[Kernel] Re: [PATCH OpenHarmony-5.10 03/18] fget: check that the fd still exists after getting a ref to it

在 2022/1/24 9:03, yiyuchangchun@126.com 写道:

From: Linus Torvalds

stable inclusion from stable-5.10.84 commit 4baba6ba56eb91a735a027f783cc4b9276b48d5b category: bugfix issue: #I4RVJ4 CVE: CVE-2021-4083

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?h=linux-5.10.y&id=4baba6ba56eb91a735a027f783cc4b9276b48d5b

Signed-off-by: Yu Changchun -------------------------------------------------

commit 054aa8d439b9185d4f5eb9a90282d1ce74772969 upstream.

Jann Horn points out that there is another possible race wrt Unix domain socket garbage collection, somewhat reminiscent of the one fixed in commit cbcf01128d0a ("af_unix: fix garbage collect vs MSG_PEEK").

See the extended comment about the garbage collection requirements added to unix_peek_fds() by that commit for details.

The race comes from how we can locklessly look up a file descriptor just as it is in the process of being closed, and with the right artificial timing (Jann added a few strategic 'mdelay(500)' calls to do that), the Unix domain socket garbage collector could see the reference count decrement of the close() happen before fget() took its reference to the file and the file was attached onto a new file descriptor.

This is all (intentionally) correct on the 'struct file *' side, with RCU lookups and lockless reference counting very much part of the design. Getting that reference count out of order isn't a problem per se.

But the garbage collector can get confused by seeing this situation of having seen a file not having any remaining external references and then seeing it being attached to an fd.

In commit cbcf01128d0a ("af_unix: fix garbage collect vs MSG_PEEK") the fix was to serialize the file descriptor install with the garbage collector by taking and releasing the unix_gc_lock.

That's not really an option here, but since this all happens when we are in the process of looking up a file descriptor, we can instead simply just re-check that the file hasn't been closed in the meantime, and just re-do the lookup if we raced with a concurrent close() of the same file descriptor.

Reported-and-tested-by: Jann Horn Acked-by: Miklos Szeredi Signed-off-by: Linus Torvalds Signed-off-by: Greg Kroah-Hartman Signed-off-by: Baokun Li Signed-off-by: Zheng Zengkai Signed-off-by: Yu Changchun --- fs/file.c | 4 ++++ 1 file changed, 4 insertions(+)

Reviewed-by: Wei Yongjun