From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-174.mta0.migadu.com (out-174.mta0.migadu.com [91.218.175.174]) (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 32BF83988F9 for ; Wed, 17 Jun 2026 09:01:50 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.174 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781686913; cv=none; b=b/1zANxZGGSGVVTVrGqj2iQ4liawCdbemUUTKMNG71sGP6RYdG3Jr2hD0vByalw2UZdG3fSeK3vNLtcbPwv3llLC9Bm+ERbeRXcYyihS341gzZ/JC7xSUcb/Ex4buA/O8faeLM6BpnEj8LCWXwuP4618lqnf+XQb+T+yG19mtrY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781686913; c=relaxed/simple; bh=goICq28k8ErnM+6DjUnV4PyQWaMNqUW8w4vTdbxMtDM=; h=From:To:Cc:Subject:Date:Message-ID:MIME-Version; b=slL5u4+morzmGyUTXs3usKoCOnkgZ7FWWRKGV3G2xrdGU55KzodrJPehqadoA4rq/GX5tX8H0cVFoe4vfXOnuHkPvQlyp9XTNK6oTx3HrTX8H2UhoYAHOiidyAQGWi1go3MvzMYuPsIMusL8ZtbQT6dJgElZsYHAL5vniQJUMj0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=ddQtveI3; arc=none smtp.client-ip=91.218.175.174 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="ddQtveI3" X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1781686909; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding; bh=QRmAg+xuFzi1RxjBmfejFM7dJeCpCksbcxaXlfltdno=; b=ddQtveI3ogFiIk9LA7O2DTIuFGZZKvzdLaBaCtEUvnRFh8Ci1w6FhDN8xO4kXzG9qEP7GO TRqM2zaVEWI5+dgIl3X0tu+AQf4OuLER72gmcQ10QQVeEIo9rcWdumdCWWk1H/CpxNVDnm PgXBuqz7INSXsWl+H/ZZ2MI/a+5X1KE= From: Qi Zheng To: akpm@linux-foundation.org, david@fromorbit.com, roman.gushchin@linux.dev, muchun.song@linux.dev Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Qi Zheng Subject: [PATCH] mm: shrinker: fix NULL pointer dereference in debugfs Date: Wed, 17 Jun 2026 17:00:52 +0800 Message-ID: <20260617090052.27325-1-qi.zheng@linux.dev> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT From: Qi Zheng The shrinker_debugfs_add() creates both "count" and "scan" debugfs files unconditionally. That assumes every shrinker implements both count_objects() and scan_objects(), which is not guaranteed. For example, the xen-backend shrinker sets count_objects() but leaves scan_objects() NULL, so writing to its scan file calls through a NULL function pointer and panics the kernel: BUG: kernel NULL pointer dereference, address: 0000000000000000 RIP: 0010:0x0 Code: Unable to access opcode bytes at 0xffffffffffffffd6. Call Trace: shrinker_debugfs_scan_write+0x12e/0x270 full_proxy_write+0x5f/0x90 vfs_write+0xde/0x420 ? filp_flush+0x75/0x90 ? filp_close+0x1d/0x30 ? do_dup2+0xb8/0x120 ksys_write+0x68/0xf0 ? filp_flush+0x75/0x90 do_syscall_64+0xb3/0x5b0 entry_SYSCALL_64_after_hwframe+0x76/0x7e The count path has the same issue in principle if a shrinker omits count_objects(). To fix it, only create "count" and "scan" debugfs files when the corresponding callbacks are present. Fixes: bbf535fd6f06 ("mm: shrinkers: add scan interface for shrinker debugfs") Signed-off-by: Qi Zheng --- mm/shrinker_debug.c | 10 ++++++---- 1 file changed, 6 insertions(+), 4 deletions(-) diff --git a/mm/shrinker_debug.c b/mm/shrinker_debug.c index cda4e86428c8..cafb56630132 100644 --- a/mm/shrinker_debug.c +++ b/mm/shrinker_debug.c @@ -183,10 +183,12 @@ int shrinker_debugfs_add(struct shrinker *shrinker) } shrinker->debugfs_entry = entry; - debugfs_create_file("count", 0440, entry, shrinker, - &shrinker_debugfs_count_fops); - debugfs_create_file("scan", 0220, entry, shrinker, - &shrinker_debugfs_scan_fops); + if (shrinker->count_objects) + debugfs_create_file("count", 0440, entry, shrinker, + &shrinker_debugfs_count_fops); + if (shrinker->scan_objects) + debugfs_create_file("scan", 0220, entry, shrinker, + &shrinker_debugfs_scan_fops); return 0; } -- 2.54.0