[SRU][F][PATCH 1/1] btrfs: trim: fix underflow in trim length to prevent access beyond device boundary

[Matthew Ruffell]

From: Qu Wenruo <wqu at suse.com>

BugLink: https://bugs.launchpad.net/bugs/1896154

[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 at vger.kernel.org # 5.4+
Signed-off-by: Qu Wenruo <wqu at suse.com>
Reviewed-by: Filipe Manana <fdmanana at suse.com>
Signed-off-by: David Sterba <dsterba at suse.com>
(backported from commit c57dd1f2f6a7cd1bb61802344f59ccdc5278c983)
[mruffell: CHUNK_STATE_MASK file changed to extent_io.h, context fixups]
Signed-off-by: Matthew Ruffell <matthew.ruffell at canonical.com>
---
 fs/btrfs/extent-tree.c | 14 ++++++++++++++
 fs/btrfs/extent_io.h   |  2 ++
 fs/btrfs/volumes.c     |  4 ++++
 3 files changed, 20 insertions(+)

diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 541497036cc2..aad83d3b8f93 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -32,6 +32,7 @@
 #include "block-rsv.h"
 #include "delalloc-space.h"
 #include "block-group.h"
+#include "rcu-string.h"
 
 #undef SCRAMBLE_DELAYED_REFS
 
@@ -5599,6 +5600,19 @@ static int btrfs_trim_free_extents(struct btrfs_device *device, u64 *trimmed)
 					    &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);
 
diff --git a/fs/btrfs/extent_io.h b/fs/btrfs/extent_io.h
index bc858c8cef0a..fcf1807cc8dd 100644
--- a/fs/btrfs/extent_io.h
+++ b/fs/btrfs/extent_io.h
@@ -35,6 +35,8 @@
  */
 #define CHUNK_ALLOCATED EXTENT_DIRTY
 #define CHUNK_TRIMMED   EXTENT_DEFRAG
+#define CHUNK_STATE_MASK			(CHUNK_ALLOCATED |		\
+						 CHUNK_TRIMMED)
 
 /*
  * flags for bio submission. The high bits indicate the compression
diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 81be71fb569e..489cc21d7c6f 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -4907,6 +4907,10 @@ int btrfs_shrink_device(struct btrfs_device *device, u64 new_size)
 	}
 
 	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,
-- 
2.27.0