kcov: ignore an out-of-range comparison count in write_comp_data()

write_comp_data() reads the comparison record count from area[0] and uses
it to index the coverage buffer:

	area = (u64 *)t->kcov_area;
	max_pos = t->kcov_size * sizeof(unsigned long);

	count = READ_ONCE(area[0]);

	/* Every record is KCOV_WORDS_PER_CMP 64-bit words. */
	start_index = 1 + count * KCOV_WORDS_PER_CMP;
	end_pos = (start_index + KCOV_WORDS_PER_CMP) * sizeof(u64);
	if (likely(end_pos <= max_pos)) {

The buffer is mmap'd writable into the collecting process, so count is
under its control and end_pos <= max_pos is its only bound.  A count that
wraps the u64 multiply leaves end_pos below max_pos, so the check passes
while the record store lands 24 bytes before the buffer, in the unmapped
vmalloc guard page, and faults:

  BUG: unable to handle page fault for address: ffa0000000b60fe8
  #PF: supervisor write access in kernel mode
  #PF: error_code(0x0002) - not-present page
  Oops: 0002 [#1] SMP KASAN NOPTI
  RIP: 0010:write_comp_data+0x7e/0xa0
  ...
  Kernel panic - not syncing: Fatal exception

Bound count first: only max_pos / (sizeof(u64) * KCOV_WORDS_PER_CMP)
records fit, so a larger count is not a valid index and is dropped. 
kcov_move_area() bounds the same untrusted count this way, and no count
the end_pos <= max_pos check accepts reaches that limit, so no valid
record is lost.

kcov is a root-only debugfs file (debugfs_create_file_unsafe("kcov", 0600,
...)) and write_comp_data() exists only under
CONFIG_KCOV_ENABLE_COMPARISONS, so this is a local, debug-kernel
robustness fix: the process corrupts its own buffer and the kernel oopses.
It crosses no privilege boundary.  Additional details at [1]

Link: https://lore.kernel.org/20260917104306.22145-1-fangxy@xiaopeng.com [1]
Fixes: ded97d2c2b ("kcov: support comparison operands collection")
Signed-off-by: Fang Xieyan <fangxy@xiaopeng.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Alexander Potapenko <glider@google.com>
Assisted-by: Hawkeye:GLM-5.3-flash
Assisted-by: Qoder:Qwen3.8-Max
Cc: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Marco Elver <elver@google.com>
Cc: Victor Chibotaru <tchibo@google.com>
Cc: <stable@vger.kernel.org>

kernel/kcov.c

@@ -253,6 +253,14 @@ static void notrace write_comp_data(u64 type, u64 arg1, u64 arg2, u64 ip)
 
 	count = READ_ONCE(area[0]);
 
+	/*
+	 * area[0] is writable by the collecting process, so count cannot be
+	 * trusted. Bound it to the records that fit, as kcov_move_area()
+	 * does, so the end_pos multiply below cannot wrap past its check.
+	 */
+	if (count >= max_pos / (sizeof(u64) * KCOV_WORDS_PER_CMP))
+		return;
+
 	/* Every record is KCOV_WORDS_PER_CMP 64-bit words. */
 	start_index = 1 + count * KCOV_WORDS_PER_CMP;
 	end_pos = (start_index + KCOV_WORDS_PER_CMP) * sizeof(u64);