From: Linus Torvalds torvalds@linux-foundation.org

stable inclusion from stable-4.19.220 commit 8bf31f9d9395b71af3ed33166a057cd3ec0c59da category: bugfix bugzilla: 185907 CVE: CVE-2021-4083

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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 jannh@google.com Acked-by: Miklos Szeredi mszeredi@redhat.com Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org Signed-off-by: Baokun Li libaokun1@huawei.com Reviewed-by: Xiu Jianfeng xiujianfeng@huawei.com Reviewed-by: Zhang Yi yi.zhang@huawei.com Signed-off-by: Yang Yingliang yangyingliang@huawei.com --- fs/file.c | 4 ++++ 1 file changed, 4 insertions(+)

diff --git a/fs/file.c b/fs/file.c index 0caadedd3cc43..20a44d0ac400b 100644 --- a/fs/file.c +++ b/fs/file.c @@ -779,6 +779,10 @@ static struct file *__fget(unsigned int fd, fmode_t mask, unsigned int refs) file = NULL; else if (!get_file_rcu_many(file, refs)) goto loop; + else if (__fcheck_files(files, fd) != file) { + fput_many(file, refs); + goto loop; + } } rcu_read_unlock();