x86/kprobes: Fix crash when probing CS CALL instructionsgrafted

When using eBPF to probe CS CALL instructions within a function,
a crash can be triggered.

The eBPF tool probes offset 257 of the __hrtimer_run_queues()
function:

<__hrtimer_run_queues+249>:  nopl   0x0(%rax,%rax,1)
<__hrtimer_run_queues+254>:  mov    %r14,%rdi
<__hrtimer_run_queues+257>:  cs call <__x86_indirect_thunk_r12>
<__hrtimer_run_queues+263>:  mov    %eax,%r12d
<__hrtimer_run_queues+266>:  xchg   %ax,%ax
<__hrtimer_run_queues+268>:  mov    %r13,%rdi

Which triggers this crash:

  BUG: unable to handle page fault for address: 00000000000f41c9
  #PF: supervisor write access in kernel mode
  #PF: error_code(0x0002) - not-present page
  PGD 0 P4D 0
  Oops: 0002 [#1] SMP NOPTI
  CPU: 1 PID: 0 Comm: swapper/1 Kdump: loaded Tainted: P
  RIP: 0010:__hrtimer_run_queues+0x106/0x230

Note that __hrtimer_run_queues+0x106 is __hrtimer_run_queues+262, which is
at the 6th byte of the above CS CALL instruction. Since the CS CALL
instruction occupies 6 bytes, the exception occurred in the middle of that
call instruction.

The root cause is that when using eBPF tools to probe in the middle of a
function, a kprobe with INT3 is used as the underlying implementation.

During single-step emulation of the original CALL instruction,
int3_emulate_call() assumes that the probed CALL instruction is 5 bytes
long. However, the actual CS-prefixed CALL instruction occupies 6 bytes,
so it constructs an incorrect exception return address. When the CPU
returns from the kprobe handler, the next instruction to be executed is at
the address of the last byte of that CS CALL instruction. Coincidentally,
starting from that address, the CPU fetches and decodes a completely
different instruction, which ultimately triggers a kernel crash.

Fix the issue by using the actual instruction length obtained from
the instruction decoder when constructing the exception return
address, rather than relying on the hardcoded CALL_INSN_SIZE macro.

[ mingo: Refined the changelog ]

Fixes: 6256e668b7af ("x86/kprobes: Use int3 instead of debug trap for single-step")
Suggested-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Jinke Han <jinkehan@didiglobal.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Acked-by: Yafang Shao <laoar.shao@gmail.com>
Acked-by: Borislav Petkov <bp@alien8.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://patch.msgid.link/20260908073742.GA10517@didi-ThinkCentre-M920t-N000

