dm verity fec: limit error correction recursion

If the hash tree itself is sufficiently corrupt in addition to data blocks,
it's possible for error correction to end up in a deep recursive loop,
which eventually causes a kernel panic.  This change limits the
recursion to a reasonable level during a single I/O operation.

Fixes: a739ff3f543a ("dm verity: add support for forward error correction")
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Cc: stable@vger.kernel.org # v4.5+

drivers/md/dm-verity-fec.c

@@ -439,6 +439,13 @@ int verity_fec_decode(struct dm_verity *v, struct dm_verity_io *io,
 	if (!verity_fec_is_enabled(v))
 		return -EOPNOTSUPP;
 
+	if (fio->level >= DM_VERITY_FEC_MAX_RECURSION) {
+		DMWARN_LIMIT("%s: FEC: recursion too deep", v->data_dev->name);
+		return -EIO;
+	}
+
+	fio->level++;
+
 	if (type == DM_VERITY_BLOCK_TYPE_METADATA)
 		block += v->data_blocks;
 
@@ -470,7 +477,7 @@ int verity_fec_decode(struct dm_verity *v, struct dm_verity_io *io,
 	if (r < 0) {
 		r = fec_decode_rsb(v, io, fio, rsb, offset, true);
 		if (r < 0)
-			return r;
+			goto done;
 	}
 
 	if (dest)
@@ -480,6 +487,8 @@ int verity_fec_decode(struct dm_verity *v, struct dm_verity_io *io,
 		r = verity_for_bv_block(v, io, iter, fec_bv_copy);
 	}
 
+done:
+	fio->level--;
 	return r;
 }
 
@@ -520,6 +529,7 @@ void verity_fec_init_io(struct dm_verity_io *io)
 	memset(fio->bufs, 0, sizeof(fio->bufs));
 	fio->nbufs = 0;
 	fio->output = NULL;
+	fio->level = 0;
 }
 
 /*

drivers/md/dm-verity-fec.h

@@ -27,6 +27,9 @@
 #define DM_VERITY_FEC_BUF_MAX \
 	(1 << (PAGE_SHIFT - DM_VERITY_FEC_BUF_RS_BITS))
 
+/* maximum recursion level for verity_fec_decode */
+#define DM_VERITY_FEC_MAX_RECURSION	4
+
 #define DM_VERITY_OPT_FEC_DEV		"use_fec_from_device"
 #define DM_VERITY_OPT_FEC_BLOCKS	"fec_blocks"
 #define DM_VERITY_OPT_FEC_START		"fec_start"
@@ -58,6 +61,7 @@ struct dm_verity_fec_io {
 	unsigned nbufs;		/* number of buffers allocated */
 	u8 *output;		/* buffer for corrected output */
 	size_t output_pos;
+	unsigned level;		/* recursion level */
 };
 
 #ifdef CONFIG_DM_VERITY_FEC