[PATCH 5.4.y] btrfs: avoid potential out-of-bounds in btrfs_encode_fh()

[Sasha Levin <sashal@kernel.org>]

From: Anderson Nascimento <anderson@allelesecurity.com>

[ Upstream commit dff4f9ff5d7f289e4545cc936362e01ed3252742 ]

The function btrfs_encode_fh() does not properly account for the three
cases it handles.

Before writing to the file handle (fh), the function only returns to the
user BTRFS_FID_SIZE_NON_CONNECTABLE (5 dwords, 20 bytes) or
BTRFS_FID_SIZE_CONNECTABLE (8 dwords, 32 bytes).

However, when a parent exists and the root ID of the parent and the
inode are different, the function writes BTRFS_FID_SIZE_CONNECTABLE_ROOT
(10 dwords, 40 bytes).

If *max_len is not large enough, this write goes out of bounds because
BTRFS_FID_SIZE_CONNECTABLE_ROOT is greater than
BTRFS_FID_SIZE_CONNECTABLE originally returned.

This results in an 8-byte out-of-bounds write at
fid->parent_root_objectid = parent_root_id.

A previous attempt to fix this issue was made but was lost.

https://lore.kernel.org/all/4CADAEEC020000780001B32C@vpn.id2.novell.com/

Although this issue does not seem to be easily triggerable, it is a
potential memory corruption bug that should be fixed. This patch
resolves the issue by ensuring the function returns the appropriate size
for all three cases and validates that *max_len is large enough before
writing any data.

Fixes: be6e8dc0ba84 ("NFS support for btrfs - v3")
CC: stable@vger.kernel.org # 3.0+
Signed-off-by: Anderson Nascimento <anderson@allelesecurity.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
[ replaced btrfs_root_id() calls with direct ->root->root_key.objectid access ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 fs/btrfs/export.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/export.c b/fs/btrfs/export.c
index 08d1d456e2f0f..c244b4cc8ba11 100644
--- a/fs/btrfs/export.c
+++ b/fs/btrfs/export.c
@@ -22,7 +22,11 @@ static int btrfs_encode_fh(struct inode *inode, u32 *fh, int *max_len,
 	int type;
 
 	if (parent && (len < BTRFS_FID_SIZE_CONNECTABLE)) {
-		*max_len = BTRFS_FID_SIZE_CONNECTABLE;
+		if (BTRFS_I(inode)->root->root_key.objectid !=
+		    BTRFS_I(parent)->root->root_key.objectid)
+			*max_len = BTRFS_FID_SIZE_CONNECTABLE_ROOT;
+		else
+			*max_len = BTRFS_FID_SIZE_CONNECTABLE;
 		return FILEID_INVALID;
 	} else if (len < BTRFS_FID_SIZE_NON_CONNECTABLE) {
 		*max_len = BTRFS_FID_SIZE_NON_CONNECTABLE;
@@ -44,6 +48,8 @@ static int btrfs_encode_fh(struct inode *inode, u32 *fh, int *max_len,
 		parent_root_id = BTRFS_I(parent)->root->root_key.objectid;
 
 		if (parent_root_id != fid->root_objectid) {
+			if (*max_len < BTRFS_FID_SIZE_CONNECTABLE_ROOT)
+				return FILEID_INVALID;
 			fid->parent_root_objectid = parent_root_id;
 			len = BTRFS_FID_SIZE_CONNECTABLE_ROOT;
 			type = FILEID_BTRFS_WITH_PARENT_ROOT;
-- 
2.51.0