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Sun, 10 Nov 2019 19:54:57 -0800 (PST) Subject: Re: [PATCH] io_uring: fix error clear of ->file_table in io_sqe_files_register() To: Bob Liu , io-uring@vger.kernel.org References: <9851837d-47f3-abfe-8c19-f518e0935b22@kernel.dk> From: Jens Axboe Message-ID: Date: Sun, 10 Nov 2019 19:54:56 -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: 7bit Sender: io-uring-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org 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 ;-) -- Jens Axboe