Re: [PATCH 1/2] kasan, mm: reset tag when access metadata

[Marco Elver <elver@google.com>]

On Tue, 27 Jul 2021 at 10:32, Kuan-Ying Lee <Kuan-Ying.Lee@mediatek.com> wrote:
>
> On Tue, 2021-07-27 at 09:10 +0200, Marco Elver wrote:
> > +Cc Catalin
> >
> > On Tue, 27 Jul 2021 at 06:00, Kuan-Ying Lee <
> > Kuan-Ying.Lee@mediatek.com> wrote:
> > >
> > > Hardware tag-based KASAN doesn't use compiler instrumentation, we
> > > can not use kasan_disable_current() to ignore tag check.
> > >
> > > Thus, we need to reset tags when accessing metadata.
> > >
> > > Signed-off-by: Kuan-Ying Lee <Kuan-Ying.Lee@mediatek.com>
> >
> > This looks reasonable, but the patch title is not saying this is
> > kmemleak, nor does the description say what the problem is. What
> > problem did you encounter? Was it a false positive?
>
> kmemleak would scan kernel memory to check memory leak.
> When it scans on the invalid slab and dereference, the issue
> will occur like below.

Please also add this info to commit message.

> So I think we should reset the tag before scanning.
>
> # echo scan > /sys/kernel/debug/kmemleak
> [  151.905804]
> ==================================================================
> [  151.907120] BUG: KASAN: out-of-bounds in scan_block+0x58/0x170
> [  151.908773] Read at addr f7ff0000c0074eb0 by task kmemleak/138
> [  151.909656] Pointer tag: [f7], memory tag: [fe]
> [  151.910195]
> [  151.910876] CPU: 7 PID: 138 Comm: kmemleak Not tainted 5.14.0-rc2-
> 00001-g8cae8cd89f05-dirty #134
> [  151.912085] Hardware name: linux,dummy-virt (DT)
> [  151.912868] Call trace:
> [  151.913211]  dump_backtrace+0x0/0x1b0
> [  151.913796]  show_stack+0x1c/0x30
> [  151.914248]  dump_stack_lvl+0x68/0x84
> [  151.914778]  print_address_description+0x7c/0x2b4
> [  151.915340]  kasan_report+0x138/0x38c
> [  151.915804]  __do_kernel_fault+0x190/0x1c4
> [  151.916386]  do_tag_check_fault+0x78/0x90
> [  151.916856]  do_mem_abort+0x44/0xb4
> [  151.917308]  el1_abort+0x40/0x60
> [  151.917754]  el1h_64_sync_handler+0xb4/0xd0
> [  151.918270]  el1h_64_sync+0x78/0x7c
> [  151.918714]  scan_block+0x58/0x170
> [  151.919157]  scan_gray_list+0xdc/0x1a0
> [  151.919626]  kmemleak_scan+0x2ac/0x560
> [  151.920129]  kmemleak_scan_thread+0xb0/0xe0
> [  151.920635]  kthread+0x154/0x160
> [  151.921115]  ret_from_fork+0x10/0x18
> [  151.921717]
> [  151.922077] Allocated by task 0:
> [  151.922523]  kasan_save_stack+0x2c/0x60
> [  151.923099]  __kasan_kmalloc+0xec/0x104
> [  151.923502]  __kmalloc+0x224/0x3c4
> [  151.924172]  __register_sysctl_paths+0x200/0x290
> [  151.924709]  register_sysctl_table+0x2c/0x40
> [  151.925175]  sysctl_init+0x20/0x34
> [  151.925665]  proc_sys_init+0x3c/0x48
> [  151.926136]  proc_root_init+0x80/0x9c
> [  151.926547]  start_kernel+0x648/0x6a4
> [  151.926987]  __primary_switched+0xc0/0xc8
> [  151.927557]
> [  151.927994] Freed by task 0:
> [  151.928340]  kasan_save_stack+0x2c/0x60
> [  151.928766]  kasan_set_track+0x2c/0x40
> [  151.929173]  kasan_set_free_info+0x44/0x54
> [  151.929568]  ____kasan_slab_free.constprop.0+0x150/0x1b0
> [  151.930063]  __kasan_slab_free+0x14/0x20
> [  151.930449]  slab_free_freelist_hook+0xa4/0x1fc
> [  151.930924]  kfree+0x1e8/0x30c
> [  151.931285]  put_fs_context+0x124/0x220
> [  151.931731]  vfs_kern_mount.part.0+0x60/0xd4
> [  151.932280]  kern_mount+0x24/0x4c
> [  151.932686]  bdev_cache_init+0x70/0x9c
> [  151.933122]  vfs_caches_init+0xdc/0xf4
> [  151.933578]  start_kernel+0x638/0x6a4
> [  151.934014]  __primary_switched+0xc0/0xc8
> [  151.934478]
> [  151.934757] The buggy address belongs to the object at
> ffff0000c0074e00
> [  151.934757]  which belongs to the cache kmalloc-256 of size 256
> [  151.935744] The buggy address is located 176 bytes inside of
> [  151.935744]  256-byte region [ffff0000c0074e00, ffff0000c0074f00)
> [  151.936702] The buggy address belongs to the page:
> [  151.937378] page:(____ptrval____) refcount:1 mapcount:0
> mapping:0000000000000000 index:0x0 pfn:0x100074
> [  151.938682] head:(____ptrval____) order:2 compound_mapcount:0
> compound_pincount:0
> [  151.939440] flags:
> 0xbfffc0000010200(slab|head|node=0|zone=2|lastcpupid=0xffff|kasantag=0x
> 0)
> [  151.940886] raw: 0bfffc0000010200 0000000000000000 dead000000000122
> f5ff0000c0002300
> [  151.941634] raw: 0000000000000000 0000000000200020 00000001ffffffff
> 0000000000000000
> [  151.942353] page dumped because: kasan: bad access detected
> [  151.942923]
> [  151.943214] Memory state around the buggy address:
> [  151.943896]  ffff0000c0074c00: f0 f0 f0 f0 f0 f0 f0 f0 f0 fe fe fe
> fe fe fe fe
> [  151.944857]  ffff0000c0074d00: fe fe fe fe fe fe fe fe fe fe fe fe
> fe fe fe fe
> [  151.945892] >ffff0000c0074e00: f7 f7 f7 f7 f7 f7 f7 f7 f7 f7 f7 fe
> fe fe fe fe
> [  151.946407]                                                     ^
> [  151.946939]  ffff0000c0074f00: fe fe fe fe fe fe fe fe fe fe fe fe
> fe fe fe fe
> [  151.947445]  ffff0000c0075000: fb fb fb fb fb fb fb fb fb fb fb fb
> fb fb fb fb
> [  151.947999]
> ==================================================================
> [  151.948524] Disabling lock debugging due to kernel taint
> [  156.434569] kmemleak: 181 new suspected memory leaks (see
> /sys/kernel/debug/kmemleak)
>
> >
> > Perhaps this should have been "kmemleak, kasan: reset pointer tags to
> > avoid false positives" ?
>
> Thanks for the suggestions.
> But I think it doesn't belong to false
> positive becuase scan block
> touched invalid metadata certainly.

It's how kmemleak works, so we knowingly access invalid memory, and
for all intents and purposes it's a false positive.

> Maybe "kmemleak, kasan: reset tags when scanning block"?

That's fine.

Thanks,
-- Marco