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

[Harry Yoo <harry.yoo@oracle.com>]

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.

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.

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>
---

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