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SubjectRe: [PATCH cgroup/for-4.15-fixes] cgroup: fix css_task_iter crash on CSS_TASK_ITER_PROC
This resolves the bug for me.

Thank you,
George

On Wed, Dec 20, 2017 at 10:13 AM, Tejun Heo <tj@kernel.org> wrote:
> Hello,
>
> Applied the following to cgroup/for-4.15-fixes. Will push out to
> linus later this week. I could reproduce the problem reliably and am
> pretty sure this is the right fix but I'd greatly appreciate if you
> guys can confirm the fix too.
>
> Thank you very much.
>
> ------ 8< ------
> From 74d0833c659a8a54735e5efdd44f4b225af68586 Mon Sep 17 00:00:00 2001
> From: Tejun Heo <tj@kernel.org>
> Date: Wed, 20 Dec 2017 07:09:19 -0800
>
> While teaching css_task_iter to handle skipping over tasks which
> aren't group leaders, bc2fb7ed089f ("cgroup: add @flags to
> css_task_iter_start() and implement CSS_TASK_ITER_PROCS") introduced a
> silly bug.
>
> CSS_TASK_ITER_PROCS is implemented by repeating
> css_task_iter_advance() while the advanced cursor is pointing to a
> non-leader thread. However, the cursor variable, @l, wasn't updated
> when the iteration has to advance to the next css_set and the
> following repetition would operate on the terminal @l from the
> previous iteration which isn't pointing to a valid task leading to
> oopses like the following or infinite looping.
>
> BUG: unable to handle kernel NULL pointer dereference at 0000000000000254
> IP: __task_pid_nr_ns+0xc7/0xf0
> PGD 0 P4D 0
> Oops: 0000 [#1] SMP
> ...
> CPU: 2 PID: 1 Comm: systemd Not tainted 4.14.4-200.fc26.x86_64 #1
> Hardware name: System manufacturer System Product Name/PRIME B350M-A, BIOS 3203 11/09/2017
> task: ffff88c4baee8000 task.stack: ffff96d5c3158000
> RIP: 0010:__task_pid_nr_ns+0xc7/0xf0
> RSP: 0018:ffff96d5c315bd50 EFLAGS: 00010206
> RAX: 0000000000000000 RBX: ffff88c4b68c6000 RCX: 0000000000000250
> RDX: ffffffffa5e47960 RSI: 0000000000000000 RDI: ffff88c490f6ab00
> RBP: ffff96d5c315bd50 R08: 0000000000001000 R09: 0000000000000005
> R10: ffff88c4be006b80 R11: ffff88c42f1b8004 R12: ffff96d5c315bf18
> R13: ffff88c42d7dd200 R14: ffff88c490f6a510 R15: ffff88c4b68c6000
> FS: 00007f9446f8ea00(0000) GS:ffff88c4be680000(0000) knlGS:0000000000000000
> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000000000000254 CR3: 00000007f956f000 CR4: 00000000003406e0
> Call Trace:
> cgroup_procs_show+0x19/0x30
> cgroup_seqfile_show+0x4c/0xb0
> kernfs_seq_show+0x21/0x30
> seq_read+0x2ec/0x3f0
> kernfs_fop_read+0x134/0x180
> __vfs_read+0x37/0x160
> ? security_file_permission+0x9b/0xc0
> vfs_read+0x8e/0x130
> SyS_read+0x55/0xc0
> entry_SYSCALL_64_fastpath+0x1a/0xa5
> RIP: 0033:0x7f94455f942d
> RSP: 002b:00007ffe81ba2d00 EFLAGS: 00000293 ORIG_RAX: 0000000000000000
> RAX: ffffffffffffffda RBX: 00005574e2233f00 RCX: 00007f94455f942d
> RDX: 0000000000001000 RSI: 00005574e2321a90 RDI: 000000000000002b
> RBP: 0000000000000000 R08: 00005574e2321a90 R09: 00005574e231de60
> R10: 00007f94458c8b38 R11: 0000000000000293 R12: 00007f94458c8ae0
> R13: 00007ffe81ba3800 R14: 0000000000000000 R15: 00005574e2116560
> Code: 04 74 0e 89 f6 48 8d 04 76 48 8d 04 c5 f0 05 00 00 48 8b bf b8 05 00 00 48 01 c7 31 c0 48 8b 0f 48 85 c9 74 18 8b b2 30 08 00 00 <3b> 71 04 77 0d 48 c1 e6 05 48 01 f1 48 3b 51 38 74 09 5d c3 8b
> RIP: __task_pid_nr_ns+0xc7/0xf0 RSP: ffff96d5c315bd50
>
> Fix it by moving the initialization of the cursor below the repeat
> label. While at it, rename it to @next for readability.
>
> Signed-off-by: Tejun Heo <tj@kernel.org>
> Fixes: bc2fb7ed089f ("cgroup: add @flags to css_task_iter_start() and implement CSS_TASK_ITER_PROCS")
> Cc: stable@vger.kernel.org # v4.14+
> Reported-by: Laura Abbott <labbott@redhat.com>
> Reported-by: Bronek Kozicki <brok@incorrekt.com>
> Reported-by: George Amanakis <gamanakis@gmail.com>
> Signed-off-by: Tejun Heo <tj@kernel.org>
> ---
> kernel/cgroup/cgroup.c | 14 ++++++--------
> 1 file changed, 6 insertions(+), 8 deletions(-)
>
> diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
> index f4c2f8c..2cf06c2 100644
> --- a/kernel/cgroup/cgroup.c
> +++ b/kernel/cgroup/cgroup.c
> @@ -4125,26 +4125,24 @@ static void css_task_iter_advance_css_set(struct css_task_iter *it)
>
> static void css_task_iter_advance(struct css_task_iter *it)
> {
> - struct list_head *l = it->task_pos;
> + struct list_head *next;
>
> lockdep_assert_held(&css_set_lock);
> - WARN_ON_ONCE(!l);
> -
> repeat:
> /*
> * Advance iterator to find next entry. cset->tasks is consumed
> * first and then ->mg_tasks. After ->mg_tasks, we move onto the
> * next cset.
> */
> - l = l->next;
> + next = it->task_pos->next;
>
> - if (l == it->tasks_head)
> - l = it->mg_tasks_head->next;
> + if (next == it->tasks_head)
> + next = it->mg_tasks_head->next;
>
> - if (l == it->mg_tasks_head)
> + if (next == it->mg_tasks_head)
> css_task_iter_advance_css_set(it);
> else
> - it->task_pos = l;
> + it->task_pos = next;
>
> /* if PROCS, skip over tasks which aren't group leaders */
> if ((it->flags & CSS_TASK_ITER_PROCS) && it->task_pos &&
> --
> 2.9.5
>

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