From patchwork Fri Jan 21 14:26:16 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Thadeu Lima de Souza Cascardo X-Patchwork-Id: 1582604 Return-Path: X-Original-To: incoming@patchwork.ozlabs.org Delivered-To: patchwork-incoming@bilbo.ozlabs.org Authentication-Results: bilbo.ozlabs.org; dkim=fail reason="signature verification failed" (2048-bit key; unprotected) header.d=canonical.com header.i=@canonical.com header.a=rsa-sha256 header.s=20210705 header.b=qYQ102Na; dkim-atps=neutral Authentication-Results: ozlabs.org; spf=none (no SPF record) smtp.mailfrom=lists.ubuntu.com (client-ip=91.189.94.19; helo=huckleberry.canonical.com; envelope-from=kernel-team-bounces@lists.ubuntu.com; receiver=) Received: from huckleberry.canonical.com (huckleberry.canonical.com [91.189.94.19]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by bilbo.ozlabs.org (Postfix) with ESMTPS id 4JgMC15bWPz9t2p for ; Sat, 22 Jan 2022 01:26:53 +1100 (AEDT) Received: from localhost ([127.0.0.1] helo=huckleberry.canonical.com) by huckleberry.canonical.com with esmtp (Exim 4.86_2) (envelope-from ) id 1nAusL-00057J-DF; Fri, 21 Jan 2022 14:26:49 +0000 Received: from smtp-relay-canonical-1.internal ([10.131.114.174] helo=smtp-relay-canonical-1.canonical.com) by huckleberry.canonical.com with esmtps (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.86_2) (envelope-from ) id 1nAusJ-00053u-18 for kernel-team@lists.ubuntu.com; Fri, 21 Jan 2022 14:26:47 +0000 Received: from localhost.localdomain (unknown [179.93.158.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by smtp-relay-canonical-1.canonical.com (Postfix) with ESMTPSA id E11663FD0D for ; Fri, 21 Jan 2022 14:26:45 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=canonical.com; s=20210705; t=1642775206; bh=6vUEl37RNMNeyZJgzOl/tSJNN3M2uAZtn9BmstjOD80=; h=From:To:Subject:Date:Message-Id:In-Reply-To:References: MIME-Version; b=qYQ102NaPdsvm2uAsjrTvXTVxOwvixSZmlvpBhDoXmQ1n4/+CqHpjOKLkttrufJlg wKcLeRaQLdqWQWYWSLUFE0GwYuxFSlXbSwshXVohYqQFEgD8m5wMHy3e2qkf/ck/L6 7H/1enJHabgGEGrCQ/WWaL6zLoiyOZYn34uwG02ZyNI6HqqJPaRmfZbe9xr/NngXU+ x1fcO7pgsxu0rGyNsEKp9EaFOYeqQuLDtRm7L18hO+ak1rIyXP9oW9olYE0GFSFLYe kLr4zpvMii+FgSZ24mUbETiAMRnPbRlPiKGQUOyaMuzK+qm5TlSw6xvABN2fnnCzKh +g2nTY7siGeJA== From: Thadeu Lima de Souza Cascardo To: kernel-team@lists.ubuntu.com Subject: [SRU Impish, Hirsute] fget: check that the fd still exists after getting a ref to it Date: Fri, 21 Jan 2022 11:26:16 -0300 Message-Id: <20220121142616.163592-4-cascardo@canonical.com> X-Mailer: git-send-email 2.32.0 In-Reply-To: <20220121142616.163592-1-cascardo@canonical.com> References: <20220121142616.163592-1-cascardo@canonical.com> MIME-Version: 1.0 X-BeenThere: kernel-team@lists.ubuntu.com X-Mailman-Version: 2.1.20 Precedence: list List-Id: Kernel team discussions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: kernel-team-bounces@lists.ubuntu.com Sender: "kernel-team" From: Linus Torvalds 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 (cherry picked from commit 054aa8d439b9185d4f5eb9a90282d1ce74772969) CVE-2021-4083 Signed-off-by: Thadeu Lima de Souza Cascardo --- fs/file.c | 4 ++++ 1 file changed, 4 insertions(+) diff --git a/fs/file.c b/fs/file.c index 3cc1867f289f..f18e4746523b 100644 --- a/fs/file.c +++ b/fs/file.c @@ -843,6 +843,10 @@ static struct file *__fget_files(struct files_struct *files, unsigned int fd, file = NULL; else if (!get_file_rcu_many(file, refs)) goto loop; + else if (files_lookup_fd_raw(files, fd) != file) { + fput_many(file, refs); + goto loop; + } } rcu_read_unlock();