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From:   Lin Ma <linma@zju.edu.cn>
To:     axboe@kernel.dk, asml.silence@gmail.com, io-uring@vger.kernel.org,
        linux-kernel@vger.kernel.org
Cc:     Lin Ma <linma@zju.edu.cn>
Subject: [PATCH v1] io_uring/filetable: fix file reference underflow
Date:   Mon, 14 Nov 2022 22:50:40 +0800
Message-Id: <20221114145040.14365-1-linma@zju.edu.cn>
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There is an interesting reference bug when -ENOMEM occurs in calling of
io_install_fixed_file(). The tracing of this bug is shown below:

commit 8c71fe750215 ("io_uring: ensure fput() called correspondingly
when direct install fails") adds an additional fput() in
io_fixed_fd_install() when io_file_bitmap_get() returns error values. In
that case, the routine will never make it to io_install_fixed_file() due
to an early return.

static int io_fixed_fd_install(...)
{
  if (alloc_slot) {
    ...
    ret = io_file_bitmap_get(ctx);
    if (unlikely(ret < 0)) {
      io_ring_submit_unlock(ctx, issue_flags);
      fput(file);
      return ret;
    }
    ...
  }
  ...
  ret = io_install_fixed_file(req, file, issue_flags, file_slot);
  ...
}

In the above scenario, the reference is okay as io_fixed_fd_install()
ensures the fput() is called when something bad happens, either via
bitmap or via inner io_install_fixed_file().

However, the commit 61c1b44a21d7 ("io_uring: fix deadlock on iowq file
slot alloc") breaks the balance because it places fput() into the common
path for both io_file_bitmap_get() and io_install_fixed_file(). Since
io_install_fixed_file() handles the fput() itself, the reference
underflow come across then.

There are some extra commits make the current code into
io_fixed_fd_install() -> __io_fixed_fd_install() ->
io_install_fixed_file()

However, the fact that there is an extra fput() is called if
io_install_fixed_file() calls fput(). Traversing through the code, I
find that the existing two callers to __io_fixed_fd_install():
io_fixed_fd_install() and io_msg_send_fd() have fput() when handling
error return, this patch simply removes the fput() in
io_install_fixed_file() to fix the bug.

Fixes: 61c1b44a21d7 ("io_uring: fix deadlock on iowq file slot alloc")
Signed-off-by: Lin Ma <linma@zju.edu.cn>
---
 io_uring/filetable.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/io_uring/filetable.c b/io_uring/filetable.c
index 7b473259f3f4..68dfc6936aa7 100644
--- a/io_uring/filetable.c
+++ b/io_uring/filetable.c
@@ -101,8 +101,6 @@ static int io_install_fixed_file(struct io_ring_ctx *ctx, struct file *file,
 err:
 	if (needs_switch)
 		io_rsrc_node_switch(ctx, ctx->file_data);
-	if (ret)
-		fput(file);
 	return ret;
 }
 
-- 
2.38.1