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Subject[PATCH 5.8 046/177] btrfs: fix NULL pointer dereference after failure to create snapshot
Date
From: Filipe Manana <fdmanana@suse.com>

[ Upstream commit 2d892ccdc163a3d2e08c5ed1cea8b61bf7e4f531 ]

When trying to get a new fs root for a snapshot during the transaction
at transaction.c:create_pending_snapshot(), if btrfs_get_new_fs_root()
fails we leave "pending->snap" pointing to an error pointer, and then
later at ioctl.c:create_snapshot() we dereference that pointer, resulting
in a crash:

[12264.614689] BUG: kernel NULL pointer dereference, address: 00000000000007c4
[12264.615650] #PF: supervisor write access in kernel mode
[12264.616487] #PF: error_code(0x0002) - not-present page
[12264.617436] PGD 0 P4D 0
[12264.618328] Oops: 0002 [#1] PREEMPT SMP DEBUG_PAGEALLOC PTI
[12264.619150] CPU: 0 PID: 2310635 Comm: fsstress Tainted: G W 5.9.0-rc3-btrfs-next-67 #1
[12264.619960] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.13.0-0-gf21b5a4aeb02-prebuilt.qemu.org 04/01/2014
[12264.621769] RIP: 0010:btrfs_mksubvol+0x438/0x4a0 [btrfs]
[12264.622528] Code: bc ef ff ff (...)
[12264.624092] RSP: 0018:ffffaa6fc7277cd8 EFLAGS: 00010282
[12264.624669] RAX: 00000000fffffff4 RBX: ffff9d3e8f151a60 RCX: 0000000000000000
[12264.625249] RDX: 0000000000000001 RSI: ffffffff9d56c9be RDI: fffffffffffffff4
[12264.625830] RBP: ffff9d3e8f151b48 R08: 0000000000000000 R09: 0000000000000000
[12264.626413] R10: 0000000000000000 R11: 0000000000000000 R12: 00000000fffffff4
[12264.626994] R13: ffff9d3ede380538 R14: ffff9d3ede380500 R15: ffff9d3f61b2eeb8
[12264.627582] FS: 00007f140d5d8200(0000) GS:ffff9d3fb5e00000(0000) knlGS:0000000000000000
[12264.628176] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[12264.628773] CR2: 00000000000007c4 CR3: 000000020f8e8004 CR4: 00000000003706f0
[12264.629379] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[12264.629994] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[12264.630594] Call Trace:
[12264.631227] btrfs_mksnapshot+0x7b/0xb0 [btrfs]
[12264.631840] __btrfs_ioctl_snap_create+0x16f/0x1a0 [btrfs]
[12264.632458] btrfs_ioctl_snap_create_v2+0xb0/0xf0 [btrfs]
[12264.633078] btrfs_ioctl+0x1864/0x3130 [btrfs]
[12264.633689] ? do_sys_openat2+0x1a7/0x2d0
[12264.634295] ? kmem_cache_free+0x147/0x3a0
[12264.634899] ? __x64_sys_ioctl+0x83/0xb0
[12264.635488] __x64_sys_ioctl+0x83/0xb0
[12264.636058] do_syscall_64+0x33/0x80
[12264.636616] entry_SYSCALL_64_after_hwframe+0x44/0xa9

(gdb) list *(btrfs_mksubvol+0x438)
0x7c7b8 is in btrfs_mksubvol (fs/btrfs/ioctl.c:858).
853 ret = 0;
854 pending_snapshot->anon_dev = 0;
855 fail:
856 /* Prevent double freeing of anon_dev */
857 if (ret && pending_snapshot->snap)
858 pending_snapshot->snap->anon_dev = 0;
859 btrfs_put_root(pending_snapshot->snap);
860 btrfs_subvolume_release_metadata(root, &pending_snapshot->block_rsv);
861 free_pending:
862 if (pending_snapshot->anon_dev)

So fix this by setting "pending->snap" to NULL if we get an error from the
call to btrfs_get_new_fs_root() at transaction.c:create_pending_snapshot().

Fixes: 2dfb1e43f57dd3 ("btrfs: preallocate anon block device at first phase of snapshot creation")
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
fs/btrfs/transaction.c | 1 +
1 file changed, 1 insertion(+)

diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
index 2710f8ddb95fb..b43ebf55b93e1 100644
--- a/fs/btrfs/transaction.c
+++ b/fs/btrfs/transaction.c
@@ -1636,6 +1636,7 @@ static noinline int create_pending_snapshot(struct btrfs_trans_handle *trans,
pending->snap = btrfs_get_new_fs_root(fs_info, objectid, pending->anon_dev);
if (IS_ERR(pending->snap)) {
ret = PTR_ERR(pending->snap);
+ pending->snap = NULL;
btrfs_abort_transaction(trans, ret);
goto fail;
}
--
2.25.1


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