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Subject[PATCH v2] block, bfq: fix possible uaf for 'bfqq->bic'
Date
From: Yu Kuai <yukuai3@huawei.com>

Our test report a uaf for 'bfqq->bic' in 5.10:

==================================================================
BUG: KASAN: use-after-free in bfq_select_queue+0x378/0xa30

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+0xe8/0x130
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
do_syscall_64+0x33/0x40
entry_SYSCALL_64_after_hwframe+0x61/0xc6

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
(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 while bfqq is detached from bic.

Fixes: 3bc5e683c67d ("bfq: Split shared queues on move between cgroups")
Suggested-by: Jan Kara <jack@suse.cz>
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
---
Changes in v2:
- Use a new solution as suggested by Jan.

block/bfq-iosched.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index a72304c728fc..b111a7b8dca6 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -386,6 +386,12 @@ static void bfq_put_stable_ref(struct bfq_queue *bfqq);

void bic_set_bfqq(struct bfq_io_cq *bic, struct bfq_queue *bfqq, bool is_sync)
{
+ struct bfq_queue *old_bfqq = bic->bfqq[is_sync];
+
+ /* Clear bic pointer if bfqq is detached from this bic */
+ if (old_bfqq && old_bfqq->bic == bic)
+ old_bfqq->bic = NULL;
+
/*
* If bfqq != NULL, then a non-stable queue merge between
* bic->bfqq and bfqq is happening here. This causes troubles
@@ -5311,7 +5317,6 @@ static void bfq_exit_icq_bfqq(struct bfq_io_cq *bic, bool is_sync)
unsigned long flags;

spin_lock_irqsave(&bfqd->lock, flags);
- bfqq->bic = NULL;
bfq_exit_bfqq(bfqd, bfqq);
bic_set_bfqq(bic, NULL, is_sync);
spin_unlock_irqrestore(&bfqd->lock, flags);
--
2.31.1
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