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

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Message-ID: <6a2bb5c7-0aab-4662-938f-38b8e2372338@gmx.com>
Date: Wed, 15 Oct 2025 18:54:37 +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 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() 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];
>