From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 389A42EAB8E; Tue, 15 Jul 2025 13:29:02 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1752586142; cv=none; b=r2FESR0oflhMvk3GZ6EnfCjVieqZ1EXxhA9jBJULNns0a7LeAOLy49dgwBIX+AUfBVIPeib7ej/UylCcwwVfrUePlCkHmrIcAiTG/e468uXIZdgE152t/7nZ2u8q/knH14lJPZ+473RB+/CpuNRUsTpQflXqNE7bkYoDCtXynYk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1752586142; c=relaxed/simple; bh=AotPO7yYcUlRa1Cg70OYHCglqQELskYLDUUMkfx4w9Y=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=NcPcNhzD6awq5Jmb01rysU9n/61U0v2R1U/+61kun9bUtqFj3BS+PGakRPQ19uZN5+8LutPyTctW5Y8THz2B5H5CI9Mv1LVItKIoxQDhBE3psGQa12/uVrIPwZ8TJgjczg6a0FTHpgSZkjJu4bWPYU1oL9GEV12NYtH9DcOmYtI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=jqzAk05f; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="jqzAk05f" Received: by smtp.kernel.org (Postfix) with ESMTPSA id C0824C4CEF1; Tue, 15 Jul 2025 13:29:01 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1752586142; bh=AotPO7yYcUlRa1Cg70OYHCglqQELskYLDUUMkfx4w9Y=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=jqzAk05flPvgGMWTw5hDVdu6J5OCK3JiynG5uBqlgAeu2mGF/5AkrQnsStnn7iRBw S9CPs/hNQrLVfD6bdm3GBRKpiArlzUtqJ5gSSQnISMsPqqtQNX1xC3rLxDw0310zzT hBIM+1IjBllup1KkkjgFkQxgerSDapITKBpQUqW8= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, NeilBrown , Christian Brauner , NeilBrown , Al Viro , Sasha Levin Subject: [PATCH 5.15 04/77] fix proc_sys_compare() handling of in-lookup dentries Date: Tue, 15 Jul 2025 15:13:03 +0200 Message-ID: <20250715130751.859031138@linuxfoundation.org> X-Mailer: git-send-email 2.50.1 In-Reply-To: <20250715130751.668489382@linuxfoundation.org> References: <20250715130751.668489382@linuxfoundation.org> User-Agent: quilt/0.68 X-stable: review X-Patchwork-Hint: ignore Precedence: bulk X-Mailing-List: patches@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 5.15-stable review patch. If anyone has any objections, please let me know. ------------------ From: Al Viro [ Upstream commit b969f9614885c20f903e1d1f9445611daf161d6d ] There's one case where ->d_compare() can be called for an in-lookup dentry; usually that's nothing special from ->d_compare() point of view, but... proc_sys_compare() is weird. The thing is, /proc/sys subdirectories can look differently for different processes. Up to and including having the same name resolve to different dentries - all of them hashed. The way it's done is ->d_compare() refusing to admit a match unless this dentry is supposed to be visible to this caller. The information needed to discriminate between them is stored in inode; it is set during proc_sys_lookup() and until it's done d_splice_alias() we really can't tell who should that dentry be visible for. Normally there's no negative dentries in /proc/sys; we can run into a dying dentry in RCU dcache lookup, but those can be safely rejected. However, ->d_compare() is also called for in-lookup dentries, before they get positive - or hashed, for that matter. In case of match we will wait until dentry leaves in-lookup state and repeat ->d_compare() afterwards. In other words, the right behaviour is to treat the name match as sufficient for in-lookup dentries; if dentry is not for us, we'll see that when we recheck once proc_sys_lookup() is done with it. While we are at it, fix the misspelled READ_ONCE and WRITE_ONCE there. Fixes: d9171b934526 ("parallel lookups machinery, part 4 (and last)") Reported-by: NeilBrown Reviewed-by: Christian Brauner Reviewed-by: NeilBrown Signed-off-by: Al Viro Signed-off-by: Sasha Levin --- fs/proc/inode.c | 2 +- fs/proc/proc_sysctl.c | 18 +++++++++++------- 2 files changed, 12 insertions(+), 8 deletions(-) diff --git a/fs/proc/inode.c b/fs/proc/inode.c index 695471fa24fe5..de5ae2e5a97df 100644 --- a/fs/proc/inode.c +++ b/fs/proc/inode.c @@ -54,7 +54,7 @@ static void proc_evict_inode(struct inode *inode) head = ei->sysctl; if (head) { - RCU_INIT_POINTER(ei->sysctl, NULL); + WRITE_ONCE(ei->sysctl, NULL); proc_sys_evict_inode(inode, head); } } diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c index 7c5d472b193f8..1a22aa43e740d 100644 --- a/fs/proc/proc_sysctl.c +++ b/fs/proc/proc_sysctl.c @@ -910,17 +910,21 @@ static int proc_sys_compare(const struct dentry *dentry, struct ctl_table_header *head; struct inode *inode; - /* Although proc doesn't have negative dentries, rcu-walk means - * that inode here can be NULL */ - /* AV: can it, indeed? */ - inode = d_inode_rcu(dentry); - if (!inode) - return 1; if (name->len != len) return 1; if (memcmp(name->name, str, len)) return 1; - head = rcu_dereference(PROC_I(inode)->sysctl); + + // false positive is fine here - we'll recheck anyway + if (d_in_lookup(dentry)) + return 0; + + inode = d_inode_rcu(dentry); + // we just might have run into dentry in the middle of __dentry_kill() + if (!inode) + return 1; + + head = READ_ONCE(PROC_I(inode)->sysctl); return !head || !sysctl_is_seen(head); } -- 2.39.5