Re: [PATCH V2] mm, slub: avoid zeroing kmalloc redzone

[David Rientjes]

On Thu, 29 Aug 2024, Peng Fan (OSS) wrote:

> From: Peng Fan <peng.fan@xxxxxxx>
> 
> Since 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 a redzone. However with
> init_on_free=1 we clear the full object->size, including the redzone.
> 
> Additionally we clear the object metadata, including the stored orig_size,
> making it zero, which makes check_object() treat the whole object as a
> redzone.
> 
> These issues lead to the following BUG report with "slub_debug=FUZ
> init_on_free=1":
> 
> [    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")
> Reviewed-by: Feng Tang <feng.tang@xxxxxxxxx>
> Signed-off-by: Peng Fan <peng.fan@xxxxxxx>

Acked-by: David Rientjes <rientjes@xxxxxxxxxx>