1 files changed, 409 insertions, 0 deletions

diff --git a/drivers/dax/fsdev.c b/drivers/dax/fsdev.c
new file mode 100644
index 000000000..598604bf5
--- /dev/null
+++ b/drivers/dax/fsdev.c
@@ -0,0 +1,409 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright(c) 2026 Micron Technology, Inc. */
+#include <linux/memremap.h>
+#include <linux/pagemap.h>
+#include <linux/module.h>
+#include <linux/device.h>
+#include <linux/cdev.h>
+#include <linux/slab.h>
+#include <linux/dax.h>
+#include <linux/uio.h>
+#include <linux/fs.h>
+#include <linux/mm.h>
+#include "dax-private.h"
+#include "bus.h"
+
+/*
+ * FS-DAX compatible devdax driver
+ *
+ * Unlike drivers/dax/device.c which pre-initializes compound folios based
+ * on device alignment (via vmemmap_shift), this driver leaves folios
+ * uninitialized similar to pmem. This allows fs-dax filesystems like famfs
+ * to work without needing special handling for pre-initialized folios.
+ *
+ * Key differences from device.c:
+ * - pgmap type is MEMORY_DEVICE_FS_DAX (not MEMORY_DEVICE_GENERIC)
+ * - vmemmap_shift is NOT set (folios remain order-0)
+ * - fs-dax can dynamically create compound folios as needed
+ * - No mmap support - all access is through fs-dax/iomap
+ */
+
+static void fsdev_write_dax(void *addr, struct page *page,
+		unsigned int off, unsigned int len)
+{
+	while (len) {
+		void *mem = kmap_local_page(page);
+		unsigned int chunk = min_t(unsigned int, len, PAGE_SIZE - off);
+
+		memcpy_flushcache(addr, mem + off, chunk);
+		kunmap_local(mem);
+		len -= chunk;
+		off = 0;
+		page++;
+		addr += chunk;
+	}
+}
+
+static long __fsdev_dax_direct_access(struct dax_device *dax_dev, pgoff_t pgoff,
+		long nr_pages, enum dax_access_mode mode, void **kaddr,
+		unsigned long *pfn)
+{
+	struct dev_dax *dev_dax = dax_get_private(dax_dev);
+	size_t size = nr_pages << PAGE_SHIFT;
+	size_t offset = pgoff << PAGE_SHIFT;
+	phys_addr_t phys;
+
+	phys = dax_pgoff_to_phys(dev_dax, pgoff, size);
+	if (phys == -1) {
+		dev_dbg(&dev_dax->dev,
+			"pgoff (%#lx) out of range\n", pgoff);
+		return -EFAULT;
+	}
+
+	if (kaddr)
+		*kaddr = __va(phys);
+
+	if (pfn)
+		*pfn = PHYS_PFN(phys);
+
+	/*
+	 * Use cached_size which was computed at probe time. The size cannot
+	 * change while the driver is bound (resize returns -EBUSY).
+	 */
+	return PHYS_PFN(min(size, dev_dax->cached_size - offset));
+}
+
+static int fsdev_dax_zero_page_range(struct dax_device *dax_dev,
+			pgoff_t pgoff, size_t nr_pages)
+{
+	void *kaddr;
+	long rc;
+
+	WARN_ONCE(nr_pages > 1, "%s: nr_pages > 1\n", __func__);
+	rc = __fsdev_dax_direct_access(dax_dev, pgoff, 1, DAX_ACCESS,
+				       &kaddr, NULL);
+	if (rc < 0)
+		return rc;
+	fsdev_write_dax(kaddr, ZERO_PAGE(0), 0, PAGE_SIZE);
+	return 0;
+}
+
+static long fsdev_dax_direct_access(struct dax_device *dax_dev,
+		pgoff_t pgoff, long nr_pages, enum dax_access_mode mode,
+		void **kaddr, unsigned long *pfn)
+{
+	return __fsdev_dax_direct_access(dax_dev, pgoff, nr_pages, mode,
+					 kaddr, pfn);
+}
+
+static size_t fsdev_dax_recovery_write(struct dax_device *dax_dev,
+		pgoff_t pgoff, void *addr, size_t bytes, struct iov_iter *i)
+{
+	return _copy_from_iter_flushcache(addr, bytes, i);
+}
+
+static const struct dax_operations dev_dax_ops = {
+	.direct_access = fsdev_dax_direct_access,
+	.zero_page_range = fsdev_dax_zero_page_range,
+	.recovery_write = fsdev_dax_recovery_write,
+};
+
+static void fsdev_cdev_del(void *cdev)
+{
+	cdev_del(cdev);
+}
+
+static void fsdev_kill(void *dev_dax)
+{
+	kill_dev_dax(dev_dax);
+}
+
+static void fsdev_clear_ops(void *data)
+{
+	struct dev_dax *dev_dax = data;
+
+	dax_set_ops(dev_dax->dax_dev, NULL);
+}
+
+static void fsdev_clear_pgmap_ops(void *data)
+{
+	struct dev_pagemap *pgmap = data;
+
+	/*
+	 * fsdev installs pgmap->ops and ->owner at probe. For a static device
+	 * the pgmap is shared and long-lived (owned by the dax bus), so
+	 * leaving fsdev's ops behind on unbind would let a later
