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Sun, 10 Nov 2019 20:09:43 -0800 (PST) Subject: Re: [PATCH] io_uring: fix error clear of ->file_table in io_sqe_files_register() To: Jackie Liu Cc: Bob Liu , io-uring@vger.kernel.org References: <9851837d-47f3-abfe-8c19-f518e0935b22@kernel.dk> From: Jens Axboe Message-ID: <834e83ff-e03c-24a0-0f50-1995d944056a@kernel.dk> Date: Sun, 10 Nov 2019 20:09:42 -0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.9.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: io-uring-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org On 11/10/19 9:02 PM, Jackie Liu wrote: > > >> 2019年11月11日 11:54，Jens Axboe 写道： >> >> On 11/10/19 4:44 PM, Bob Liu wrote: >>> On 11/10/19 11:46 PM, Jens Axboe wrote: >>>> syzbot reports that when using failslab and friends, we can get a double >>>> free in io_sqe_files_unregister(): >>>> >>>> BUG: KASAN: double-free or invalid-free in >>>> io_sqe_files_unregister+0x20b/0x300 fs/io_uring.c:3185 >>>> >>>> CPU: 1 PID: 8819 Comm: syz-executor452 Not tainted 5.4.0-rc6-next-20191108 >>>> 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+0x197/0x210 lib/dump_stack.c:118 >>>> print_address_description.constprop.0.cold+0xd4/0x30b mm/kasan/report.c:374 >>>> kasan_report_invalid_free+0x65/0xa0 mm/kasan/report.c:468 >>>> __kasan_slab_free+0x13a/0x150 mm/kasan/common.c:450 >>>> kasan_slab_free+0xe/0x10 mm/kasan/common.c:480 >>>> __cache_free mm/slab.c:3426 [inline] >>>> kfree+0x10a/0x2c0 mm/slab.c:3757 >>>> io_sqe_files_unregister+0x20b/0x300 fs/io_uring.c:3185 >>>> io_ring_ctx_free fs/io_uring.c:3998 [inline] >>>> io_ring_ctx_wait_and_kill+0x348/0x700 fs/io_uring.c:4060 >>>> io_uring_release+0x42/0x50 fs/io_uring.c:4068 >>>> __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 >>>> exit_task_work include/linux/task_work.h:22 [inline] >>>> do_exit+0x904/0x2e60 kernel/exit.c:817 >>>> do_group_exit+0x135/0x360 kernel/exit.c:921 >>>> __do_sys_exit_group kernel/exit.c:932 [inline] >>>> __se_sys_exit_group kernel/exit.c:930 [inline] >>>> __x64_sys_exit_group+0x44/0x50 kernel/exit.c:930 >>>> do_syscall_64+0xfa/0x760 arch/x86/entry/common.c:290 >>>> entry_SYSCALL_64_after_hwframe+0x49/0xbe >>>> RIP: 0033:0x43f2c8 >>>> Code: 31 b8 c5 f7 ff ff 48 8b 5c 24 28 48 8b 6c 24 30 4c 8b 64 24 38 4c 8b >>>> 6c 24 40 4c 8b 74 24 48 4c 8b 7c 24 50 48 83 c4 58 c3 66 <0f> 1f 84 00 00 >>>> 00 00 00 48 8d 35 59 ca 00 00 0f b6 d2 48 89 fb 48 >>>> RSP: 002b:00007ffd5b976008 EFLAGS: 00000246 ORIG_RAX: 00000000000000e7 >>>> RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 000000000043f2c8 >>>> RDX: 0000000000000000 RSI: 000000000000003c RDI: 0000000000000000 >>>> RBP: 00000000004bf0a8 R08: 00000000000000e7 R09: ffffffffffffffd0 >>>> R10: 0000000000000001 R11: 0000000000000246 R12: 0000000000000001 >>>> R13: 00000000006d1180 R14: 0000000000000000 R15: 0000000000000000 >>>> >>>> This happens if we fail allocating the file tables. For that case we do >>>> free the file table correctly, but we forget to set it to NULL. This >>>> means that ring teardown will see it as being non-NULL, and attempt to >>>> free it again. >>>> >>>> Fix this by clearing the file_table pointer if we free the table. >>>> >>>> Reported-by: syzbot+3254bc44113ae1e331ee@syzkaller.appspotmail.com >>>> Fixes: 65e19f54d29c ("io_uring: support for larger fixed file sets") >>>> Signed-off-by: Jens Axboe >>>> >>> >>> Reviewed-by: Bob Liu >> >> Thanks, added. >> >>> By the way, there are many place(besides io_uring.c) which need to set >>> pointer to NULL after free. I saw similar fix from time to time. >>> >>> Do you think a safe_free() is worth? e.g >>> #define SAFE_FREE(p) { if (p) { free(p); (p)=NULL; } } >> >> Hmm not sure, and would probably be better as: >> >> kfree_safe(&ptr); >> >> or something instead. I seem to recall discussions about that ages ago, >> probably worth while to try and search and see if you can find those. I >> suspect Linus hates it, reasons not remembered ;-) >> > > I think this may be a worthwhile solution, but kfree can handle NULL, we can > set it to NULL directly after free is finished. > > void kfree_safe(const void *ptr) > { > kfree(ptr); > ptr = NULL; > } Sure, but that doesn't change the ptr in the caller, which was my point. You need to pass in a pointer to the pointer for that, otherwise clearing it in kfree_safe() is pointless: void kfree_safe(const void **ptr) { kfree(*ptr); *ptr = NULL; } and then you run into all sorts of fun, since void ** isn't the same as 'struct foo **'. -- Jens Axboe