Re: [PATCH] io_uring: prevent io_put_identity() from freeing a static identity

[Tadeusz Struk <tadeusz.struk@linaro.org>]

On 11/15/21 08:38, Tadeusz Struk wrote:
> On 11/3/21 18:21, Tadeusz Struk wrote:
>> Note: this applies to 5.10 stable only. It doesn't trigger on anything
>> above 5.10 as the code there has been substantially reworked. This also
>> doesn't apply to any stable kernel below 5.10 afaict.
>>
>> Syzbot found a bug: KASAN: invalid-free in io_dismantle_req
>> https://syzkaller.appspot.com/bug?id=123d9a852fc88ba573ffcb2dbcf4f9576c3b0559
>>
>> The test submits bunch of io_uring writes and exits, which then triggers
>> uring_task_cancel() and io_put_identity(), which in some corner cases,
>> tries to free a static identity. This causes a panic as shown in the
>> trace below:
>>
>>   BUG: KASAN: double-free or invalid-free in kfree+0xd5/0x310
>>   CPU: 0 PID: 4618 Comm: repro Not tainted 5.10.76-05281-g4944ec82ebb9-dirty #17
>>   Call Trace:
>>    dump_stack_lvl+0x1b2/0x21b
>>    print_address_description+0x8d/0x3b0
>>    kasan_report_invalid_free+0x58/0x130
>>    ____kasan_slab_free+0x14b/0x170
>>    __kasan_slab_free+0x11/0x20
>>    slab_free_freelist_hook+0xcc/0x1a0
>>    kfree+0xd5/0x310
>>    io_dismantle_req+0x9b0/0xd90
>>    io_do_iopoll+0x13a4/0x23e0
>>    io_iopoll_try_reap_events+0x116/0x290
>>    io_uring_cancel_task_requests+0x197d/0x1ee0
>>    io_uring_flush+0x170/0x6d0
>>    filp_close+0xb0/0x150
>>    put_files_struct+0x1d4/0x350
>>    exit_files+0x80/0xa0
>>    do_exit+0x6d9/0x2390
>>    do_group_exit+0x16a/0x2d0
>>    get_signal+0x133e/0x1f80
>>    arch_do_signal+0x7b/0x610
>>    exit_to_user_mode_prepare+0xaa/0xe0
>>    syscall_exit_to_user_mode+0x24/0x40
>>    do_syscall_64+0x3d/0x70
>>    entry_SYSCALL_64_after_hwframe+0x44/0xa9
>>
>>   Allocated by task 4611:
>>    ____kasan_kmalloc+0xcd/0x100
>>    __kasan_kmalloc+0x9/0x10
>>    kmem_cache_alloc_trace+0x208/0x390
>>    io_uring_alloc_task_context+0x57/0x550
>>    io_uring_add_task_file+0x1f7/0x290
>>    io_uring_create+0x2195/0x3490
>>    __x64_sys_io_uring_setup+0x1bf/0x280
>>    do_syscall_64+0x31/0x70
>>    entry_SYSCALL_64_after_hwframe+0x44/0xa9
>>
>>   The buggy address belongs to the object at ffff88810732b500
>>    which belongs to the cache kmalloc-192 of size 192
>>   The buggy address is located 88 bytes inside of
>>    192-byte region [ffff88810732b500, ffff88810732b5c0)
>>   Kernel panic - not syncing: panic_on_warn set ...
>>
>> This issue bisected to this commit:
>> commit 186725a80c4e ("io_uring: fix skipping disabling sqo on exec")
>>
>> Simple reverting the offending commit doesn't work as it hits some
>> other, related issues like:
>>
>> /* sqo_dead check is for when this happens after cancellation */
>> WARN_ON_ONCE(ctx->sqo_task == current && !ctx->sqo_dead &&
>>          !xa_load(&tctx->xa, (unsigned long)file));
>>
>>   ------------[ cut here ]------------
