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Haoran Luo 67f0d6d988 tracing: Fix bug in rb_per_cpu_empty() that might cause deadloop.
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
(2734d6c1b1), 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.

Link: https://lore.kernel.org/linux-trace-devel/YPaNxsIlb2yjSi5Y@aegistudio/
Link: https://lore.kernel.org/linux-trace-devel/YPgrN85WL9VyrZ55@aegistudio

Cc: stable@vger.kernel.org
Fixes: bf41a158ca ("ring-buffer: make reentrant")
Suggested-by: Linus Torvalds <torvalds@linuxfoundation.org>
Signed-off-by: Haoran Luo <www@aegistudio.net>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
2021-07-22 11:52:33 -04:00
arch ARM: SoC fixes for v5.14 2021-07-17 15:58:24 -07:00
block block-5.14-2021-07-08 2021-07-09 12:05:33 -07:00
certs Kbuild updates for v5.13 (2nd) 2021-05-08 10:00:11 -07:00
crypto Merge branch 'linus' of git://git.kernel.org/pub/scm/linux/kernel/git/herbert/crypto-2.6 2021-07-09 11:00:44 -07:00
Documentation Kbuild fixes for v5.14 2021-07-18 11:10:30 -07:00
drivers ARM: SoC fixes for v5.14 2021-07-17 15:58:24 -07:00
fs Fixes for 5.14-rc: 2021-07-18 11:27:25 -07:00
include ARM: SoC fixes for v5.14 2021-07-17 15:58:24 -07:00
init Revert "mm/slub: use stackdepot to save stack trace in objects" 2021-07-17 13:27:00 -07:00
ipc Merge branch 'akpm' (patches from Andrew) 2021-07-02 12:08:10 -07:00
kernel tracing: Fix bug in rb_per_cpu_empty() that might cause deadloop. 2021-07-22 11:52:33 -04:00
lib lib/test_hmm: remove set but unused page variable 2021-07-15 10:13:49 -07:00
LICENSES LICENSES/dual/CC-BY-4.0: Git rid of "smart quotes" 2021-07-15 06:31:24 -06:00
mm Revert "mm/slub: use stackdepot to save stack trace in objects" 2021-07-17 13:27:00 -07:00
net Networking fixes for 5.14-rc2, including fixes from bpf and netfilter. 2021-07-14 09:24:32 -07:00
samples Networking fixes for 5.14-rc2, including fixes from bpf and netfilter. 2021-07-14 09:24:32 -07:00
scripts Kbuild fixes for v5.14 2021-07-18 11:10:30 -07:00
security asm-generic/unaligned: Unify asm/unaligned.h around struct helper 2021-07-02 12:43:40 -07:00
sound ASoC: Mediatek: MT8183: Fix fall-through warning for Clang 2021-07-13 14:58:18 -05:00
tools perf tools fixes for v5.14: 1st batch 2021-07-18 12:20:27 -07:00
usr .gitignore: prefix local generated files with a slash 2021-05-02 00:43:35 +09:00
virt * Allow again loading KVM on 32-bit non-PAE builds 2021-07-15 11:56:07 -07:00
.clang-format clang-format: Update with the latest for_each macro list 2021-05-12 23:32:39 +02:00
.cocciconfig
.get_maintainer.ignore Opt out of scripts/get_maintainer.pl 2019-05-16 10:53:40 -07:00
.gitattributes .gitattributes: use 'dts' diff driver for dts files 2019-12-04 19:44:11 -08:00
.gitignore .gitignore: ignore only top-level modules.builtin 2021-05-02 00:43:35 +09:00
.mailmap m68k updates for v5.14 2021-06-28 14:01:03 -07:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS MAINTAINERS: move Murali Karicheri to credits 2021-04-29 15:47:30 -07:00
Kbuild kbuild: rename hostprogs-y/always to hostprogs/always-y 2020-02-04 01:53:07 +09:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS IOMMU Fixes for Linux v5.14-rc1 2021-07-15 11:50:15 -07:00
Makefile Linux 5.14-rc2 2021-07-18 14:13:49 -07:00
README Drop all 00-INDEX files from Documentation/ 2018-09-09 15:08:58 -06:00

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.