Re: [PATCH v2] lib/alloc_tag: do not acquire non-existent lock in alloc_tag_top_users()

[Suren Baghdasaryan <surenb@google.com>]

On Fri, Jun 20, 2025 at 8:43 PM David Wang <00107082@163.com> wrote:
>
>
> At 2025-06-21 03:53:05, "Harry Yoo" <harry.yoo@oracle.com> wrote:
> >alloc_tag_top_users() attempts to lock alloc_tag_cttype->mod_lock
> >even when the alloc_tag_cttype is not allocated because:
> >
> >  1) alloc tagging is disabled because mem profiling is disabled
> >     (!alloc_tag_cttype)
> >  2) alloc tagging is enabled, but not yet initialized (!alloc_tag_cttype)
> >  3) alloc tagging is enabled, but failed initialization
> >     (!alloc_tag_cttype or IS_ERR(alloc_tag_cttype))
> >
> >In all cases, alloc_tag_cttype is not allocated, and therefore
> >alloc_tag_top_users() should not attempt to acquire the semaphore.
> >
> >This leads to a crash on memory allocation failure by attempting to
> >acquire a non-existent semaphore:
> >
> >  Oops: general protection fault, probably for non-canonical address 0xdffffc000000001b: 0000 [#3] SMP KASAN NOPTI
> >  KASAN: null-ptr-deref in range [0x00000000000000d8-0x00000000000000df]
> >  CPU: 2 UID: 0 PID: 1 Comm: systemd Tainted: G      D             6.16.0-rc2 #1 VOLUNTARY
> >  Tainted: [D]=DIE
> >  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
> >  RIP: 0010:down_read_trylock+0xaa/0x3b0
> >  Code: d0 7c 08 84 d2 0f 85 a0 02 00 00 8b 0d df 31 dd 04 85 c9 75 29 48 b8 00 00 00 00 00 fc ff df 48 8d 6b 68 48 89 ea 48 c1 ea 03 <80> 3c 02 00 0f 85 88 02 00 00 48 3b 5b 68 0f 85 53 01 00 00 65 ff
> >  RSP: 0000:ffff8881002ce9b8 EFLAGS: 00010016
> >  RAX: dffffc0000000000 RBX: 0000000000000070 RCX: 0000000000000000
> >  RDX: 000000000000001b RSI: 000000000000000a RDI: 0000000000000070
> >  RBP: 00000000000000d8 R08: 0000000000000001 R09: ffffed107dde49d1
> >  R10: ffff8883eef24e8b R11: ffff8881002cec20 R12: 1ffff11020059d37
> >  R13: 00000000003fff7b R14: ffff8881002cec20 R15: dffffc0000000000
> >  FS:  00007f963f21d940(0000) GS:ffff888458ca6000(0000) knlGS:0000000000000000
> >  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> >  CR2: 00007f963f5edf71 CR3: 000000010672c000 CR4: 0000000000350ef0
> >  Call Trace:
> >   <TASK>
> >   codetag_trylock_module_list+0xd/0x20
> >   alloc_tag_top_users+0x369/0x4b0
> >   __show_mem+0x1cd/0x6e0
> >   warn_alloc+0x2b1/0x390
> >   __alloc_frozen_pages_noprof+0x12b9/0x21a0
> >   alloc_pages_mpol+0x135/0x3e0
> >   alloc_slab_page+0x82/0xe0
> >   new_slab+0x212/0x240
> >   ___slab_alloc+0x82a/0xe00
> >   </TASK>
> >
> >As David Wang points out, this issue became easier to trigger after commit
> >780138b12381 ("alloc_tag: check mem_profiling_support in alloc_tag_init").
> >
> >Before the commit, the issue occurred only when it failed to allocate
> >and initialize alloc_tag_cttype or if a memory allocation fails before
> >alloc_tag_init() is called. After the commit, it can be easily triggered
> >when memory profiling is compiled but disabled at boot.

