[v2] Bluetooth: Fix atomicity violation in {min,max}_key_size_set

Commit Message

Gui-Dong Han Dec. 22, 2023, 3:12 p.m. UTC

In min_key_size_set():
    if (val > hdev->le_max_key_size || val < SMP_MIN_ENC_KEY_SIZE)
        return -EINVAL;
    hci_dev_lock(hdev);
    hdev->le_min_key_size = val;
    hci_dev_unlock(hdev);

In max_key_size_set():
    if (val > SMP_MAX_ENC_KEY_SIZE || val < hdev->le_min_key_size)
        return -EINVAL;
    hci_dev_lock(hdev);
    hdev->le_max_key_size = val;
    hci_dev_unlock(hdev);

The atomicity violation occurs due to concurrent execution of set_min and
set_max funcs.Consider a scenario where setmin writes a new, valid 'min'
value, and concurrently, setmax writes a value that is greater than the
old 'min' but smaller than the new 'min'. In this case, setmax might check
against the old 'min' value (before acquiring the lock) but write its
value after the 'min' has been updated by setmin. This leads to a
situation where the 'max' value ends up being smaller than the 'min'
value, which is an inconsistency.

This possible bug is found by an experimental static analysis tool
developed by our team, BassCheck[1]. This tool analyzes the locking APIs
to extract function pairs that can be concurrently executed, and then
analyzes the instructions in the paired functions to identify possible
concurrency bugs including data races and atomicity violations. The above
possible bug is reported when our tool analyzes the source code of
Linux 5.17.

To resolve this issue, it is suggested to encompass the validity checks
within the locked sections in both set_min and set_max funcs. The
modification ensures that the validation of 'val' against the
current min/max values is atomic, thus maintaining the integrity of the
settings. With this patch applied, our tool no longer reports the bug,
with the kernel configuration allyesconfig for x86_64. Due to the lack of
associated hardware, we cannot test the patch in runtime testing, and just
verify it according to the code logic.

[1] https://sites.google.com/view/basscheck/

Fixes: 18f81241b74f ("Bluetooth: Move {min,max}_key_size debugfs ...")
Cc: [e-mail]
Signed-off-by: Gui-Dong Han <[e-mail]>
---
v2:
* Adjust the format to pass the CI.
---
 net/bluetooth/hci_debugfs.c | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

Comments

[e-mail] Dec. 22, 2023, 3:58 p.m. UTC | #1

This is automated email and please do not reply to this email!

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=812520

---Test result---

Test Summary:
CheckPatch                    PASS      1.55 seconds
GitLint                       PASS      0.54 seconds
SubjectPrefix                 PASS      0.07 seconds
BuildKernel                   PASS      29.23 seconds
CheckAllWarning               PASS      31.03 seconds
CheckSparse                   PASS      36.97 seconds
CheckSmatch                   PASS      101.71 seconds
BuildKernel32                 PASS      27.72 seconds
TestRunnerSetup               PASS      447.68 seconds
TestRunner_l2cap-tester       PASS      23.94 seconds
TestRunner_iso-tester         PASS      46.71 seconds
TestRunner_bnep-tester        PASS      6.99 seconds
TestRunner_mgmt-tester        PASS      161.13 seconds
TestRunner_rfcomm-tester      PASS      10.85 seconds
TestRunner_sco-tester         PASS      14.50 seconds
TestRunner_ioctl-tester       PASS      12.27 seconds
TestRunner_mesh-tester        PASS      8.83 seconds
TestRunner_smp-tester         PASS      9.77 seconds
TestRunner_userchan-tester    PASS      7.22 seconds
IncrementalBuild              PASS      25.57 seconds



---
Regards,
Linux Bluetooth

Gui-Dong Han Jan. 11, 2024, 8:45 a.m. UTC | #2

Dear All, I hope this message finds you well. Following up on my 
previous email regarding the patch, I want to highlight an important 
aspect. In /net/bluetooth/hci_debugfs.c, there are four similar bugs 
related to atomicity violations, potentially leading to minimum values 
exceeding maximum ones. 
https://patchwork.kernel.org/project/bluetooth/patch/[e-mail]/ 
https://patchwork.kernel.org/project/bluetooth/patch/[e-mail]/ 
https://patchwork.kernel.org/project/bluetooth/patch/[e-mail]/ 
https://patchwork.kernel.org/project/bluetooth/patch/[e-mail]/ 
I kindly request your review of these issues alongside the original 
patch. Understanding your busy schedules, your prompt attention to this 
consolidated review would be greatly appreciated. Thank you for your 
time and consideration. Best regards, Han

Patch

diff --git a/net/bluetooth/hci_debugfs.c b/net/bluetooth/hci_debugfs.c
index 6b7741f6e95b..3ffbf3f25363 100644
--- a/net/bluetooth/hci_debugfs.c
+++ b/net/bluetooth/hci_debugfs.c
@@ -1045,11 +1045,13 @@  DEFINE_DEBUGFS_ATTRIBUTE(adv_max_interval_fops, adv_max_interval_get,
 static int min_key_size_set(void *data, u64 val)
 {
 	struct hci_dev *hdev = data;
-
-	if (val > hdev->le_max_key_size || val < SMP_MIN_ENC_KEY_SIZE)
+
+	hci_dev_lock(hdev);
+	if (val > hdev->le_max_key_size || val < SMP_MIN_ENC_KEY_SIZE) {
+		hci_dev_unlock(hdev);
 		return -EINVAL;
+	}
 
-	hci_dev_lock(hdev);
 	hdev->le_min_key_size = val;
 	hci_dev_unlock(hdev);
 
@@ -1073,11 +1075,13 @@  DEFINE_DEBUGFS_ATTRIBUTE(min_key_size_fops, min_key_size_get,
 static int max_key_size_set(void *data, u64 val)
 {
 	struct hci_dev *hdev = data;
-
-	if (val > SMP_MAX_ENC_KEY_SIZE || val < hdev->le_min_key_size)
+
+	hci_dev_lock(hdev);
+	if (val > SMP_MAX_ENC_KEY_SIZE || val < hdev->le_min_key_size) {
+		hci_dev_unlock(hdev);
 		return -EINVAL;
+	}
 
-	hci_dev_lock(hdev);
 	hdev->le_max_key_size = val;
 	hci_dev_unlock(hdev);