+	 * memory_failure -- after rebind to another driver, or after this
+	 * module is unloaded -- dispatch through a stale or freed
+	 * ->memory_failure handler. Clear them so the pgmap carries no fsdev
+	 * state once we are unbound.
+	 */
+	pgmap->ops = NULL;
+	pgmap->owner = NULL;
+}
+
+/*
+ * Page map operations for FS-DAX mode
+ * Similar to fsdax_pagemap_ops in drivers/nvdimm/pmem.c
+ *
+ * Note: folio_free callback is not needed for MEMORY_DEVICE_FS_DAX.
+ * The core mm code in free_zone_device_folio() handles the wake_up_var()
+ * directly for this memory type.
+ */
+static u64 fsdev_pfn_to_offset(struct dev_dax *dev_dax, unsigned long pfn)
+{
+	phys_addr_t phys = PFN_PHYS(pfn);
+	u64 offset = 0;
+
+	for (int i = 0; i < dev_dax->nr_range; i++) {
+		struct range *range = &dev_dax->ranges[i].range;
+
+		if (phys >= range->start && phys <= range->end)
+			return offset + (phys - range->start);
+		offset += range_len(range);
+	}
+	return -1ULL;
+}
+
+static int fsdev_pagemap_memory_failure(struct dev_pagemap *pgmap,
+		unsigned long pfn, unsigned long nr_pages, int mf_flags)
+{
+	struct dev_dax *dev_dax = pgmap->owner;
+	u64 offset = fsdev_pfn_to_offset(dev_dax, pfn);
+	u64 len = nr_pages << PAGE_SHIFT;
+
+	return dax_holder_notify_failure(dev_dax->dax_dev, offset,
+					 len, mf_flags);
+}
+
+static const struct dev_pagemap_ops fsdev_pagemap_ops = {
+	.memory_failure		= fsdev_pagemap_memory_failure,
+};
+
+/*
+ * Clear any stale folio state from pages in the given range.
+ * This is necessary because device_dax pre-initializes compound folios
+ * based on vmemmap_shift, and that state may persist after driver unbind.
+ * Since fsdev_dax uses MEMORY_DEVICE_FS_DAX without vmemmap_shift, fs-dax
+ * expects to find clean order-0 folios that it can build into compound
+ * folios on demand.
+ *
+ * At probe time, no filesystem should be mounted yet, so all mappings
+ * are stale and must be cleared along with compound state.
+ */
+static void fsdev_clear_folio_state(struct dev_dax *dev_dax)
+{
+	for (int i = 0; i < dev_dax->nr_range; i++) {
+		struct range *range = &dev_dax->ranges[i].range;
+		unsigned long pfn = PHYS_PFN(range->start);
+		unsigned long end_pfn = PHYS_PFN(range->end) + 1;
+
+		while (pfn < end_pfn) {
+			struct folio *folio = pfn_folio(pfn);
+			int order = dax_folio_reset_order(folio);
+
+			pfn += 1UL << order;
+		}
+	}
+}
+
+static void fsdev_clear_folio_state_action(void *data)
+{
+	fsdev_clear_folio_state(data);
+}
+
+static int fsdev_open(struct inode *inode, struct file *filp)
+{
+	struct dax_device *dax_dev = inode_dax(inode);
+	struct dev_dax *dev_dax = dax_get_private(dax_dev);
+
+	filp->private_data = dev_dax;
+
+	return 0;
+}
+
+static int fsdev_release(struct inode *inode, struct file *filp)
+{
+	return 0;
+}
+
+static const struct file_operations fsdev_fops = {
+	.llseek = noop_llseek,
+	.owner = THIS_MODULE,
+	.open = fsdev_open,
+	.release = fsdev_release,
+};
+
+/*
+ * Acquire the dev_pagemap for probe: the static (pre-populated) one if
+ * present, or a devm-allocated one for the dynamic case. Note that
+ * dev_dax->pgmap is not set here; fsdev_dax_probe() sets it only once
+ * probe succeeds, so a failed probe never leaves a dangling pointer
+ * to a devres-freed pgmap.
+ */
+static struct dev_pagemap *fsdev_acquire_pgmap(struct dev_dax *dev_dax)
+{
+	struct device *dev = &dev_dax->dev;
+	struct dev_pagemap *pgmap;
+	size_t pgmap_size;
+
+	if (static_dev_dax(dev_dax)) {
+		if (dev_dax->nr_range > 1) {
+			dev_warn(dev,
+				 "static pgmap / multi-range device conflict\n");
+			return ERR_PTR(-EINVAL);
+		}
+
+		pgmap = dev_dax->pgmap;
+		pgmap->vmemmap_shift = 0;
+		return pgmap;
+	}
+
+	if (dev_dax->pgmap) {
+		dev_warn(dev, "dynamic-dax with pre-populated page map\n");
+		return ERR_PTR(-EINVAL);
+	}
+
+	pgmap_size = struct_size(pgmap, ranges, dev_dax->nr_range - 1);
