[PATCH v2] Bluetooth: eir: Fix stack OOB write when prepending the Flags AD

[Weiming Shi <bestswngs@gmail.com>]

eir_create_adv_data() builds the advertising data into a fixed-size
buffer ("size", 31 for the legacy path). It may prepend a 3-byte "Flags"
AD structure (LE_AD_NO_BREDR on an LE-only controller) and then copies
the per-instance data without checking that it still fits:

	memcpy(ptr, adv->adv_data, adv->adv_data_len);

tlv_data_max_len() only reserves those 3 bytes when the user-supplied
flags carry a managed-flags bit, so an instance added with flags == 0 is
accepted with adv_data_len up to the full buffer. At advertise time the
flags are still prepended, and the memcpy() writes 3 + adv_data_len
bytes into the size-byte buffer:

  BUG: KASAN: stack-out-of-bounds in eir_create_adv_data (net/bluetooth/eir.c:301)
  Write of size 31 at addr ffff88800a547bdc by task kworker/u9:0/65
  Workqueue: hci0 hci_cmd_sync_work
   __asan_memcpy (mm/kasan/shadow.c:106)
   eir_create_adv_data (net/bluetooth/eir.c:301)
   hci_update_adv_data_sync (net/bluetooth/hci_sync.c:1310)
   hci_schedule_adv_instance_sync (net/bluetooth/hci_sync.c:1817)
   hci_cmd_sync_work (net/bluetooth/hci_sync.c:332)
  This frame has 1 object:
   [32, 64) 'cp'

The "Flags" structure is added by the kernel, not requested by
userspace, so only prepend it when it fits together with the instance
advertising data; when there is no room for both, drop the flags rather
than the user-provided data.

Reachable by a local user with CAP_NET_ADMIN owning an LE-only
controller on the legacy advertising path.

Fixes: b44133ff03be ("Bluetooth: Support the "discoverable" adv flag")
Reported-by: Xiang Mei <xmei5@asu.edu>
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Weiming Shi <bestswngs@gmail.com>
---
v2: drop the kernel-added "Flags" AD when it does not fit with the instance data,
    instead of dropping the user data .

 net/bluetooth/eir.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/net/bluetooth/eir.c b/net/bluetooth/eir.c
index 3f72111ba651..1de5f9df6eec 100644
--- a/net/bluetooth/eir.c
+++ b/net/bluetooth/eir.c
@@ -283,10 +283,12 @@ u8 eir_create_adv_data(struct hci_dev *hdev, u8 instance, u8 *ptr, u8 size)
 		if (!flags)
 			flags |= mgmt_get_adv_discov_flags(hdev);
 
-		/* If flags would still be empty, then there is no need to
-		 * include the "Flags" AD field".
+		/* Only add the "Flags" if it fits together with the instance
+		 * advertising data; drop it rather than overflow the buffer.
 		 */
-		if (flags && (ad_len + eir_precalc_len(1) <= size)) {
+		if (flags &&
+		    (ad_len + eir_precalc_len(1) +
+		     (adv ? adv->adv_data_len : 0) <= size)) {
 			ptr[0] = 0x02;
 			ptr[1] = EIR_FLAGS;
 			ptr[2] = flags;
-- 
2.43.0