linux-kernel - [PATCH] kernel: auditfilter: Fix a possible null-pointer dereference in audit_watch

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Date:   Tue, 23 Jul 2019 20:49:51 +0800
From:   Jia-Ju Bai <baijiaju1990@...il.com>
To:     paul@...l-moore.com, eparis@...hat.com
Cc:     linux-audit@...hat.com, linux-kernel@...r.kernel.org,
        Jia-Ju Bai <baijiaju1990@...il.com>
Subject: [PATCH] kernel: auditfilter: Fix a possible null-pointer dereference in audit_watch_path()

In audit_find_rule(), there is an if statement on line 894 to check
whether entry->rule.watch is NULL:
    else if (entry->rule.watch)

If entry->rule.watch is NULL, audit_compare_rule on 910 is called:
    audit_compare_rule(&entry->rule, &e->rule))

In audit_compare_rule(), a->watch is used on line 720:
    if (strcmp(audit_watch_path(a->watch), ...)

In this case, a->watch is NULL, and audit_watch_path() will use:
    watch->path

Thus, a possible null-pointer dereference may occur in this case.

To fix this possible bug, an if statement is added in
audit_compare_rule() to check a->watch before using a->watch.

This bug is found by a static analysis tool STCheck written by us.

Signed-off-by: Jia-Ju Bai <baijiaju1990@...il.com>
---
 kernel/auditfilter.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/auditfilter.c b/kernel/auditfilter.c
index b0126e9c0743..b0ad17b14609 100644
--- a/kernel/auditfilter.c
+++ b/kernel/auditfilter.c
@@ -717,6 +717,8 @@ static int audit_compare_rule(struct audit_krule *a, struct audit_krule *b)
 				return 1;
 			break;
 		case AUDIT_WATCH:
+			if (!a->watch)
+				break;
 			if (strcmp(audit_watch_path(a->watch),
 				   audit_watch_path(b->watch)))
 				return 1;
-- 
2.17.0