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Subject: Re: KASAN: use-after-free Read in io_uring_setup
From:   Jens Axboe <axboe@kernel.dk>
To:     syzbot <syzbot+6f03d895a6cd0d06187f@syzkaller.appspotmail.com>,
        linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com,
        viro@zeniv.linux.org.uk
References: <0000000000007c4f500595f35bf4@google.com>
 <883b3cf9-1d92-a22f-e946-0936d09d36c0@kernel.dk>
Message-ID: <36878490-a616-d5d6-3fd7-6d222cc41ba4@kernel.dk>
Date:   Mon, 28 Oct 2019 09:15:25 -0600
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On 10/28/19 8:09 AM, Jens Axboe wrote:
> On 10/28/19 1:22 AM, syzbot wrote:
>> Hello,
>>
>> syzbot found the following crash on:
>>
>> HEAD commit:    5a1e843c Merge tag 'mips_fixes_5.4_3' of git://git.kernel...
>> git tree:       upstream
>> console output: https://syzkaller.appspot.com/x/log.txt?x=10e2001f600000
>> kernel config:  https://syzkaller.appspot.com/x/.config?x=420126a10fdda0f1
>> dashboard link: https://syzkaller.appspot.com/bug?extid=6f03d895a6cd0d06187f
>> compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
>> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=11d4fa97600000
>>
>> IMPORTANT: if you fix the bug, please add the following tag to the commit:
>> Reported-by: syzbot+6f03d895a6cd0d06187f@syzkaller.appspotmail.com
>>
>> ==================================================================
>> BUG: KASAN: use-after-free in io_uring_create fs/io_uring.c:3842 [inline]
>> BUG: KASAN: use-after-free in io_uring_setup+0x1877/0x18c0
>> fs/io_uring.c:3881
>> Read of size 8 at addr ffff888082284048 by task syz-executor.5/11342
>>
>> CPU: 1 PID: 11342 Comm: syz-executor.5 Not tainted 5.4.0-rc4+ #0
>> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
>> Google 01/01/2011
>> Call Trace:
>>     __dump_stack lib/dump_stack.c:77 [inline]
>>     dump_stack+0x172/0x1f0 lib/dump_stack.c:113
>>     print_address_description.constprop.0.cold+0xd4/0x30b mm/kasan/report.c:374
>>     __kasan_report.cold+0x1b/0x41 mm/kasan/report.c:506
>>     kasan_report+0x12/0x20 mm/kasan/common.c:634
>>     __asan_report_load8_noabort+0x14/0x20 mm/kasan/generic_report.c:132
>>     io_uring_create fs/io_uring.c:3842 [inline]
>>     io_uring_setup+0x1877/0x18c0 fs/io_uring.c:3881
>>     __do_sys_io_uring_setup fs/io_uring.c:3894 [inline]
>>     __se_sys_io_uring_setup fs/io_uring.c:3891 [inline]
>>     __x64_sys_io_uring_setup+0x54/0x80 fs/io_uring.c:3891
>>     do_syscall_64+0xfa/0x760 arch/x86/entry/common.c:290
>>     entry_SYSCALL_64_after_hwframe+0x49/0xbe
>> RIP: 0033:0x459f39
>> Code: ad b6 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7
>> 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff
>> ff 0f 83 7b b6 fb ff c3 66 2e 0f 1f 84 00 00 00 00
>> RSP: 002b:00007f313e126c78 EFLAGS: 00000246 ORIG_RAX: 00000000000001a9
>> RAX: ffffffffffffffda RBX: 0000000000000002 RCX: 0000000000459f39
>> RDX: 0000000000000000 RSI: 00000000200005c0 RDI: 000000040000000e
>> RBP: 000000000075bf20 R08: 0000000000000000 R09: 0000000000000000
>> R10: 0000000000000000 R11: 0000000000000246 R12: 00007f313e1276d4
>> R13: 00000000004c1512 R14: 00000000004d4da8 R15: 00000000ffffffff
>>
>> Allocated by task 11342:
>>     save_stack+0x23/0x90 mm/kasan/common.c:69
>>     set_track mm/kasan/common.c:77 [inline]
>>     __kasan_kmalloc mm/kasan/common.c:510 [inline]
>>     __kasan_kmalloc.constprop.0+0xcf/0xe0 mm/kasan/common.c:483
>>     kasan_kmalloc+0x9/0x10 mm/kasan/common.c:524
>>     kmem_cache_alloc_trace+0x158/0x790 mm/slab.c:3550
>>     kmalloc include/linux/slab.h:556 [inline]
>>     kzalloc include/linux/slab.h:690 [inline]
>>     io_ring_ctx_alloc fs/io_uring.c:393 [inline]
>>     io_uring_create fs/io_uring.c:3811 [inline]
>>     io_uring_setup+0xec6/0x18c0 fs/io_uring.c:3881
>>     __do_sys_io_uring_setup fs/io_uring.c:3894 [inline]
>>     __se_sys_io_uring_setup fs/io_uring.c:3891 [inline]
>>     __x64_sys_io_uring_setup+0x54/0x80 fs/io_uring.c:3891
>>     do_syscall_64+0xfa/0x760 arch/x86/entry/common.c:290
>>     entry_SYSCALL_64_after_hwframe+0x49/0xbe
>>
>> Freed by task 11335:
>>     save_stack+0x23/0x90 mm/kasan/common.c:69
>>     set_track mm/kasan/common.c:77 [inline]
>>     kasan_set_free_info mm/kasan/common.c:332 [inline]
>>     __kasan_slab_free+0x102/0x150 mm/kasan/common.c:471
>>     kasan_slab_free+0xe/0x10 mm/kasan/common.c:480
>>     __cache_free mm/slab.c:3425 [inline]
>>     kfree+0x10a/0x2c0 mm/slab.c:3756
>>     io_ring_ctx_free fs/io_uring.c:3552 [inline]
>>     io_ring_ctx_wait_and_kill+0x4d7/0x6c0 fs/io_uring.c:3592
>>     io_uring_release+0x42/0x50 fs/io_uring.c:3600
>>     __fput+0x2ff/0x890 fs/file_table.c:280
>>     ____fput+0x16/0x20 fs/file_table.c:313
>>     task_work_run+0x145/0x1c0 kernel/task_work.c:113
>>     tracehook_notify_resume include/linux/tracehook.h:188 [inline]
>>     exit_to_usermode_loop+0x316/0x380 arch/x86/entry/common.c:163
>>     prepare_exit_to_usermode arch/x86/entry/common.c:194 [inline]
>>     syscall_return_slowpath arch/x86/entry/common.c:274 [inline]
>>     do_syscall_64+0x65f/0x760 arch/x86/entry/common.c:300
>>     entry_SYSCALL_64_after_hwframe+0x49/0xbe
>>
>> The buggy address belongs to the object at ffff888082284000
>>     which belongs to the cache kmalloc-2k of size 2048
>> The buggy address is located 72 bytes inside of
>>     2048-byte region [ffff888082284000, ffff888082284800)
>> The buggy address belongs to the page:
>> page:ffffea000208a100 refcount:1 mapcount:0 mapping:ffff8880aa400e00
>> index:0x0
>> flags: 0x1fffc0000000200(slab)
>> raw: 01fffc0000000200 ffffea0002a1bc88 ffffea00023fa248 ffff8880aa400e00
>> raw: 0000000000000000 ffff888082284000 0000000100000001 0000000000000000
>> page dumped because: kasan: bad access detected
>>
>> Memory state around the buggy address:
>>     ffff888082283f00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>>     ffff888082283f80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>>> ffff888082284000: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>>                                                  ^
>>     ffff888082284080: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>>     ffff888082284100: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>> ==================================================================
> 
> Interesting, looks like a malicious case that attempts to close the
> fd as soon as it's installed. As a result of that, the rest of the
> setup will be done on a ring that's already torn down. The below should
> fix that.
> 
> Totally untested, haven't tried the reproducer yet.