+	pgmap = devm_kzalloc(dev, pgmap_size, GFP_KERNEL);
+	if (!pgmap)
+		return ERR_PTR(-ENOMEM);
+
+	pgmap->nr_range = dev_dax->nr_range;
+	for (int i = 0; i < dev_dax->nr_range; i++)
+		pgmap->ranges[i] = dev_dax->ranges[i].range;
+
+	return pgmap;
+}
+
+static int fsdev_dax_probe(struct dev_dax *dev_dax)
+{
+	struct dax_device *dax_dev = dev_dax->dax_dev;
+	struct device *dev = &dev_dax->dev;
+	struct dev_pagemap *pgmap;
+	struct inode *inode;
+	u64 data_offset = 0;
+	struct cdev *cdev;
+	void *addr;
+	int rc, i;
+
+	pgmap = fsdev_acquire_pgmap(dev_dax);
+	if (IS_ERR(pgmap))
+		return PTR_ERR(pgmap);
+
+	for (i = 0; i < dev_dax->nr_range; i++) {
+		struct range *range = &dev_dax->ranges[i].range;
+
+		if (!devm_request_mem_region(dev, range->start,
+					range_len(range), dev_name(dev))) {
+			dev_warn(dev, "mapping%d: %#llx-%#llx could not reserve range\n",
+				 i, range->start, range->end);
+			return -EBUSY;
+		}
+	}
+
+	/* Cache size now; it cannot change while driver is bound */
+	dev_dax->cached_size = 0;
+	for (i = 0; i < dev_dax->nr_range; i++)
+		dev_dax->cached_size += range_len(&dev_dax->ranges[i].range);
+
+	/*
+	 * Use MEMORY_DEVICE_FS_DAX without setting vmemmap_shift, leaving
+	 * folios at order-0. Unlike device.c (MEMORY_DEVICE_GENERIC), this
+	 * lets fs-dax dynamically build compound folios as needed, similar
+	 * to pmem behavior.
+	 */
+	pgmap->type = MEMORY_DEVICE_FS_DAX;
+	pgmap->ops = &fsdev_pagemap_ops;
+	pgmap->owner = dev_dax;
+
+	addr = devm_memremap_pages(dev, pgmap);
+	if (IS_ERR(addr))
+		return PTR_ERR(addr);
+
+	/* Drop fsdev's pgmap->ops/owner on unbind so no stale ops survive. */
+	rc = devm_add_action_or_reset(dev, fsdev_clear_pgmap_ops, pgmap);
+	if (rc)
+		return rc;
+
+	/*
+	 * Clear any stale compound folio state left over from a previous
+	 * driver (e.g., device_dax with vmemmap_shift). Also register this
+	 * as a devm action so folio state is cleared on unbind, ensuring
+	 * clean pages for subsequent drivers (e.g., kmem for system-ram).
+	 */
+	fsdev_clear_folio_state(dev_dax);
+	rc = devm_add_action_or_reset(dev, fsdev_clear_folio_state_action,
+				      dev_dax);
+	if (rc)
+		return rc;
+
+	/* Detect whether the data is at a non-zero offset into the memory */
+	if (pgmap->range.start != dev_dax->ranges[0].range.start) {
+		u64 phys = dev_dax->ranges[0].range.start;
+		u64 pgmap_phys = pgmap[0].range.start;
+
+		if (pgmap_phys > phys) {
+			dev_err(dev, "pgmap start %#llx exceeds data start %#llx\n",
+				pgmap_phys, phys);
+			return -EINVAL;
+		}
+		data_offset = phys - pgmap_phys;
+
+		pr_debug("%s: offset detected phys=%llx pgmap_phys=%llx offset=%llx\n",
+		       __func__, phys, pgmap_phys, data_offset);
+	}
+
+	inode = dax_inode(dax_dev);
+	cdev = inode->i_cdev;
+	cdev_init(cdev, &fsdev_fops);
+	cdev->owner = dev->driver->owner;
+	cdev_set_parent(cdev, &dev->kobj);
+	rc = cdev_add(cdev, dev->devt, 1);
+	if (rc)
+		return rc;
+
+	rc = devm_add_action_or_reset(dev, fsdev_cdev_del, cdev);
+	if (rc)
+		return rc;
+
+	/* Set the dax operations for fs-dax access path */
+	rc = dax_set_ops(dax_dev, &dev_dax_ops);
+	if (rc)
+		return rc;
+
+	rc = devm_add_action_or_reset(dev, fsdev_clear_ops, dev_dax);
+	if (rc)
+		return rc;
+
+	run_dax(dax_dev);
+	rc = devm_add_action_or_reset(dev, fsdev_kill, dev_dax);
+	if (rc)
+		return rc;
+
+	/* Probe can no longer fail; expose the pgmap via dev_dax */
+	dev_dax->pgmap = pgmap;
+	return 0;
+}
+
+static struct dax_device_driver fsdev_dax_driver = {
+	.probe = fsdev_dax_probe,
+	.type = DAXDRV_FSDEV_TYPE,
+};
+
+static int __init dax_init(void)
+{
+	return dax_driver_register(&fsdev_dax_driver);
+}
+
+static void __exit dax_exit(void)
+{
+	dax_driver_unregister(&fsdev_dax_driver);
+}
+
+MODULE_AUTHOR("John Groves");
+MODULE_DESCRIPTION("FS-DAX Device: fs-dax compatible devdax driver");
+MODULE_LICENSE("GPL");
+module_init(dax_init);
+module_exit(dax_exit);
+MODULE_ALIAS_DAX_DEVICE(0);