzone 0xffff000010032858-0xffff00001003285f=0xcc
> [    0.000000] Slab 0xfffffdffc0400c80 objects=36 used=23 fp=0xffff000010032a18 flags=0x3fffe0000000200(workingset|node=0|zone=0|lastcpupid=0x1ffff)
> [    0.000000] Object 0xffff000010032858 @offset=2136 fp=0xffff0000100328c8
> [    0.000000]
> [    0.000000] Redzone  ffff000010032850: cc cc cc cc cc cc cc cc                          ........
> [    0.000000] Object   ffff000010032858: cc cc cc cc cc cc cc cc                          ........
> [    0.000000] Redzone  ffff000010032860: cc cc cc cc cc cc cc cc                          ........
> [    0.000000] Padding  ffff0000100328b4: 00 00 00 00 00 00 00 00 00 00 00 00              ............
> [    0.000000] CPU: 0 UID: 0 PID: 0 Comm: swapper/0 Not tainted 6.11.0-rc3-next-20240814-00004-g61844c55c3f4 #144
> [    0.000000] Hardware name: NXP i.MX95 19X19 board (DT)
> [    0.000000] Call trace:
> [    0.000000]  dump_backtrace+0x90/0xe8
> [    0.000000]  show_stack+0x18/0x24
> [    0.000000]  dump_stack_lvl+0x74/0x8c
> [    0.000000]  dump_stack+0x18/0x24
> [    0.000000]  print_trailer+0x150/0x218
> [    0.000000]  check_object+0xe4/0x454
> [    0.000000]  free_to_partial_list+0x2f8/0x5ec
> 
> To address the issue, use orig_size to clear the used area. And restore
> the value of orig_size after clear the remaining area.
> 
> When CONFIG_SLUB_DEBUG not defined, (get_orig_size()' directly returns
> s->object_size. So when using memset to init the area, the size can simply
> be orig_size, as orig_size returns object_size when CONFIG_SLUB_DEBUG not
> enabled. And orig_size can never be bigger than object_size.
> 
> Fixes: 946fa0dbf2d8 ("mm/slub: extend redzone check to extra allocated kmalloc space than requested")

Thanks for the fix! I missed to test the 'init_on_free' case back then.

Reviewed-by: Feng Tang <feng.tang@intel.com>

with one small nit below

> Signed-off-by: Peng Fan <peng.fan@nxp.com>
> ---
> 
> RFC->V1:
>  Update commit log (Per Hyeonggon)
>  Use orig_size to do memset(Per Hyeonggon)
>  Add get_orig_size and set_orig_size when CONFIG_SLUB_DEBUG not enabled(kernel test robot)
>  https://lore.kernel.org/all/20240819064115.385086-1-peng.fan@oss.nxp.com/
> 
>  mm/slub.c | 20 ++++++++++++++++++--
>  1 file changed, 18 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 94f5a4143825..a5fbeb2835b1 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -1895,6 +1895,15 @@ static inline void inc_slabs_node(struct kmem_cache *s, int node,
>  static inline void dec_slabs_node(struct kmem_cache *s, int node,
>  							int objects) {}
>  
> +static inline unsigned int get_orig_size(struct kmem_cache *s, void *object)
> +{
> +	return s->object_size;
> +}
> +
> +static inline void set_orig_size(struct kmem_cache *s, void *object,
> +				 unsigned int orig_size)
> +{}

Current get_orig_size() and set_orig_size() are protected by
CONFIG_SLUB_DEUG=y macro, and with this patch, they will be called
in both ON and OFF case. Maybe we can just lift those existing
functions out of the "#ifdef CONFIG_SLUB_DEBUG" protection?

Thanks,
Feng