ring-buffer: Fix deadloop issue on reading trace_pipe

In the Linux kernel, the following vulnerability has been resolved:

ring-buffer: Fix deadloop issue on reading trace_pipe

Soft lockup occurs when reading file 'trace_pipe':

watchdog: BUG: soft lockup - CPU#6 stuck for 22s! [cat:4488]

[...]

RIP: 0010:ring_buffer_empty_cpu+0xed/0x170

RSP: 0018:ffff88810dd6fc48 EFLAGS: 00000246

RAX: 0000000000000000 RBX: 0000000000000246 RCX: ffffffff93d1aaeb

RDX: ffff88810a280040 RSI: 0000000000000008 RDI: ffff88811164b218

RBP: ffff88811164b218 R08: 0000000000000000 R09: ffff88815156600f

R10: ffffed102a2acc01 R11: 0000000000000001 R12: 0000000051651901

R13: 0000000000000000 R14: ffff888115e49500 R15: 0000000000000000

[...]

CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033

CR2: 00007f8d853c2000 CR3: 000000010dcd8000 CR4: 00000000000006e0

DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000

DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400

Call Trace:

__find_next_entry+0x1a8/0x4b0

? peek_next_entry+0x250/0x250

? down_write+0xa5/0x120

? down_write_killable+0x130/0x130

trace_find_next_entry_inc+0x3b/0x1d0

tracing_read_pipe+0x423/0xae0

? tracing_splice_read_pipe+0xcb0/0xcb0

vfs_read+0x16b/0x490

ksys_read+0x105/0x210

? __ia32_sys_pwrite64+0x200/0x200

? switch_fpu_return+0x108/0x220

do_syscall_64+0x33/0x40

entry_SYSCALL_64_after_hwframe+0x61/0xc6

Through the vmcore, I found it's because in tracing_read_pipe(),

ring_buffer_empty_cpu() found some buffer is not empty but then it

cannot read anything due to "rb_num_of_entries() == 0" always true,

Then it infinitely loop the procedure due to user buffer not been

filled, see following code path:

tracing_read_pipe() {

... ...

waitagain:

tracing_wait_pipe() // 1. find non-empty buffer here

trace_find_next_entry_inc() // 2. loop here try to find an entry

__find_next_entry()

ring_buffer_empty_cpu(); // 3. find non-empty buffer

peek_next_entry() // 4. but peek always return NULL

ring_buffer_peek()

rb_buffer_peek()

rb_get_reader_page()

// 5. because rb_num_of_entries() == 0 always true here

// then return NULL

// 6. user buffer not been filled so goto 'waitgain'

// and eventually leads to an deadloop in kernel!!!

}

By some analyzing, I found that when resetting ringbuffer, the 'entries'

of its pages are not all cleared (see rb_reset_cpu()). Then when reducing

the ringbuffer, and if some reduced pages exist dirty 'entries' data, they

will be added into 'cpu_buffer->overrun' (see rb_remove_pages()), which

cause wrong 'overrun' count and eventually cause the deadloop issue.

To fix it, we need to clear every pages in rb_reset_cpu().