Still haven't managed to reproduce this, but the below is simpler. As long
as we don't touch the ring after having installed fd, we should be safe
from these kinds of games.


diff --git a/fs/io_uring.c b/fs/io_uring.c
index ba1431046c98..c11c4157a4c2 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -3829,10 +3829,6 @@ static int io_uring_create(unsigned entries, struct io_uring_params *p)
 	if (ret)
 		goto err;
 
-	ret = io_uring_get_fd(ctx);
-	if (ret < 0)
-		goto err;
-
 	memset(&p->sq_off, 0, sizeof(p->sq_off));
 	p->sq_off.head = offsetof(struct io_rings, sq.head);
 	p->sq_off.tail = offsetof(struct io_rings, sq.tail);
@@ -3850,6 +3846,14 @@ static int io_uring_create(unsigned entries, struct io_uring_params *p)
 	p->cq_off.overflow = offsetof(struct io_rings, cq_overflow);
 	p->cq_off.cqes = offsetof(struct io_rings, cqes);
 
+	/*
+	 * Install ring fd as the very last thing, so we don't risk someone
+	 * having closed it before we finish setup
+	 */
+	ret = io_uring_get_fd(ctx);
+	if (ret < 0)
+		goto err;
+
 	p->features = IORING_FEAT_SINGLE_MMAP;
 	return ret;
 err:

-- 
Jens Axboe