Thanks for the fix and sorry about the delay with reviewing it.

> >
> >To properly determine whether alloc_tag_init() has been called and
> >its data structures initialized, verify that alloc_tag_cttype is a valid
> >pointer before acquiring the semaphore. If the variable is NULL or an error
> >value, it has not been properly initialized. In such a case, just skip
> >and do not attempt acquire the semaphore.

nit: s/attempt acquire/attempt to acquire

> >
> >Reported-by: kernel test robot <oliver.sang@intel.com>
> >Closes: https://lore.kernel.org/oe-lkp/202506181351.bba867dd-lkp@intel.com
> >Fixes: 780138b12381 ("alloc_tag: check mem_profiling_support in alloc_tag_init")
> >Fixes: 1438d349d16b ("lib: add memory allocations report in show_mem()")
> >Cc: stable@vger.kernel.org
> >Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
>
> Just notice another thread can be closed as well:
> https://lore.kernel.org/all/202506131711.5b41931c-lkp@intel.com/
> This coincide with scenario #1, where OOM happened with
> CONFIG_MEM_ALLOC_PROFILING=y
> # CONFIG_MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT is not set
> # CONFIG_MEM_ALLOC_PROFILING_DEBUG is not set
>
> >---
> >
> >v1 -> v2:
> >
> >- v1 fixed the bug only when MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT=n.
> >
> >  v2 now fixes the bug even when MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT=y.
> >  I didn't expect alloc_tag_cttype to be NULL when
> >  mem_profiling_support is true, but as David points out (Thanks David!)
> >  if a memory allocation fails before alloc_tag_init(), it can be NULL.
> >
> >  So instead of indirectly checking mem_profiling_support, just directly
> >  check if alloc_tag_cttype is allocated.
> >
> >- Closes: https://lore.kernel.org/oe-lkp/202505071555.e757f1e0-lkp@intel.com
> >  tag was removed because it was not a crash and not relevant to this
> >  patch.
> >
> >- Added Cc: stable because, if an allocation fails before
> >  alloc_tag_init(), it can be triggered even prior-780138b12381.
> >  I verified that the bug can be triggered in v6.12 and fixed by this
> >  patch.
> >
> >  It should be quite difficult to trigger in practice, though.
> >  Maybe I'm a bit paranoid?
> >
> > lib/alloc_tag.c | 4 +++-
> > 1 file changed, 3 insertions(+), 1 deletion(-)
> >
> >diff --git a/lib/alloc_tag.c b/lib/alloc_tag.c
> >index 66a4628185f7..d8ec4c03b7d2 100644
> >--- a/lib/alloc_tag.c
> >+++ b/lib/alloc_tag.c
> >@@ -124,7 +124,9 @@ size_t alloc_tag_top_users(struct codetag_bytes *tags, size_t count, bool can_sl
> >       struct codetag_bytes n;
> >       unsigned int i, nr = 0;
> >
> >-      if (can_sleep)
> >+      if (IS_ERR_OR_NULL(alloc_tag_cttype))
> >+              return 0;

So, AFAIKT alloc_tag_cttype will be NULL when memory profiling is
disabled and it will be ENOMEM if codetag_register_type() fails. I
think it would be good to add a pr_warn() in the alloc_tag_init() when
codetag_register_type() fails so that the user can determine the
reason why show_mem() report is missing allocation tag information.

> >+      else if (can_sleep)

nit: the above extra "else" is not really needed. The following should
work just fine, is more readable and produces less churn:

+      if (IS_ERR_OR_NULL(alloc_tag_cttype))
+              return 0;
+
      if (can_sleep)
               codetag_lock_module_list(alloc_tag_cttype, true);
       else if (!codetag_trylock_module_list(alloc_tag_cttype))
               return 0;

> >               codetag_lock_module_list(alloc_tag_cttype, true);
> >       else if (!codetag_trylock_module_list(alloc_tag_cttype))
> >               return 0;
> >--
> >2.43.0