Re: general protection fault in __do_sys_io_uring_register

[Pavel Begunkov <asml.silence@gmail.com>]

On 14/10/2020 14:35, Jens Axboe wrote:
> On 10/14/20 6:01 AM, syzbot wrote:
>> Hello,
>>
>> syzbot found the following issue on:
>>
>> HEAD commit:    029f56db Merge tag 'x86_asm_for_v5.10' of git://git.kernel..
>> git tree:       upstream
>> console output: https://syzkaller.appspot.com/x/log.txt?x=13b5c678500000
>> kernel config:  https://syzkaller.appspot.com/x/.config?x=d4ce0764b8e2dd3f
>> dashboard link: https://syzkaller.appspot.com/bug?extid=f4ebcc98223dafd8991e
>> compiler:       gcc (GCC) 10.1.0-syz 20200507
>>
>> Unfortunately, I don't have any reproducer for this issue yet.
>>
>> IMPORTANT: if you fix the issue, please add the following tag to the commit:
>> Reported-by: syzbot+f4ebcc98223dafd8991e@syzkaller.appspotmail.com
>>
>> general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] PREEMPT SMP KASAN
>> KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
>> CPU: 1 PID: 8927 Comm: syz-executor.3 Not tainted 5.9.0-syzkaller #0
>> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
>> RIP: 0010:io_file_from_index fs/io_uring.c:5963 [inline]
>> RIP: 0010:io_sqe_files_register fs/io_uring.c:7369 [inline]
>> RIP: 0010:__io_uring_register fs/io_uring.c:9463 [inline]
>> RIP: 0010:__do_sys_io_uring_register+0x2fd2/0x3ee0 fs/io_uring.c:9553
>> Code: ec 03 49 c1 ee 03 49 01 ec 49 01 ee e8 57 61 9c ff 41 80 3c 24 00 0f 85 9b 09 00 00 4d 8b af b8 01 00 00 4c 89 e8 48 c1 e8 03 <80> 3c 28 00 0f 85 76 09 00 00 49 8b 55 00 89 d8 c1 f8 09 48 98 4c
>> RSP: 0018:ffffc90009137d68 EFLAGS: 00010246
>> RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffffc9000ef2a000
>> RDX: 0000000000040000 RSI: ffffffff81d81dd9 RDI: 0000000000000005
>> RBP: dffffc0000000000 R08: 0000000000000001 R09: 0000000000000000
>> R10: 0000000000000000 R11: 0000000000000000 R12: ffffed1012882a37
>> R13: 0000000000000000 R14: ffffed1012882a38 R15: ffff888094415000
>> FS:  00007f4266f3c700(0000) GS:ffff8880ae500000(0000) knlGS:0000000000000000
>> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> CR2: 000000000118c000 CR3: 000000008e57d000 CR4: 00000000001506e0
>> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>> Call Trace:
>>  do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
>>  entry_SYSCALL_64_after_hwframe+0x44/0xa9
>> RIP: 0033:0x45de59
>> Code: 0d b4 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 db b3 fb ff c3 66 2e 0f 1f 84 00 00 00 00
>> RSP: 002b:00007f4266f3bc78 EFLAGS: 00000246 ORIG_RAX: 00000000000001ab
>> RAX: ffffffffffffffda RBX: 00000000000083c0 RCX: 000000000045de59
>> RDX: 0000000020000280 RSI: 0000000000000002 RDI: 0000000000000005
>> RBP: 000000000118bf68 R08: 0000000000000000 R09: 0000000000000000
>> R10: 40000000000000a1 R11: 0000000000000246 R12: 000000000118bf2c
>> R13: 00007fff2fa4f12f R14: 00007f4266f3c9c0 R15: 000000000118bf2c
>> Modules linked in:
>> ---[ end trace 2a40a195e2d5e6e6 ]---
>> RIP: 0010:io_file_from_index fs/io_uring.c:5963 [inline]
>> RIP: 0010:io_sqe_files_register fs/io_uring.c:7369 [inline]
>> RIP: 0010:__io_uring_register fs/io_uring.c:9463 [inline]
>> RIP: 0010:__do_sys_io_uring_register+0x2fd2/0x3ee0 fs/io_uring.c:9553
>> Code: ec 03 49 c1 ee 03 49 01 ec 49 01 ee e8 57 61 9c ff 41 80 3c 24 00 0f 85 9b 09 00 00 4d 8b af b8 01 00 00 4c 89 e8 48 c1 e8 03 <80> 3c 28 00 0f 85 76 09 00 00 49 8b 55 00 89 d8 c1 f8 09 48 98 4c
>> RSP: 0018:ffffc90009137d68 EFLAGS: 00010246
>> RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffffc9000ef2a000
>> RDX: 0000000000040000 RSI: ffffffff81d81dd9 RDI: 0000000000000005
>> RBP: dffffc0000000000 R08: 0000000000000001 R09: 0000000000000000
>> R10: 0000000000000000 R11: 0000000000000000 R12: ffffed1012882a37
>> R13: 0000000000000000 R14: ffffed1012882a38 R15: ffff888094415000
>> FS:  00007f4266f3c700(0000) GS:ffff8880ae400000(0000) knlGS:0000000000000000
>> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> CR2: 000000000074a918 CR3: 000000008e57d000 CR4: 00000000001506f0
>> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> 
> I think this should fix it.

("io_uring: clean file_data access in files_register") is reported
already a second time, that's a really unfortunate commit...

The fix looks good. Though, in the future I'd prefer to remove the
ctx dependency there, e.g. __io_file_from_index(file_table).

> 
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index fc6de6b4784e..528eced8ee1f 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -7311,6 +7311,7 @@ static int io_sqe_files_register(struct io_ring_ctx *ctx, void __user *arg,
>  
>  	if (io_sqe_alloc_file_tables(file_data, nr_tables, nr_args))
>  		goto out_ref;
> +	ctx->file_data = file_data;
>  
>  	for (i = 0; i < nr_args; i++, ctx->nr_user_files++) {
>  		struct fixed_file_table *table;
> @@ -7345,7 +7346,6 @@ static int io_sqe_files_register(struct io_ring_ctx *ctx, void __user *arg,
>  		table->files[index] = file;
>  	}
>  
> -	ctx->file_data = file_data;
>  	ret = io_sqe_files_scm(ctx);
>  	if (ret) {
>  		io_sqe_files_unregister(ctx);
> @@ -7378,6 +7378,7 @@ static int io_sqe_files_register(struct io_ring_ctx *ctx, void __user *arg,
>  out_free:
>  	kfree(file_data->table);
>  	kfree(file_data);
> +	ctx->file_data = NULL;
>  	return ret;
>  }
>  
> 

-- 
Pavel Begunkov