[PATCH v3] Bluetooth: MGMT: validate advertising TLV before type checks

[Zhang Cen <rollkingzzc@gmail.com>]

tlv_data_is_valid() reads each advertising data field length from
data[i], then inspects data[i + 1] for managed EIR types before
checking that the current field still fits inside the supplied buffer.

A malformed field whose length byte is the last byte of the buffer can
therefore make the parser read one byte past the advertising data.

KASAN reported the following when a malformed MGMT_OP_ADD_ADVERTISING
request reached that path:

  BUG: KASAN: vmalloc-out-of-bounds in tlv_data_is_valid()
  Read of size 1
  Call trace:
    tlv_data_is_valid()
    add_advertising()
    hci_mgmt_cmd()
    hci_sock_sendmsg()

Move the existing element-length check before any type-octet inspection
so each non-empty element is proven to contain its type byte before the
parser looks at data[i + 1].

Fixes: 2bb36870e8cb ("Bluetooth: Unify advertising instance flags check")
Reviewed-by: Paul Menzel <pmenzel@molgen.mpg.de>
Signed-off-by: Zhang Cen <rollkingzzc@gmail.com>
---
v3:
- Move the KASAN excerpt earlier in the commit message.
- Add Reviewed-by from Paul Menzel.

 net/bluetooth/mgmt.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/net/bluetooth/mgmt.c b/net/bluetooth/mgmt.c
index de5bd6b637b20..027b266ccc747 100644
--- a/net/bluetooth/mgmt.c
+++ b/net/bluetooth/mgmt.c
@@ -8638,6 +8638,12 @@ static bool tlv_data_is_valid(struct hci_dev *hdev, u32 adv_flags, u8 *data,
 		if (!cur_len)
 			continue;
 
+		/* If the current field length would exceed the total data
+		 * length, then it's invalid.
+		 */
+		if (i + cur_len >= len)
+			return false;
+
 		if (data[i + 1] == EIR_FLAGS &&
 		    (!is_adv_data || flags_managed(adv_flags)))
 			return false;
@@ -8654,12 +8660,6 @@ static bool tlv_data_is_valid(struct hci_dev *hdev, u32 adv_flags, u8 *data,
 		if (data[i + 1] == EIR_APPEARANCE &&
 		    appearance_managed(adv_flags))
 			return false;
-
-		/* If the current field length would exceed the total data
-		 * length, then it's invalid.
-		 */
-		if (i + cur_len >= len)
-			return false;
 	}
 
 	return true;
-- 
2.43.0