From: Runyu Xiao Date: Thu, 28 May 2026 16:52:01 +0000 (+0800) Subject: coresight: etb10: restore atomic_t for shared reading state X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=fa09f08ede3db3050ae16ae1ed92c902d0cada23;p=thirdparty%2Fkernel%2Fstable.git coresight: etb10: restore atomic_t for shared reading state The etb10 miscdevice uses drvdata->reading as a shared exclusivity gate for userspace buffer access. etb_open() claims that gate with local_cmpxchg(), and etb_release() clears it with local_set(). That gate is shared per-device state rather than CPU-local state. A running system can reach it whenever /dev/ is opened, closed, and reopened by different tasks while the device remains registered, so the same drvdata->reading variable may be claimed on one CPU and later cleared on another. This code used to use atomic_t for the same gate, but commit 27b10da8fff2 ("coresight: etb10: moving to local atomic operations") changed it to local_t even though the access pattern remained cross-task and cross-CPU. Restore atomic_t together with atomic_cmpxchg() and atomic_set() so the exclusivity gate again uses a primitive intended for shared state. The issue was found on Linux v6.18.21 by our static analysis tool while scanning surviving local_t-on-shared-state sites, and then manually reviewed against the live etb10 file-op path. It was runtime-validated with a reproducible QEMU no-device KCSAN PoC that kept the same report-local contract: 1. use one shared struct etb_drvdata carrier and its drvdata->reading gate; 2. call etb_open() and etb_release() sequentially on that gate to confirm the original claim/clear path; 3. bind the open side to CPU0 and the release side to CPU1 for the same gate to show cross-CPU ownership; 4. run bound workers that repeatedly race etb_open() and etb_release() on the same gate until KCSAN reports a target hit. The harness recorded: L1 passed open=1 release=1 reading_after_open=1 reading_after_release=0 L2 passed open_cpu=0 release_cpu=1 cross_cpu_release=1 reading_after=0 open_ret=0 Representative KCSAN excerpt from the no-device validation run: BUG: KCSAN: data-race in etb_open.constprop.0.isra.0 [vuln_msv] write to 0xffffffffc0003810 of 4 bytes by task 216 on cpu 1: etb_open.constprop.0.isra.0+0x38/0x80 [vuln_msv] l3_worker_thread_fn+0x4f/0xf0 [vuln_msv] kthread+0x17e/0x1c0 ret_from_fork+0x22/0x30 read to 0xffffffffc0003810 of 4 bytes by task 215 on cpu 0: etb_open.constprop.0.isra.0+0x18/0x80 [vuln_msv] l3_worker_thread_fn+0x4f/0xf0 [vuln_msv] kthread+0x17e/0x1c0 ret_from_fork+0x22/0x30 value changed: 0x00000000 -> 0x00000001 Reported by Kernel Concurrency Sanitizer on: CPU: 0 PID: 215 Comm: etb10_l3_a Tainted: G O 6.1.66 #2 This no-device harness is not a real ETB10 hardware end-to-end run, but it preserves the same shared drvdata->reading gate and the same etb_open()/etb_release() claim/clear contract. No real ETB10 hardware was available for runtime testing. Build-tested with: make olddefconfig make -j"$(nproc)" drivers/hwtracing/coresight/coresight-etb10.o Fixes: 27b10da8fff2 ("coresight: etb10: moving to local atomic operations") Cc: stable@vger.kernel.org Signed-off-by: Runyu Xiao Reviewed-by: James Clark Signed-off-by: Suzuki K Poulose Link: https://lore.kernel.org/r/20260528165201.319452-1-runyu.xiao@seu.edu.cn --- diff --git a/drivers/hwtracing/coresight/coresight-etb10.c b/drivers/hwtracing/coresight/coresight-etb10.c index b952a1d47f12..a827f76b8144 100644 --- a/drivers/hwtracing/coresight/coresight-etb10.c +++ b/drivers/hwtracing/coresight/coresight-etb10.c @@ -83,7 +83,7 @@ struct etb_drvdata { struct coresight_device *csdev; struct miscdevice miscdev; raw_spinlock_t spinlock; - local_t reading; + atomic_t reading; pid_t pid; u8 *buf; u32 buffer_depth; @@ -601,7 +601,7 @@ static int etb_open(struct inode *inode, struct file *file) struct etb_drvdata *drvdata = container_of(file->private_data, struct etb_drvdata, miscdev); - if (local_cmpxchg(&drvdata->reading, 0, 1)) + if (atomic_cmpxchg(&drvdata->reading, 0, 1)) return -EBUSY; dev_dbg(&drvdata->csdev->dev, "%s: successfully opened\n", __func__); @@ -639,7 +639,7 @@ static int etb_release(struct inode *inode, struct file *file) { struct etb_drvdata *drvdata = container_of(file->private_data, struct etb_drvdata, miscdev); - local_set(&drvdata->reading, 0); + atomic_set(&drvdata->reading, 0); dev_dbg(&drvdata->csdev->dev, "%s: released\n", __func__); return 0;