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, 179 insertions, 0 deletions

diff --git a/drivers/mtd/mtdsuper.c b/drivers/mtd/mtdsuper.c
new file mode 100644
index 000000000..c709ff7aa
--- /dev/null
+++ b/drivers/mtd/mtdsuper.c
@@ -0,0 +1,179 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/* MTD-based superblock management
+ *
+ * Copyright © 2001-2007 Red Hat, Inc. All Rights Reserved.
+ * Copyright © 2001-2010 David Woodhouse <dwmw2@infradead.org>
+ *
+ * Written by:  David Howells <dhowells@redhat.com>
+ *              David Woodhouse <dwmw2@infradead.org>
+ */
+
+#include <linux/mtd/super.h>
+#include <linux/namei.h>
+#include <linux/export.h>
+#include <linux/ctype.h>
+#include <linux/slab.h>
+#include <linux/major.h>
+#include <linux/backing-dev.h>
+#include <linux/blkdev.h>
+#include <linux/fs_context.h>
+#include "mtdcore.h"
+
+/*
+ * get a superblock on an MTD-backed filesystem
+ */
+static int mtd_get_sb(struct fs_context *fc,
+		      struct mtd_info *mtd,
+		      int (*fill_super)(struct super_block *,
+					struct fs_context *))
+{
+	struct super_block *sb;
+	int ret;
+
+	sb = sget_dev(fc, MKDEV(MTD_BLOCK_MAJOR, mtd->index));
+	if (IS_ERR(sb))
+		return PTR_ERR(sb);
+
+	if (sb->s_root) {
+		/* new mountpoint for an already mounted superblock */
+		pr_debug("MTDSB: Device %d (\"%s\") is already mounted\n",
+			 mtd->index, mtd->name);
+		put_mtd_device(mtd);
+	} else {
+		/* fresh new superblock */
+		pr_debug("MTDSB: New superblock for device %d (\"%s\")\n",
+			 mtd->index, mtd->name);
+
+		/*
+		 * Would usually have been set with @sb_lock held but in
+		 * contrast to sb->s_bdev that's checked with only
+		 * @sb_lock held, nothing checks sb->s_mtd without also
+		 * holding sb->s_umount and we're holding sb->s_umount
+		 * here.
+		 */
+		sb->s_mtd = mtd;
+		sb->s_bdi = bdi_get(mtd_bdi);
+
+		ret = fill_super(sb, fc);
+		if (ret < 0)
+			goto error_sb;
+
+		sb->s_flags |= SB_ACTIVE;
+	}
+
+	BUG_ON(fc->root);
+	fc->root = dget(sb->s_root);
+	return 0;
+
+error_sb:
+	deactivate_locked_super(sb);
+	return ret;
+}
+
+/*
+ * get a superblock on an MTD-backed filesystem by MTD device number
+ */
+static int mtd_get_sb_by_nr(struct fs_context *fc, int mtdnr,
+			    int (*fill_super)(struct super_block *,
+					      struct fs_context *))
+{
+	struct mtd_info *mtd;
+
+	mtd = get_mtd_device(NULL, mtdnr);
+	if (IS_ERR(mtd)) {
+		errorf(fc, "MTDSB: Device #%u doesn't appear to exist\n", mtdnr);
+		return PTR_ERR(mtd);
+	}
+
+	return mtd_get_sb(fc, mtd, fill_super);
+}
+
+/**
+ * get_tree_mtd - Get a superblock based on a single MTD device
+ * @fc: The filesystem context holding the parameters
+ * @fill_super: Helper to initialise a new superblock
+ */
+int get_tree_mtd(struct fs_context *fc,
+	      int (*fill_super)(struct super_block *sb,
+				struct fs_context *fc))
+{
+#ifdef CONFIG_BLOCK
+	dev_t dev;
+	int ret;
+#endif
+	int mtdnr;
+
+	if (!fc->source)
+		return invalf(fc, "No source specified");
+
+	pr_debug("MTDSB: dev_name \"%s\"\n", fc->source);
+
+	/* the preferred way of mounting in future; especially when
+	 * CONFIG_BLOCK=n - we specify the underlying MTD device by number or
+	 * by name, so that we don't require block device support to be present
+	 * in the kernel.
+	 */
+	if (fc->source[0] == 'm' &&
+	    fc->source[1] == 't' &&
+	    fc->source[2] == 'd') {
+		if (fc->source[3] == ':') {
+			struct mtd_info *mtd;
+
+			/* mount by MTD device name */
+			pr_debug("MTDSB: mtd:%%s, name \"%s\"\n",
+				 fc->source + 4);
+
+			mtd = get_mtd_device_nm(fc->source + 4);
+			if (!IS_ERR(mtd))
+				return mtd_get_sb(fc, mtd, fill_super);
+
+			errorf(fc, "MTD: MTD device with name \"%s\" not found",
+			       fc->source + 4);
+
+		} else if (isdigit(fc->source[3])) {
+			/* mount by MTD device number name */
+			unsigned int mtdnr_val;
+
+			if (kstrtouint(fc->source + 3, 0, &mtdnr_val) == 0) {
+				mtdnr = mtdnr_val;
+				/* It was a valid number */
+				pr_debug("MTDSB: mtd%%d, mtdnr %u\n", mtdnr_val);
+				return mtd_get_sb_by_nr(fc, mtdnr, fill_super);
+			}
+		}
+	}
+
+#ifdef CONFIG_BLOCK
+	/* try the old way - the hack where we allowed users to mount
+	 * /dev/mtdblock$(n) but didn't actually _use_ the blockdev
+	 */
+	ret = lookup_bdev(fc->source, &dev);
+	if (ret) {
+		errorf(fc, "MTD: Couldn't look up '%s': %d", fc->source, ret);
+		return ret;
+	}
+	pr_debug("MTDSB: lookup_bdev() returned 0\n");
+
+	if (MAJOR(dev) == MTD_BLOCK_MAJOR)
+		return mtd_get_sb_by_nr(fc, MINOR(dev), fill_super);
+
+#endif /* CONFIG_BLOCK */
+
+	if (!(fc->sb_flags & SB_SILENT))
+		errorf(fc, "MTD: Attempt to mount non-MTD device \"%s\"",
+		       fc->source);
+	return -EINVAL;
+}
+EXPORT_SYMBOL_GPL(get_tree_mtd);
+
+/*
+ * destroy an MTD-based superblock
+ */
+void kill_mtd_super(struct super_block *sb)
+{
+	generic_shutdown_super(sb);
+	put_mtd_device(sb->s_mtd);
+	sb->s_mtd = NULL;
+}
+
+EXPORT_SYMBOL_GPL(kill_mtd_super);