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

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Message-ID: <CAPjX3FdquokJ380EuYVf-m4M5o=_PwQkg+OJN-cOSHkCc-N97w@mail.gmail.com>
Date: Wed, 15 Oct 2025 09:59:20 +0200
From: Daniel Vacek <neelx@...e.com>
To: Dewei Meng <mengdewei@...oftware.com.cn>
Cc: clm@...com, dsterba@...e.com, 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()

On Wed, 15 Oct 2025 at 09:39, Dewei Meng <mengdewei@...oftware.com.cn> wrote:
>
> If fs_info->super_copy or fs_info->super_for_commit is NULL 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(+)

Looks good to me.
Reviewed-by: Daniel Vacek <neelx@...e.com>

Thank you

>
> 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;
> +
>         while (!list_empty(&fs_info->allocated_roots)) {
>                 char buf[BTRFS_ROOT_NAME_BUF_LEN];
>
> --
> 2.43.5
>
>