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Pavel Begunkov
2c32395d81
There is an unlikely but possible race using a freed context. That's
because req->task_work.func() can free a request, but we won't
necessarily find a completion in submit_state.comp and so all ctx refs
may be put by the time we do mutex_lock(&ctx->uring_ctx);
There are several reasons why it can miss going through
submit_state.comp: 1) req->task_work.func() didn't complete it itself,
but punted to iowq (e.g. reissue) and it got freed later, or a similar
situation with it overflowing and getting flushed by someone else, or
being submitted to IRQ completion, 2) As we don't hold the uring_lock,
someone else can do io_submit_flush_completions() and put our ref.
3) Bugs and code obscurities, e.g. failing to propagate issue_flags
properly.
One example is as follows
CPU1 | CPU2
=======================================================================
@req->task_work.func() |
-> @req overflwed, |
so submit_state.comp,nr==0 |
| flush overflows, and free @req
| ctx refs == 0, free it
ctx is dead, but we do |
lock + flush + unlock |
So take a ctx reference for each new ctx we see in __tctx_task_work(),
and do release it until we do all our flushing.
Fixes: 65453d1efb ("io_uring: enable req cache for task_work items")
Reported-by: syzbot+a157ac7c03a56397f553@syzkaller.appspotmail.com
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
[axboe: fold in my one-liner and fix ref mismatch]
Signed-off-by: Jens Axboe <axboe@kernel.dk>
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.