Re: [PATCH] block, bfq: fix possible uaf for 'bfqq->bic'
From: Yu Kuai
Date: Tue Dec 13 2022 - 05:32:33 EST
Hi, Jan!
在 2022/12/12 21:35, Jan Kara 写道:
On Sat 10-12-22 18:25:37, Yu Kuai wrote:
From: Yu Kuai <yukuai3@xxxxxxxxxx>
Our test report a uaf for 'bfqq->bic' in 5.10:
==================================================================
BUG: KASAN: use-after-free in bfq_select_queue+0x378/0xa30
Read of size 8 at addr ffff88810efb42d8 by task fsstress/2318352
CPU: 6 PID: 2318352 Comm: fsstress Kdump: loaded Not tainted 5.10.0-60.18.0.50.h602.kasan.eulerosv2r11.x86_64 #1
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.12.1-0-ga5cab58-20220320_160524-szxrtosci10000 04/01/2014
Call Trace:
...
bfq_select_queue+0x378/0xa30
__bfq_dispatch_request+0x1c4/0x220
bfq_dispatch_request+0xe8/0x130
__blk_mq_do_dispatch_sched+0x3f4/0x560
blk_mq_do_dispatch_sched+0x62/0xb0
__blk_mq_sched_dispatch_requests+0x215/0x2a0
blk_mq_sched_dispatch_requests+0x8f/0xd0
__blk_mq_run_hw_queue+0x98/0x180
__blk_mq_delay_run_hw_queue+0x22b/0x240
blk_mq_run_hw_queue+0xe3/0x190
blk_mq_sched_insert_requests+0x107/0x200
blk_mq_flush_plug_list+0x26e/0x3c0
blk_finish_plug+0x63/0x90
__iomap_dio_rw+0x7b5/0x910
iomap_dio_rw+0x36/0x80
ext4_dio_read_iter+0x146/0x190 [ext4]
ext4_file_read_iter+0x1e2/0x230 [ext4]
new_sync_read+0x29f/0x400
vfs_read+0x24e/0x2d0
ksys_read+0xd5/0x1b0
Perhaps we can trim this UAF report a bit to what I've left above? That
should be enough to give idea about the problem.
Yes, of course.
Commit 3bc5e683c67d ("bfq: Split shared queues on move between cgroups")
changes that move process to a new cgroup will allocate a new bfqq to
use, however, the old bfqq and new bfqq can point to the same bic:
1) Initial state, two process with io in the same cgroup.
Process 1 Process 2
(BIC1) (BIC2)
| Λ | Λ
| | | |
V | V |
bfqq1 bfqq2
2) bfqq1 is merged to bfqq2.
Process 1 Process 2（cg1)
(BIC1) (BIC2)
| |
\-------------\|
V
bfqq1 bfqq2(coop)
3) Process 1 exit, then issue new io(denoce IOA) from Process 2.
(BIC2)
| Λ
| |
V |
bfqq2(coop)
4) Before IOA is completed, move Process 2 to another cgroup and issue io.
Process 2
(BIC2)
Λ
|\--------------\
| V
bfqq2 bfqq3
Now that BIC2 points to bfqq3, while bfqq2 and bfqq3 both point to BIC2.
If all the requests are completed, and Process 2 exit, BIC2 will be
freed while there is no guarantee that bfqq2 will be freed before BIC2.
Fix the problem by clearing bfqq->bic if process references is decreased
to zero, since that they are not related anymore.
Fixes: 3bc5e683c67d ("bfq: Split shared queues on move between cgroups")
Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
Thanks for the analysis and the patch! I agree this is a problem.
diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index a72304c728fc..6eada99d1b34 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -3036,6 +3036,14 @@ void bfq_release_process_ref(struct bfq_data *bfqd, struct bfq_queue *bfqq)
bfq_reassign_last_bfqq(bfqq, NULL);
+ /*
+ * __bfq_bic_change_cgroup() just reset bic->bfqq so that a new bfqq
+ * will be created to handle new io, while old bfqq will stay around
+ * until all the requests are completed. It's unsafe to keep bfqq->bic
+ * since they are not related anymore.
+ */
+ if (bfqq_process_refs(bfqq) == 1)
+ bfqq->bic = NULL;
bfq_put_queue(bfqq);
Rather than changing bfq_release_process_ref() I think it would be more
logical to change bic_set_bfqq() like:
struct bfq_queue *old_bfqq = bic->bfqq[is_sync];
/* Clear bic pointer if we are detaching bfqq from its bic */
if (old_bfqq && old_bfqq->bic == bic)
old_bfqq->bic = NULL;
And then we can also remove several explicit bfqq->bic = NULL statements
from bfq code.
Yes, I agree. I'll send a new patch soon.
Thanks,
Kuai
Honza