>>   WARNING: CPU: 1 PID: 5622 at fs/io_uring.c:8960 io_uring_flush+0x5bc/0x6d0
>>   Modules linked in:
>>   CPU: 1 PID: 5622 Comm: repro Not tainted 5.10.76-05281-g4944ec82ebb9-dirty #16
>>   Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-6.fc35 
>> 04/01/2014
>>   RIP: 0010:io_uring_flush+0x5bc/0x6d0
>>   Call Trace:
>>   filp_close+0xb0/0x150
>>   put_files_struct+0x1d4/0x350
>>   reset_files_struct+0x88/0xa0
>>   bprm_execve+0x7f2/0x9f0
>>   do_execveat_common+0x46f/0x5d0
>>   __x64_sys_execve+0x92/0xb0
>>   do_syscall_64+0x31/0x70
>>   entry_SYSCALL_64_after_hwframe+0x44/0xa9
>>
>> Changing __io_uring_task_cancel() to call io_disable_sqo_submit() directly,
>> as the comment suggests, only if __io_uring_files_cancel() is not executed
>> seems to fix the issue.
>>
>> Cc: Jens Axboe <axboe@kernel.dk>
>> Cc: Alexander Viro <viro@zeniv.linux.org.uk>
>> Cc: <io-uring@vger.kernel.org>
>> Cc: <linux-fsdevel@vger.kernel.org>
>> Cc: <linux-kernel@vger.kernel.org>
>> Cc: <stable@vger.kernel.org>
>> Reported-by: syzbot+6055980d041c8ac23307@syzkaller.appspotmail.com
>> Signed-off-by: Tadeusz Struk <tadeusz.struk@linaro.org>
>> ---
>>   fs/io_uring.c | 21 +++++++++++++++++----
>>   1 file changed, 17 insertions(+), 4 deletions(-)
>>
>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>> index 0736487165da..fcf9ffe9b209 100644
>> --- a/fs/io_uring.c
>> +++ b/fs/io_uring.c
>> @@ -8882,20 +8882,18 @@ void __io_uring_task_cancel(void)
>>       struct io_uring_task *tctx = current->io_uring;
>>       DEFINE_WAIT(wait);
>>       s64 inflight;
>> +    int canceled = 0;
>>       /* make sure overflow events are dropped */
>>       atomic_inc(&tctx->in_idle);
>> -    /* trigger io_disable_sqo_submit() */
>> -    if (tctx->sqpoll)
>> -        __io_uring_files_cancel(NULL);
>> -
>>       do {
>>           /* read completions before cancelations */
>>           inflight = tctx_inflight(tctx);
>>           if (!inflight)
>>               break;
>>           __io_uring_files_cancel(NULL);
>> +        canceled = 1;
>>           prepare_to_wait(&tctx->wait, &wait, TASK_UNINTERRUPTIBLE);
>> @@ -8909,6 +8907,21 @@ void __io_uring_task_cancel(void)
>>           finish_wait(&tctx->wait, &wait);
>>       } while (1);
>> +    /*
>> +     * trigger io_disable_sqo_submit()
>> +     * if not already done by __io_uring_files_cancel()
>> +     */
>> +    if (tctx->sqpoll && !canceled) {
>> +        struct file *file;
>> +        unsigned long index;
>> +
>> +        xa_for_each(&tctx->xa, index, file) {
>> +            struct io_ring_ctx *ctx = file->private_data;
>> +
>> +            io_disable_sqo_submit(ctx);
>> +        }
>> +    }
>> +
>>       atomic_dec(&tctx->in_idle);
>>       io_uring_remove_task_files(tctx);
>>
> 
> Hi,
> Any comments on this one? It needs to be ACK'ed by the maintainer before
> it is applied to 5.10 stable.
> 

This still triggers on 5.10.85. Anyone want to have a look?

-- 
Thanks,
Tadeusz