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SubjectRe: [PATCH] rbtree: fix the red root
On Fri, Jan 11, 2019 at 11:51:45AM -0500, Qian Cai wrote:
> 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.

Good catch, acked. After thinking through the logic a bit your solution
seems like the simplest fix.

Now, I didn't do _extensive_ testing but a quick compile and bootup of
the patch with CONFIG_KASAN_INLINE enabled has yet to throw any GFPs,
so take that as you will.

Reviewed-by: Joey Pabalinas <joeypabalinas@gmail.com>
Tested-by: Joey Pabalinas <joeypabalinas@gmail.com>

> /*
> * 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)
>

--
Cheers,
Joey Pabalinas
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