In the Linux kernel, the following vulnerability has been resolved:

ftrace: Fix invalid address access in lookup_rec() when index is 0

KASAN reported follow problem:

 BUG: KASAN: use-after-free in lookup_rec
 Read of size 8 at addr ffff000199270ff0 by task modprobe
 CPU: 2 Comm: modprobe
 Call trace:
  kasan_report
  __asan_load8
  lookup_rec
  ftrace_location
  arch_check_ftrace_location
  check_kprobe_address_safe
  register_kprobe

When checking pg->records[pg->index - 1].ip in lookup_rec(), it can get a
pg which is newly added to ftrace_pages_start in ftrace_process_locs().
Before the first pg->index++, index is 0 and accessing pg->records[-1].ip
will cause this problem.

Don't check the ip when pg->index is 0.

https://git.kernel.org/stable/c/2a0d71fabfeb349216d33f001a6421b1768bd3a9

https://git.kernel.org/stable/c/2de28e5ce34b22b73b833a21e2c45ae3aade3964

https://git.kernel.org/stable/c/4f84f31f63416b0f02fc146ffdc4ab32723eb7e8

https://git.kernel.org/stable/c/7569ee04b0e3b32df79f64db3a7138573edad9bc

https://git.kernel.org/stable/c/83c3b2f4e7c61367c7b24551f4c6eb94bbdda283

https://git.kernel.org/stable/c/ac58b88ccbbb8e9fb83e137cee04a856b1ea6635

https://git.kernel.org/stable/c/ee92fa443358f4fc0017c1d0d325c27b37802504

https://git.kernel.org/stable/c/f1bd8b7fd890d87d0dc4dedc6287ea34dd07c0b4