[PATCH mm-stable] uprobes: Revert ref_ctr_offset in uprobe_unregister error path

[Jiri Olsa <jolsa@kernel.org>]

From: Jiri Olsa <olsajiri@gmail.com>

There's error path that could lead to inactive uprobe:

  1) uprobe_register succeeds - updates instruction to int3 and
     changes ref_ctr from 0 to 1
  2) uprobe_unregister fails  - int3 stays in place, but ref_ctr
     is changed to 0 (it's not restored to 1 in the fail path)
     uprobe is leaked
  3) another uprobe_register comes and re-uses the leaked uprobe
     and succeds - but int3 is already in place, so ref_ctr update
     is skipped and it stays 0 - uprobe CAN NOT be triggered now
  4) uprobe_unregister fails because ref_ctr value is unexpected

Fixing this by reverting the updated ref_ctr value back to 1 in step 2),
which is the case when uprobe_unregister fails (int3 stays in place),
but we have already updated refctr.

The new scenario will go as follows:

  1) uprobe_register succeeds - updates instruction to int3 and
     changes ref_ctr from 0 to 1
  2) uprobe_unregister fails  - int3 stays in place and ref_ctr
     is reverted to 1..  uprobe is leaked
  3) another uprobe_register comes and re-uses the leaked uprobe
     and succeds - but int3 is already in place, so ref_ctr update
     is skipped and it stays 1 - uprobe CAN be triggered now
  4) uprobe_unregister succeeds

Suggested-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
---
Please note it's based on mm-stable branch, because it has the
latest uprobe_write_opcode rewrite changes.

 kernel/events/uprobes.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index 4c965ba77f9f..84ee7b590861 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -581,8 +581,8 @@ int uprobe_write_opcode(struct arch_uprobe *auprobe, struct vm_area_struct *vma,
 
 out:
 	/* Revert back reference counter if instruction update failed. */
-	if (ret < 0 && is_register && ref_ctr_updated)
-		update_ref_ctr(uprobe, mm, -1);
+	if (ret < 0 && ref_ctr_updated)
+		update_ref_ctr(uprobe, mm, is_register ? -1 : 1);
 
 	/* try collapse pmd for compound page */
 	if (ret > 0)
-- 
2.49.0