From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-ej1-f42.google.com (mail-ej1-f42.google.com [209.85.218.42]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 5496F16FF29 for ; Tue, 28 May 2024 14:45:00 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.218.42 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1716907502; cv=none; b=Cua4YXbxXHDBjmlYL0LdCMSfH0go6tONXDmpklvWdFzYWw97eB0CTWSElSVkG7vq9QCSj8645hFXY5qYc+BXeV+Nt2nbfRE95yYhSlrC2M2UBMyK+YrX+mvfMxKhrz5jby2klXOn38cmGNkHzCgOZKt70Ark5rT/DBqjOOeyFPs= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1716907502; c=relaxed/simple; bh=u5DGMExmMELV1xy8yLiiNHEKToOQqClZz2C5kQWXUNA=; h=From:To:Cc:Subject:Date:Message-Id:MIME-Version; b=cYWPhexPfYzBW1eM3NHZnjD/9vCkb/1xIzCnG9EhpeNAMkS71n2bdOQyAXY0Pcuwn38wN2rGp0D+0Esoc1+/kNzQnRgYJDN7CrawGVJ5ZunhfxMgzzUo4GBmfkDfwKYWBy16Tz4hauOh8K9lifnqrcNQIdL5P/B/oz/0HBQeWE0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=ionos.com; spf=pass smtp.mailfrom=ionos.com; dkim=pass (2048-bit key) header.d=ionos.com header.i=@ionos.com header.b=XGlqYIti; arc=none smtp.client-ip=209.85.218.42 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=ionos.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=ionos.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=ionos.com header.i=@ionos.com header.b="XGlqYIti" Received: by mail-ej1-f42.google.com with SMTP id a640c23a62f3a-a6303e13ffeso110307166b.3 for ; Tue, 28 May 2024 07:45:00 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=ionos.com; s=google; t=1716907499; x=1717512299; darn=lists.linux.dev; h=content-transfer-encoding:mime-version:message-id:date:subject:cc :to:from:from:to:cc:subject:date:message-id:reply-to; bh=TG5W+RrKzluwPnH1A1A5fDJr2fjufgKg3mZALw2M2DI=; b=XGlqYItiu2+HiXuKSOMjOD2PmnhIc0L4Ieh4A2Pysn3IHwgIbUnT9zYxjp8avkvsRx abNZBLvu6EKM0E6RbSUgawIFkBAN5jFysDPLA51uaHZ/c0I8nLFpEORu55jrHZ5nZdE2 ApbPNV0hrPnS5M4Xn+4CpXVNbbAJo84WGiGlUHWSHmNoa52ffA8NlyNC0B0knxPj38Wx wqWaCIB8W4O0WOLCNQj6SlBxEfBuhEZT5Xa8dxzp6q+JpKoXZHbPJyVIINdg97nSN932 ZgOOEMHkkzV8Dzmzh3wucmXM1dFdVqDlGLuOZXOWGOANRkZ1JtHGgjiG5OxQbs6Tcunl ZOAQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1716907499; x=1717512299; h=content-transfer-encoding:mime-version:message-id:date:subject:cc :to:from:x-gm-message-state:from:to:cc:subject:date:message-id :reply-to; bh=TG5W+RrKzluwPnH1A1A5fDJr2fjufgKg3mZALw2M2DI=; b=Y4B5u+g5fth5GmgRDDIHMbRxG4Kqvyrm0VfUYmoz7/OMP7m8Z4DfH1h5k+HHjh2lDn Ba5onFuvja5DsNrzu3tk8EmEGCW7ElYgVjmYJlcr8MnabvfaRhqLLIWoKWhtOEpYKV5p Bq7lSBYVXOC88EMB3zKZqa5Ufs9WFtABoI2t75jCNxIlGLh80mQYuirDP9SHM5GJG8Vw x1s6VmC8dYqwjdOEXfql1eYYGq21t1iG/s61ZLzBqjIAM1Q6fE68cEKB9ku49y3XM96v KYUuFBKT4s+L+HEXsd+XMoD8oKsNM5cjHaZ1+w0MC18unwKwKlzLVWG126vpPmPiQtOw rcMA== X-Forwarded-Encrypted: i=1; AJvYcCVJKPclRUOCBMtiE+H84Kb52Q6A1PQsZuXuGVQuZxzRn7JURLUH4DfMRxKgUcd94NNfmUb1wVIOKKxhsBgBzyK3jSoERDY= X-Gm-Message-State: AOJu0Yw96JWKY7pdFLkLyuy3Tf+czd6O/flizNAyRg5SMU+voKVkh8Xd KJQwvtqA7v7WIU3280ZW/bO0fYoS5x30AjdULQXUaUtHr+ndzOYqnNo6yz025jo= X-Google-Smtp-Source: AGHT+IGcfIH5RPh4OnyVqTq4ATc6SBA5sJNh2KrF2k5/BA6AVB02Dz6lBLYJ9g6apMqOwqcdLBUyzg== X-Received: by 2002:a17:906:4453:b0:a59:9b75:b84 with SMTP id a640c23a62f3a-a62646cd4bfmr828803266b.35.1716907498607; Tue, 28 May 2024 07:44:58 -0700 (PDT) Received: from raven.blarg.de (p200300dc6f06e100023064fffe740809.dip0.t-ipconnect.de. [2003:dc:6f06:e100:230:64ff:fe74:809]) by smtp.gmail.com with ESMTPSA