| Message ID | YPeZeEP3oSEUHscl@aegistudio (mailing list archive) |
|---|---|
| State | Superseded |
| Headers | show |
| Series | [v2,1/1] tracing: fix bug in rb_per_cpu_empty() that might cause deadloop. | expand |
On Wed, 21 Jul 2021 03:50:16 +0000 Haoran Luo <www@aegistudio.net> wrote: > + /* > + * If writers are committing on the head page, we just need > + * to care about there're committed data, and the reader will > + * swap reader page with head page when it is to read data. > + */ > + if (commit == head) > + return rb_page_commit(commit) == 0; > + > + /* > + * Writers are committing on page other than reader page and > + * head page, there should always be content to read. > + */ > + return false; > } > I like the patch, but could you swap the above two operations. I find that return conditional in the if condition rather ugly. That is, can you instead have: /* * If writers are committing on a page other than the reader * page and head page, there should always be content to read. */ if (commit != head) return false; /* * Writers are committing on the head page, we just need * to care about there're committed data, and the reader will * swap reader page with head page when it is to read data. */ return rb_page_commit(commit) == 0; -- Steve
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c index d1463eac11a3..fb9129029a32 100644 --- a/kernel/trace/ring_buffer.c +++ b/kernel/trace/ring_buffer.c @@ -3880,10 +3880,30 @@ static bool rb_per_cpu_empty(struct ring_buffer_per_cpu *cpu_buffer) if (unlikely(!head)) return true; - return reader->read == rb_page_commit(reader) && - (commit == reader || - (commit == head && - head->read == rb_page_commit(commit))); + /* Reader should exhaust content in reader page */ + if (reader->read != rb_page_commit(reader)) + return false; + + /* + * If writers are committing on the reader page, knowing all + * committed content has been read, the ring buffer is empty. + */ + if (commit == reader) + return true; + + /* + * If writers are committing on the head page, we just need + * to care about there're committed data, and the reader will + * swap reader page with head page when it is to read data. + */ + if (commit == head) + return rb_page_commit(commit) == 0; + + /* + * Writers are committing on page other than reader page and + * head page, there should always be content to read. + */ + return false; } /**
The "rb_per_cpu_empty()" misinterpret the condition (as not-empty) when "head_page" and "commit_page" of "struct ring_buffer_per_cpu" points to the same buffer page, whose "buffer_data_page" is empty and "read" field is non-zero. An error scenario could be constructed as followed (kernel perspective): 1. All pages in the buffer has been accessed by reader(s) so that all of them will have non-zero "read" field. 2. Read and clear all buffer pages so that "rb_num_of_entries()" will return 0 rendering there's no more data to read. It is also required that the "read_page", "commit_page" and "tail_page" points to the same page, while "head_page" is the next page of them. 3. Invoke "ring_buffer_lock_reserve()" with large enough "length" so that it shot pass the end of current tail buffer page. Now the "head_page", "commit_page" and "tail_page" points to the same page. 4. Discard current event with "ring_buffer_discard_commit()", so that "head_page", "commit_page" and "tail_page" points to a page whose buffer data page is now empty. When the error scenario has been constructed, "tracing_read_pipe" will be trapped inside a deadloop: "trace_empty()" returns 0 since "rb_per_cpu_empty()" returns 0 when it hits the CPU containing such constructed ring buffer. Then "trace_find_next_entry_inc()" always return NULL since "rb_num_of_entries()" reports there's no more entry to read. Finally "trace_seq_to_user()" returns "-EBUSY" spanking "tracing_read_pipe" back to the start of the "waitagain" loop. I've also written a proof-of-concept script to construct the scenario and trigger the bug automatically, you can use it to trace and validate my reasoning above: https://github.com/aegistudio/RingBufferDetonator.git Tests has been carried out on linux kernel 5.14-rc2 (2734d6c1b1a089fb593ef6a23d4b70903526fe0c), my fixed version of kernel (for testing whether my update fixes the bug) and some older kernels (for range of affected kernels). Test result is also attached to the proof-of-concept repository. Signed-off-by: Haoran Luo <www@aegistudio.net> --- kernel/trace/ring_buffer.c | 28 ++++++++++++++++++++++++---- 1 file changed, 24 insertions(+), 4 deletions(-)