ring-buffer: Fix deadloop issue on reading trace_pipe

Overview

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().

Affected

• Linux >= a5fb833172eca69136e9ee1ada778e404086ab8a < 0a29dae5786d263016a9aceb1e56bf3fd4cc6fa0
• Linux >= a5fb833172eca69136e9ee1ada778e404086ab8a < a55e8a3596048c2f7b574049aeb1885b5abba1cc
• Linux >= a5fb833172eca69136e9ee1ada778e404086ab8a < e84829522fc72bb43556b31575731de0440ac0dd
• Linux >= a5fb833172eca69136e9ee1ada778e404086ab8a < 5e68f1f3a20fe9b6bde018e353269fbfa289609c
• Linux >= a5fb833172eca69136e9ee1ada778e404086ab8a < bb14a93bccc92766b1d9302c6bcbea17d4bce306
• Linux >= a5fb833172eca69136e9ee1ada778e404086ab8a < 8b0b63fdac6b70a45614e7d4b30e5bbb93deb007
• Linux >= a5fb833172eca69136e9ee1ada778e404086ab8a < 27bdd93e44cc28dd9b94893fae146b83d4f5b31e
• Linux >= a5fb833172eca69136e9ee1ada778e404086ab8a < 7e42907f3a7b4ce3a2d1757f6d78336984daf8f5
• Linux 3.6

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.