Re: [PATCH v2] rbtree: fix the red root

[David Lechner]

On 1/11/19 8:58 PM, Michel Lespinasse wrote:
> On Fri, Jan 11, 2019 at 3:47 PM David Lechner <david@xxxxxxxxxxxxxx> wrote:
> > On 1/11/19 2:58 PM, Qian Cai wrote:
> > > A GPF 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; <-- GPF was 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 (the root can't be red) and need a fix
> > > up, and also add a regression test for it. This looks like was
> > > introduced by 6d58452dc06 where it no longer always paint the root as
> > > black.
> > >
> > > Fixes: 6d58452dc06 (rbtree: adjust root color in rb_insert_color() only
> > > when necessary)
> > > Reported-by: Esme <esploit@xxxxxxxxxxxxx>
> > > Tested-by: Joey Pabalinas <joeypabalinas@xxxxxxxxx>
> > > Signed-off-by: Qian Cai <cai@xxxxxx>
> > > ---
> >
> > Tested-by: David Lechner <david@xxxxxxxxxxxxxx>
> > FWIW, this fixed the following crash for me:
> >
> > Unable to handle kernel NULL pointer dereference at virtual address 00000004
>
> Just to clarify, do you have a way to reproduce this crash without the fix ?

I am starting to suspect that my crash was caused by some new code
in the drm-misc-next tree that might be causing a memory corruption.
It threw me off that the stack trace didn't contain anything related
to drm.

See: https://patchwork.freedesktop.org/patch/276719/

> I don't think the fix is correct, because it just silently ignores a
> corrupted rbtree (red root node). But the code that creates this
> situation certainly needs to be fixed - having a reproduceable test
> case would certainly help here.
>
> Regarding 6d58452dc06, the reasoning was that this code expects to be
> called after inserting a new (red) leaf into an rbtree that had all of
> its data structure invariants satisfied. So in this context, it should
> not be necessary to always reset the root to black, as this should
> already be the case...