kernel/linux
0
Fork 0
mirror of https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ synced 2026-11-10 20:48:12 +00:00
Code Issues Releases Packages Activity
No description
1,402,638 commits 117 branches 5,827 tags 6.3 GiB
  • C 96.8%
  • Assembly 0.9%
  • Rust 0.7%
  • Shell 0.6%
  • Python 0.5%
  • Other 0.3%
Find a file
Jiayuan Chen 7466ae2aee bpf: Fix race in cpumap on PREEMPT_RT 
[ Upstream commit 869c63d597 ]

On PREEMPT_RT kernels, the per-CPU xdp_bulk_queue (bq) can be accessed
concurrently by multiple preemptible tasks on the same CPU.

The original code assumes bq_enqueue() and __cpu_map_flush() run
atomically with respect to each other on the same CPU, relying on
local_bh_disable() to prevent preemption. However, on PREEMPT_RT,
local_bh_disable() only calls migrate_disable() (when
PREEMPT_RT_NEEDS_BH_LOCK is not set) and does not disable
preemption, which allows CFS scheduling to preempt a task during
bq_flush_to_queue(), enabling another task on the same CPU to enter
bq_enqueue() and operate on the same per-CPU bq concurrently.

This leads to several races:

1. Double __list_del_clearprev(): after bq->count is reset in
   bq_flush_to_queue(), a preempting task can call bq_enqueue() ->
   bq_flush_to_queue() on the same bq when bq->count reaches
   CPU_MAP_BULK_SIZE. Both tasks then call __list_del_clearprev()
   on the same bq->flush_node, the second call dereferences the
   prev pointer that was already set to NULL by the first.

2. bq->count and bq->q[] races: concurrent bq_enqueue() can corrupt
   the packet queue while bq_flush_to_queue() is processing it.

The race between task A (__cpu_map_flush -> bq_flush_to_queue) and
task B (bq_enqueue -> bq_flush_to_queue) on the same CPU:

  Task A (xdp_do_flush)          Task B (cpu_map_enqueue)
  ----------------------         ------------------------
  bq_flush_to_queue(bq)
    spin_lock(&q->producer_lock)
    /* flush bq->q[] to ptr_ring */
    bq->count = 0
    spin_unlock(&q->producer_lock)
                                   bq_enqueue(rcpu, xdpf)
    <-- CFS preempts Task A -->      bq->q[bq->count++] = xdpf
                                     /* ... more enqueues until full ... */
                                     bq_flush_to_queue(bq)
                                       spin_lock(&q->producer_lock)
                                       /* flush to ptr_ring */
                                       spin_unlock(&q->producer_lock)
                                       __list_del_clearprev(flush_node)
                                         /* sets flush_node.prev = NULL */
    <-- Task A resumes -->
    __list_del_clearprev(flush_node)
      flush_node.prev->next = ...
      /* prev is NULL -> kernel oops */

Fix this by adding a local_lock_t to xdp_bulk_queue and acquiring it
in bq_enqueue() and __cpu_map_flush(). These paths already run under
local_bh_disable(), so use local_lock_nested_bh() which on non-RT is
a pure annotation with no overhead, and on PREEMPT_RT provides a
per-CPU sleeping lock that serializes access to the bq.

To reproduce, insert an mdelay(100) between bq->count = 0 and
__list_del_clearprev() in bq_flush_to_queue(), then run reproducer
provided by syzkaller.

Fixes: 3253cb49cb ("softirq: Allow to drop the softirq-BKL lock on PREEMPT_RT")
Reported-by: syzbot+2b3391f44313b3983e91@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/69369331.a70a0220.38f243.009d.GAE@google.com/T/
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Jiayuan Chen <jiayuan.chen@shopee.com>
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Link: https://lore.kernel.org/r/20260225121459.183121-2-jiayuan.chen@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
History 2026-03-12 07:09:22 -04:00
arch
block
certs
crypto
Documentation
drivers
fs
include
init
io_uring
ipc
kernel
lib
LICENSES
mm
net
rust
samples
scripts
security
sound
tools
usr
virt
.clang-format
.clippy.toml
.cocciconfig
.editorconfig
.get_maintainer.ignore
.gitattributes
.gitignore
.mailmap
.pylintrc
.rustfmt.toml
COPYING
CREDITS
Kbuild
Kconfig
MAINTAINERS
Makefile
README

README 

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 reStructuredText 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.