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

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

[Weiming Shi]

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

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

[bluez.test.bot]

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Dear submitter,

Thank you for submitting the patches to the linux bluetooth mailing list.
This is a CI test results with your patch series:
PW Link:https://patchwork.kernel.org/project/bluetooth/list/?series=1104768

---Test result---

Test Summary:
CheckPatch                    FAIL      0.69 seconds
VerifyFixes                   PASS      0.11 seconds
VerifySignedoff               PASS      0.11 seconds
GitLint                       FAIL      0.28 seconds
SubjectPrefix                 PASS      0.11 seconds
BuildKernel                   PASS      24.67 seconds
CheckAllWarning               PASS      27.02 seconds
CheckSparse                   PASS      26.04 seconds
BuildKernel32                 PASS      24.01 seconds
TestRunnerSetup               PASS      529.67 seconds
TestRunner_l2cap-tester       PASS      60.82 seconds
TestRunner_iso-tester         PASS      80.07 seconds
TestRunner_bnep-tester        PASS      18.82 seconds
TestRunner_mgmt-tester        FAIL      215.41 seconds
TestRunner_rfcomm-tester      PASS      25.46 seconds
TestRunner_sco-tester         PASS      32.57 seconds
TestRunner_ioctl-tester       PASS      26.03 seconds
TestRunner_mesh-tester        FAIL      25.87 seconds
TestRunner_smp-tester         PASS      27.08 seconds
TestRunner_userchan-tester    PASS      19.66 seconds
TestRunner_6lowpan-tester     PASS      22.53 seconds
IncrementalBuild              PASS      23.19 seconds

Details
##############################
Test: CheckPatch - FAIL
Desc: Run checkpatch.pl script
Output:
[v2] Bluetooth: eir: Fix stack OOB write when prepending the Flags AD
WARNING: Reported-by: should be immediately followed by Closes: with a URL to the report
#136: 
Reported-by: Xiang Mei <xmei5@asu.edu>
Assisted-by: Claude:claude-opus-4-8

total: 0 errors, 1 warnings, 0 checks, 15 lines checked

NOTE: For some of the reported defects, checkpatch may be able to
      mechanically convert to the typical style using --fix or --fix-inplace.

/github/workspace/src/patch/14607518.patch has style problems, please review.

NOTE: Ignored message types: UNKNOWN_COMMIT_ID

NOTE: If any of the errors are false positives, please report
      them to the maintainer, see CHECKPATCH in MAINTAINERS.


##############################
Test: GitLint - FAIL
Desc: Run gitlint
Output:
[v2] Bluetooth: eir: Fix stack OOB write when prepending the Flags AD

8: B3 Line contains hard tab characters (\t): "	memcpy(ptr, adv->adv_data, adv->adv_data_len);"
16: B1 Line exceeds max length (82>80): "  BUG: KASAN: stack-out-of-bounds in eir_create_adv_data (net/bluetooth/eir.c:301)"
39: B1 Line exceeds max length (81>80): "v2: drop the kernel-added "Flags" AD when it does not fit with the instance data,"
##############################
Test: TestRunner_mgmt-tester - FAIL
Desc: Run mgmt-tester with test-runner
Output:
Total: 494, Passed: 489 (99.0%), Failed: 1, Not Run: 4

Failed Test Cases
Read Exp Feature - Success                           Failed       0.247 seconds
##############################
Test: TestRunner_mesh-tester - FAIL
Desc: Run mesh-tester with test-runner
Output:
Total: 10, Passed: 8 (80.0%), Failed: 2, Not Run: 0

Failed Test Cases
Mesh - Send cancel - 1                               Timed out    2.452 seconds
Mesh - Send cancel - 2                               Timed out    1.990 seconds


https://github.com/bluez/bluetooth-next/pull/275

---
Regards,
Linux Bluetooth

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

[patchwork-bot+bluetooth]

Hello:

This patch was applied to bluetooth/bluetooth-next.git (master)
by Luiz Augusto von Dentz <luiz.von.dentz@intel.com>:

On Wed,  3 Jun 2026 01:06:21 +0800 you wrote:
> 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);
> 
> [...]

Here is the summary with links:
  - [v2] Bluetooth: eir: Fix stack OOB write when prepending the Flags AD
    https://git.kernel.org/bluetooth/bluetooth-next/c/83dc982fad52

You are awesome, thank you!
-- 
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