[PATCH 5.7 034/204] btrfs: trim: fix underflow in trim length to prevent access beyond device boundary

Greg Kroah-Hartman
2020-08-20T13:10:35Z

From: Qu Wenruo <wqu@suse.com>

commit c57dd1f2f6a7cd1bb61802344f59ccdc5278c983 upstream.

[BUG]
The following script can lead to tons of beyond device boundary access:

mkfs.btrfs -f $dev -b 10G
mount $dev $mnt
trimfs $mnt
btrfs filesystem resize 1:-1G $mnt
trimfs $mnt

[CAUSE]
Since commit 929be17a9b49 ("btrfs: Switch btrfs_trim_free_extents to
find_first_clear_extent_bit"), we try to avoid trimming ranges that's
already trimmed.

So we check device->alloc_state by finding the first range which doesn't
have CHUNK_TRIMMED and CHUNK_ALLOCATED not set.

But if we shrunk the device, that bits are not cleared, thus we could
easily got a range starts beyond the shrunk device size.

This results the returned @start and @end are all beyond device size,
then we call "end = min(end, device->total_bytes -1);" making @end
smaller than device size.

Then finally we goes "len = end - start + 1", totally underflow the
result, and lead to the beyond-device-boundary access.

[FIX]
This patch will fix the problem in two ways:

- Clear CHUNK_TRIMMED | CHUNK_ALLOCATED bits when shrinking device
This is the root fix

- Add extra safety check when trimming free device extents
We check and warn if the returned range is already beyond current
device.

Link: https://github.com/kdave/btrfs-progs/issues/282
Fixes: 929be17a9b49 ("btrfs: Switch btrfs_trim_free_extents to find_first_clear_extent_bit")
CC: stable@vger.kernel.org # 5.4+
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
fs/btrfs/extent-io-tree.h | 2 ++
fs/btrfs/extent-tree.c | 14 ++++++++++++++
fs/btrfs/volumes.c | 4 ++++
3 files changed, 20 insertions(+)

--- a/fs/btrfs/extent-io-tree.h
+++ b/fs/btrfs/extent-io-tree.h
@@ -34,6 +34,8 @@ struct io_failure_record;
*/
#define CHUNK_ALLOCATED EXTENT_DIRTY
#define CHUNK_TRIMMED EXTENT_DEFRAG
+#define CHUNK_STATE_MASK (CHUNK_ALLOCATED | \
+ CHUNK_TRIMMED)

enum {
IO_TREE_FS_PINNED_EXTENTS,
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -33,6 +33,7 @@
#include "delalloc-space.h"
#include "block-group.h"
#include "discard.h"
+#include "rcu-string.h"

#undef SCRAMBLE_DELAYED_REFS

@@ -5685,6 +5686,19 @@ static int btrfs_trim_free_extents(struc
&start, &end,
CHUNK_TRIMMED | CHUNK_ALLOCATED);

+ /* Check if there are any CHUNK_* bits left */
+ if (start > device->total_bytes) {
+ WARN_ON(IS_ENABLED(CONFIG_BTRFS_DEBUG));
+ btrfs_warn_in_rcu(fs_info,
+"ignoring attempt to trim beyond device size: offset %llu length %llu device %s device size %llu",
+ start, end - start + 1,
+ rcu_str_deref(device->name),
+ device->total_bytes);
+ mutex_unlock(&fs_info->chunk_mutex);
+ ret = 0;
+ break;
+ }
+
/* Ensure we skip the reserved area in the first 1M */
start = max_t(u64, start, SZ_1M);

--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -4724,6 +4724,10 @@ again:
}

mutex_lock(&fs_info->chunk_mutex);
+ /* Clear all state bits beyond the shrunk device size */
+ clear_extent_bits(&device->alloc_state, new_size, (u64)-1,
+ CHUNK_STATE_MASK);
+
btrfs_device_set_disk_total_bytes(device, new_size);
if (list_empty(&device->post_commit_list))
list_add_tail(&device->post_commit_list,