Re: [PATCH v2] btrfs: Fix NULL pointer access in btrfs_check_leaked_roots()

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

在 2025/10/16 16:40, Dewei Meng 写道:
> If fs_info->super_copy or fs_info->super_for_commit allocated failed in
> btrfs_get_tree_subvol(), then no need to call btrfs_free_fs_info().
> Otherwise btrfs_check_leaked_roots() would access NULL pointer because
> fs_info->allocated_roots had not been initialised.
> 
> 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
>    [...]
> 
> Fixes: 3bb17a25bcb0 ("btrfs: add get_tree callback for new mount API")
> Signed-off-by: Dewei Meng <mengdewei@cqsoftware.com.cn>
> Reviewed-by: Daniel Vacek <neelx@suse.com>

Reviewed-by: Qu Wenruo <wqu@suse.com>

Thanks,
Qu

> ---
> V1 -> V2:
> - Revise the patch description to make it easier to read
> - Delete btrfs_free_fs_info() when super_copy/super_for_commit allocated
>    failed, instead of NULL pointer check in btrfs_check_leaked_roots()
> 
>   fs/btrfs/super.c | 4 +++-
>   1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
> index d6e496436539..dc95e697b22b 100644
> --- a/fs/btrfs/super.c
> +++ b/fs/btrfs/super.c
> @@ -2069,7 +2069,9 @@ static int btrfs_get_tree_subvol(struct fs_context *fc)
>   	fs_info->super_copy = kzalloc(BTRFS_SUPER_INFO_SIZE, GFP_KERNEL);
>   	fs_info->super_for_commit = kzalloc(BTRFS_SUPER_INFO_SIZE, GFP_KERNEL);
>   	if (!fs_info->super_copy || !fs_info->super_for_commit) {
> -		btrfs_free_fs_info(fs_info);
> +		kfree(fs_info->super_copy);
> +		kfree(fs_info->super_for_commit);
> +		kvfree(fs_info);
>   		return -ENOMEM;
>   	}
>   	btrfs_init_fs_info(fs_info);