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Tetsuo Handa
a2fb8222cd
commit2eed77fdcbupstream. syzbot is reporting KCOV state corruption on PREEMPT_RT kernels, for the temporary storage used for saving/restoring remote KCOV state is currently allocated as the per-CPU area. On PREEMPT_RT kernels, softirq handlers run as preemptible task threads (e.g., ksoftirqd). If a softirq context preempts a task running a remote KCOV session, it safely saves the task's state into the per-CPU area. However, if that softirq thread is subsequently preempted by a higher- priority softirq thread on the same CPU, the second softirq will overwrite the same per-CPU area, permanently destroying the original task's KCOV state. Fix this data corruption by moving the temporary storage from the per-CPU area to the per-thread area. Since each softirq thread now owns its own task context, nested softirq preemption no longer causes data overwrites. Note that while the temporary storage is now on a per-thread basis, the per-CPU kcov_percpu_data.lock must be retained, for we need to ensure that kcov_remote_start() and kcov_remote_stop() operate atomically without racing against asynchronous interrupts that manipulate the current task's KCOV state. It is likely that GFP_KERNEL allocation by vmalloc_node() in kcov_init() has already called panic() before returning NULL, for there will be no OOM-killable userspace processes when __init function of built-in module runs. But this patch also fixes crashing the kernel when vmalloc_node() in kcov_init() returned NULL, for kcov_init() left per-CPU irq_area == NULL but kcov_remote_start() depends on per-CPU irq_area != NULL, resulting in (1) doing vmalloc() in kcov_remote_start() despite !in_task() context (2) out-of-array-bounds access if (1) succeeded but kcov->remote_size < CONFIG_KCOV_IRQ_AREA_SIZE (3) always leak memory allocated by (1), eventually killing all OOM-killable userspace processes problems. Link: https://lore.kernel.org/43552d09-2ce2-4b19-b0d3-a2d1ab952145@I-love.SAKURA.ne.jp Reported-by: syzbot+3f51ad7ac3ae57a6fdcc@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=3f51ad7ac3ae57a6fdcc Reported-by: syzbot+47cf95ca1f9dcca872c8@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=47cf95ca1f9dcca872c8 Reported-by: syzbot+8a173e13208949931dc7@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=8a173e13208949931dc7 Reported-by: syzbot+90984d3713722683112e@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=90984d3713722683112e Analyzed-by: AI Mode in Google Search (no mail address) Fixes:5ff3b30ab5("kcov: collect coverage from interrupts") Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> Reviewed-by: Alexander Potapenko <glider@google.com> Cc: Alan Stern <stern@rowland.harvard.edu> Cc: Andrey Konovalov <andreyknvl@gmail.com> Cc: Christoph Hellwig <hch@infradead.org> Cc: Clark Williams <williams@redhat.com> Cc: Dmitry Vyukov <dvyukov@google.com> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Cc: Marco Elver <elver@google.com> Cc: Mark Brown <broonie@kernel.org> Cc: Roman Gushchin <roman.gushchin@linux.dev> Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de> Cc: <stable@vger.kernel.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>