[PATCH] btrfs: reloc: Fix NULL pointer dereference due to expanded reloc_root lifespan

[Qu Wenruo <wqu@suse.com>]

Commit 514e70150b60 ("btrfs: relocation: Delay reloc tree deletion after
merge_reloc_roots()") expands the life span of root->reloc_root.

This breaks certain check on fs_info->reloc_ctl.
Before that commit, if we have a root with valid reloc_root, then it's
ensured to have fs_info->reloc_ctl.

But now since reloc_root doesn't always mean a valid fs_info->reloc_ctl,
such check is unreliable and can cause the following NULL pointer
dereference:
BUG: unable to handle kernel NULL pointer dereference at 00000000000005c1
IP: btrfs_reloc_pre_snapshot+0x20/0x50 [btrfs]
PGD 0 P4D 0
Oops: 0000 [#1] SMP PTI
CPU: 0 PID: 10379 Comm: snapperd Not tainted 4.12.14-lp150.12.53-default #1 openSUSE Leap 15.0
Call Trace:
 create_pending_snapshot+0xd7/0xfc0 [btrfs]
 create_pending_snapshots+0x8e/0xb0 [btrfs]
 btrfs_commit_transaction+0x2ac/0x8f0 [btrfs]
 btrfs_mksubvol+0x561/0x570 [btrfs]
 btrfs_ioctl_snap_create_transid+0x189/0x190 [btrfs]
 btrfs_ioctl_snap_create_v2+0x102/0x150 [btrfs]
 btrfs_ioctl+0x5c9/0x1e60 [btrfs]
 do_vfs_ioctl+0x90/0x5f0
 SyS_ioctl+0x74/0x80
 do_syscall_64+0x7b/0x150
 entry_SYSCALL_64_after_hwframe+0x3d/0xa2
RIP: 0033:0x7fd7cdab8467

Fix it by explicitly checking fs_info->reloc_ctl other than using the
implied root->reloc_root.

Fixes: 514e70150b60 ("btrfs: relocation: Delay reloc tree deletion after merge_reloc_roots()")
Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 fs/btrfs/relocation.c | 12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
index ddf028509931..351fa506dc9b 100644
--- a/fs/btrfs/relocation.c
+++ b/fs/btrfs/relocation.c
@@ -4667,14 +4667,12 @@ int btrfs_reloc_cow_block(struct btrfs_trans_handle *trans,
 void btrfs_reloc_pre_snapshot(struct btrfs_pending_snapshot *pending,
 			      u64 *bytes_to_reserve)
 {
-	struct btrfs_root *root;
-	struct reloc_control *rc;
+	struct btrfs_root *root = pending->root;
+	struct reloc_control *rc = root->fs_info->reloc_ctl;
 
-	root = pending->root;
-	if (!root->reloc_root)
+	if (!root->reloc_root || !rc)
 		return;
 
-	rc = root->fs_info->reloc_ctl;
 	if (!rc->merge_reloc_tree)
 		return;
 
@@ -4703,10 +4701,10 @@ int btrfs_reloc_post_snapshot(struct btrfs_trans_handle *trans,
 	struct btrfs_root *root = pending->root;
 	struct btrfs_root *reloc_root;
 	struct btrfs_root *new_root;
-	struct reloc_control *rc;
+	struct reloc_control *rc = root->fs_info->reloc_ctl;
 	int ret;
 
-	if (!root->reloc_root)
+	if (!root->reloc_root || !rc)
 		return 0;
 
 	rc = root->fs_info->reloc_ctl;
-- 
2.21.0