[PATCH 5.4 50/60] ftrace: Fix invalid address access in lookup_rec() when index is 0

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From: Chen Zhongjin <chenzhongjin@xxxxxxxxxx>

commit ee92fa443358f4fc0017c1d0d325c27b37802504 upstream.

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.

Link: https://lore.kernel.org/linux-trace-kernel/20230309080230.36064-1-chenzhongjin@xxxxxxxxxx

Cc: stable@xxxxxxxxxxxxxxx
Fixes: 9644302e3315 ("ftrace: Speed up search by skipping pages by address")
Suggested-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
Signed-off-by: Chen Zhongjin <chenzhongjin@xxxxxxxxxx>
Signed-off-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 kernel/trace/ftrace.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -1557,7 +1557,8 @@ unsigned long ftrace_location_range(unsi
 	key.flags = end;	/* overload flags, as it is unsigned long */
 
 	for (pg = ftrace_pages_start; pg; pg = pg->next) {
-		if (end < pg->records[0].ip ||
+		if (pg->index == 0 ||
+		    end < pg->records[0].ip ||
 		    start >= (pg->records[pg->index - 1].ip + MCOUNT_INSN_SIZE))
 			continue;
 		rec = bsearch(&key, pg->records, pg->index,





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