From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 636D73370E4 for ; Wed, 15 Jul 2026 20:16:07 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784146568; cv=none; b=oDKnobg5Pkg1ht3vCY7x14IgrW2nCos+E+I/NK0Wh9HppR4Ygg3QS2Fmu2qS/WDLNipOFJOH24JLP4OlR8ZyLTSC1eRXWQqMk2fKQGkdczQTIP68u0C/AVkHzZuZomD1lEQwILXcvHWJleObhXwDs34oq2wbH5nJ9pz8i4gRt5M= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784146568; c=relaxed/simple; bh=jdxuCCD4Bk1qymC4XwWErZVgBlJ+43IGalblXkk9Ekc=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=gzwPGM/3jeIQ1Qllr13Y9cK50xlJn3AqTu/GRFN/bUwZ2hFOofFY/KQTcLTsEFI9cz21V9a22gimihS09sz/9FdXX1hjHZqPQQjuLDP72AcPtBk9CHzw6CYy4qUCkoQh7tniLWv2S5SCfx2wyERqRNewwL3Aiib7zZXGgVByGS8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=hKWCcoj7; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="hKWCcoj7" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 2AC801F00A3D; Wed, 15 Jul 2026 20:16:06 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1784146567; bh=Q/zpUPMgM/yIL2RAy8a+UWHqCrVs8X+9CpmOS/v7ans=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=hKWCcoj7CwCqfY1pL3B2aRdvRYErqqqtsJXQ0ZD93pwQiNNgFE2BkN3qGMoVT69SI IgKRWHfVAnYrVhByBWDlP15oKDRB0LX1f4zBLPMg+BxoZCr5vGwL+5W/lYhlGMlhMa rxPd9uP9OmWKfkMvwR5FqsOJ/Z/zkFoDEbQfL6UXIEZh+AIZjew4V0EWshqe/FFIRa mKXGXD/yHtmM9PHzS4xmkzVj8LQCZ3cnHUv4Tb1X2h7VLpxr6aH6vfH/4Dl/tnFF7Y XxF2UFjfCVquDDjeceyedh0g9CS7e00JXXN9+VgiLc0BdyPTk/wEE7LEjqcn4fWGbp XotaKhn8SXctg== From: Sasha Levin To: stable@vger.kernel.org Cc: Qi Zheng , Muchun Song , Dave Chinner , Roman Gushchin , Andrew Morton , Sasha Levin Subject: [PATCH 6.1.y 2/2] mm: shrinker: fix NULL pointer dereference in debugfs Date: Wed, 15 Jul 2026 16:16:03 -0400 Message-ID: <20260715201603.63084-2-sashal@kernel.org> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20260715201603.63084-1-sashal@kernel.org> References: <2026071340-handed-risk-625c@gregkh> <20260715201603.63084-1-sashal@kernel.org> Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: Qi Zheng [ Upstream commit e30453c61e185e914fde83c650e268067b140218 ] 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. Link: https://lore.kernel.org/20260617090052.27325-1-qi.zheng@linux.dev Fixes: bbf535fd6f06 ("mm: shrinkers: add scan interface for shrinker debugfs") Signed-off-by: Qi Zheng Reviewed-by: Muchun Song Cc: Dave Chinner Cc: Qi Zheng Cc: Roman Gushchin Cc: Signed-off-by: Andrew Morton Signed-off-by: Sasha Levin --- 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 6133c8d00f116b..9f3fabe1bf30cd 100644 --- a/mm/shrinker_debug.c +++ b/mm/shrinker_debug.c @@ -198,10 +198,12 @@ 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; } -- 2.53.0