Re:[PATCH v3] lib/alloc_tag: do not acquire non-existent lock in alloc_tag_top_users()

["David Wang" <00107082@163.com>]

At 2025-06-24 15:25:13, "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.
>
>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 to acquire the semaphore.
>
>Reported-by: kernel test robot <oliver.sang@intel.com>
>Closes: https://lore.kernel.org/oe-lkp/202506181351.bba867dd-lkp@intel.com
>Closes: https://lore.kernel.org/oe-lkp/202506131711.5b41931c-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>
>---
>
>@Suren: I did not add another pr_warn() because every error path in
>alloc_tag_init() already has pr_err().
>
>v2 -> v3:
>- Added another Closes: tag (David)
>- Moved the condition into a standalone if block for better readability
>  (Suren)
>- Typo fix (Suren)
>
> lib/alloc_tag.c | 3 +++
> 1 file changed, 3 insertions(+)
>
>diff --git a/lib/alloc_tag.c b/lib/alloc_tag.c
>index 41ccfb035b7b..e9b33848700a 100644
>--- a/lib/alloc_tag.c
>+++ b/lib/alloc_tag.c
>@@ -127,6 +127,9 @@ 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 (IS_ERR_OR_NULL(alloc_tag_cttype))

Should a warning  added here? indicating  codetag module not ready yet and the memory failure happened during boot:
 if (mem_profiling_support) pr_warn("...


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