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From:   Qian Cai <cai@lca.pw>
To:     akpm@linux-foundation.org
Cc:     esploit@protonmail.ch, jejb@linux.ibm.com, dgilbert@interlog.com,
        martin.petersen@oracle.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Qian Cai <cai@lca.pw>
Subject: [PATCH] rbtree: fix the red root
Date:   Fri, 11 Jan 2019 11:51:45 -0500
Message-Id: <20190111165145.23628-1-cai@lca.pw>
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A GFP was reported,

kasan: CONFIG_KASAN_INLINE enabled
kasan: GPF could be caused by NULL-ptr deref or user memory access
general protection fault: 0000 [#1] SMP KASAN
        kasan_die_handler.cold.22+0x11/0x31
        notifier_call_chain+0x17b/0x390
        atomic_notifier_call_chain+0xa7/0x1b0
        notify_die+0x1be/0x2e0
        do_general_protection+0x13e/0x330
        general_protection+0x1e/0x30
        rb_insert_color+0x189/0x1480
        create_object+0x785/0xca0
        kmemleak_alloc+0x2f/0x50
        kmem_cache_alloc+0x1b9/0x3c0
        getname_flags+0xdb/0x5d0
        getname+0x1e/0x20
        do_sys_open+0x3a1/0x7d0
        __x64_sys_open+0x7e/0xc0
        do_syscall_64+0x1b3/0x820
        entry_SYSCALL_64_after_hwframe+0x49/0xbe

It turned out,

gparent = rb_red_parent(parent);
tmp = gparent->rb_right; <-- GFP triggered here.

Apparently, "gparent" is NULL which indicates "parent" is rbtree's root
which is red. Otherwise, it will be treated properly a few lines above.

/*
 * If there is a black parent, we are done.
 * Otherwise, take some corrective action as,
 * per 4), we don't want a red root or two
 * consecutive red nodes.
 */
if(rb_is_black(parent))
	break;

Hence, it violates the rule #1 and need a fix up.

Reported-by: Esme <esploit@protonmail.ch>
Signed-off-by: Qian Cai <cai@lca.pw>
---
 lib/rbtree.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/lib/rbtree.c b/lib/rbtree.c
index d3ff682fd4b8..acc969ad8de9 100644
--- a/lib/rbtree.c
+++ b/lib/rbtree.c
@@ -127,6 +127,13 @@ __rb_insert(struct rb_node *node, struct rb_root *root,
 			break;
 
 		gparent = rb_red_parent(parent);
+		if (unlikely(!gparent)) {
+			/*
+			 * The root is red so correct it.
+			 */
+			rb_set_parent_color(parent, NULL, RB_BLACK);
+			break;
+		}
 
 		tmp = gparent->rb_right;
 		if (parent != tmp) {	/* parent == gparent->rb_left */
-- 
2.17.2 (Apple Git-113)