linux-kernel - Re: [PATCH] btrfs: Fix NULL pointer access in btrfs_check_leaked

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Message-ID: <74bc8d0a-70a6-4f4a-b6a1-ecdf1b2e3f3b@gmx.com>
Date: Wed, 15 Oct 2025 18:56:37 +1030
From: Qu Wenruo <quwenruo.btrfs@....com>
To: Dewei Meng <mengdewei@...oftware.com.cn>, clm@...com, dsterba@...e.com
Cc: linux-btrfs@...r.kernel.org, linux-kernel@...r.kernel.org,
 josef@...icpanda.com
Subject: Re: [PATCH] btrfs: Fix NULL pointer access in
 btrfs_check_leaked_roots()



在 2025/10/15 18:54, Qu Wenruo 写道:
> 
> 
> 在 2025/10/15 17:54, Dewei Meng 写道:
>> If fs_info->super_copy or fs_info->super_for_commit is NULL in
>> btrfs_get_tree_subvol(),
> 
> Please reorganize this sentence. It would be way more easier to read by 
> just saying something like "If memory allocation failed for fs_info- 
>  >super_copy or fs_info->super_for_commit in btrfs_get_tree_subvol()".
> 
>> the btrfs_check_leaked_roots() will get the
>> btrfs_root list entry using the fs_info->allocated_roots->next
>> which is NULL.
>>
>> syzkaller reported the following information:
>>    ------------[ cut here ]------------
>>    BUG: unable to handle page fault for address: fffffffffffffbb0
>>    #PF: supervisor read access in kernel mode
>>    #PF: error_code(0x0000) - not-present page
>>    PGD 64c9067 P4D 64c9067 PUD 64cb067 PMD 0
>>    Oops: Oops: 0000 [#1] SMP KASAN PTI
>>    CPU: 0 UID: 0 PID: 1402 Comm: syz.1.35 Not tainted 6.15.8 #4 
>> PREEMPT(lazy)
>>    Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), (...)
>>    RIP: 0010:arch_atomic_read arch/x86/include/asm/atomic.h:23 [inline]
>>    RIP: 0010:raw_atomic_read include/linux/atomic/atomic-arch- 
>> fallback.h:457 [inline]
>>    RIP: 0010:atomic_read include/linux/atomic/atomic-instrumented.h:33 
>> [inline]
>>    RIP: 0010:refcount_read include/linux/refcount.h:170 [inline]
>>    RIP: 0010:btrfs_check_leaked_roots+0x18f/0x2c0 fs/btrfs/disk-io.c:1230
>>    [...]
>>    Call Trace:
>>     <TASK>
>>     btrfs_free_fs_info+0x310/0x410 fs/btrfs/disk-io.c:1280
>>     btrfs_get_tree_subvol+0x592/0x6b0 fs/btrfs/super.c:2029
>>     btrfs_get_tree+0x63/0x80 fs/btrfs/super.c:2097
>>     vfs_get_tree+0x98/0x320 fs/super.c:1759
>>     do_new_mount+0x357/0x660 fs/namespace.c:3899
>>     path_mount+0x716/0x19c0 fs/namespace.c:4226
>>     do_mount fs/namespace.c:4239 [inline]
>>     __do_sys_mount fs/namespace.c:4450 [inline]
>>     __se_sys_mount fs/namespace.c:4427 [inline]
>>     __x64_sys_mount+0x28c/0x310 fs/namespace.c:4427
>>     do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>>     do_syscall_64+0x92/0x180 arch/x86/entry/syscall_64.c:94
>>     entry_SYSCALL_64_after_hwframe+0x76/0x7e
>>    RIP: 0033:0x7f032eaffa8d
>>    [...]
>>
>> This should check if the fs_info->allocated_roots->next is NULL before
>> accessing it.
>>
>> Fixes: 3bb17a25bcb0 ("btrfs: add get_tree callback for new mount API")
>> Signed-off-by: Dewei Meng <mengdewei@...oftware.com.cn>
>> ---
>>   fs/btrfs/disk-io.c | 3 +++
>>   1 file changed, 3 insertions(+)
>>
>> diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
>> index 0aa7e5d1b05f..76db7f98187a 100644
>> --- a/fs/btrfs/disk-io.c
>> +++ b/fs/btrfs/disk-io.c
>> @@ -1213,6 +1213,9 @@ void btrfs_check_leaked_roots(const struct 
>> btrfs_fs_info *fs_info)
>>   #ifdef CONFIG_BTRFS_DEBUG
>>       struct btrfs_root *root;
>> +    if (!fs_info->allocated_roots.next)
>> +        return;
>> +
> 
> The check looks too adhoc to me.
> 
> It would be much easier to just call kvfree()

Of course with kfree() on the super_copy/super_for_commit before 
kvfree() on the fs_info.

> in the error handling of 
> super_copy/super_for_commit allocation, we do not and should not call 
> btrfs_free_fs_info() before calling btrfs_init_fs_info().
> 
> Thanks,
> Qu
>>       while (!list_empty(&fs_info->allocated_roots)) {
>>           char buf[BTRFS_ROOT_NAME_BUF_LEN];
> 
>