id a640c23a62f3a-a626c817bffsm623787766b.32.2024.05.28.07.44.58 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 28 May 2024 07:44:58 -0700 (PDT) From: Max Kellermann To: dhowells@redhat.com, jlayton@kernel.org, netfs@lists.linux.dev, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org Cc: Max Kellermann , stable@vger.kernel.org Subject: [PATCH] fs/netfs/fscache_cookie: add missing "n_accesses" check Date: Tue, 28 May 2024 16:44:45 +0200 Message-Id: <20240528144445.3268304-1-max.kellermann@ionos.com> X-Mailer: git-send-email 2.39.2 Precedence: bulk X-Mailing-List: netfs@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit This fixes a NULL pointer dereference bug due to a data race which looks like this: BUG: kernel NULL pointer dereference, address: 0000000000000008 #PF: supervisor read access in kernel mode #PF: error_code(0x0000) - not-present page PGD 0 P4D 0 Oops: 0000 [#1] SMP PTI CPU: 33 PID: 16573 Comm: kworker/u97:799 Not tainted 6.8.7-cm4all1-hp+ #43 Hardware name: HP ProLiant DL380 Gen9/ProLiant DL380 Gen9, BIOS P89 10/17/2018 Workqueue: events_unbound netfs_rreq_write_to_cache_work RIP: 0010:cachefiles_prepare_write+0x30/0xa0 Code: 57 41 56 45 89 ce 41 55 49 89 cd 41 54 49 89 d4 55 53 48 89 fb 48 83 ec 08 48 8b 47 08 48 83 7f 10 00 48 89 34 24 48 8b 68 20 <48> 8b 45 08 4c 8b 38 74 45 49 8b 7f 50 e8 4e a9 b0 ff 48 8b 73 10 RSP: 0018:ffffb4e78113bde0 EFLAGS: 00010286 RAX: ffff976126be6d10 RBX: ffff97615cdb8438 RCX: 0000000000020000 RDX: ffff97605e6c4c68 RSI: ffff97605e6c4c60 RDI: ffff97615cdb8438 RBP: 0000000000000000 R08: 0000000000278333 R09: 0000000000000001 R10: ffff97605e6c4600 R11: 0000000000000001 R12: ffff97605e6c4c68 R13: 0000000000020000 R14: 0000000000000001 R15: ffff976064fe2c00 FS: 0000000000000000(0000) GS:ffff9776dfd40000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 0000000000000008 CR3: 000000005942c002 CR4: 00000000001706f0 Call Trace: ? __die+0x1f/0x70 ? page_fault_oops+0x15d/0x440 ? search_module_extables+0xe/0x40 ? fixup_exception+0x22/0x2f0 ? exc_page_fault+0x5f/0x100 ? asm_exc_page_fault+0x22/0x30 ? cachefiles_prepare_write+0x30/0xa0 netfs_rreq_write_to_cache_work+0x135/0x2e0 process_one_work+0x137/0x2c0 worker_thread+0x2e9/0x400 ? __pfx_worker_thread+0x10/0x10 kthread+0xcc/0x100 ? __pfx_kthread+0x10/0x10 ret_from_fork+0x30/0x50 ? __pfx_kthread+0x10/0x10 ret_from_fork_asm+0x1b/0x30 Modules linked in: CR2: 0000000000000008 ---[ end trace 0000000000000000 ]--- This happened because fscache_cookie_state_machine() was slow and was still running while another process invoked fscache_unuse_cookie(); this led to a fscache_cookie_lru_do_one() call, setting the FSCACHE_COOKIE_DO_LRU_DISCARD flag, which was picked up by fscache_cookie_state_machine(), withdrawing the cookie via cachefiles_withdraw_cookie(), clearing cookie->cache_priv. At the same time, yet another process invoked cachefiles_prepare_write(), which found a NULL pointer in this code line: struct cachefiles_object *object = cachefiles_cres_object(cres); The next line crashes, obviously: struct cachefiles_cache *cache = object->volume->cache; During cachefiles_prepare_write(), the "n_accesses" counter is non-zero (via fscache_begin_operation()). The cookie must not be withdrawn until it drops to zero. The counter is checked by fscache_cookie_state_machine() before switching to FSCACHE_COOKIE_STATE_RELINQUISHING and FSCACHE_COOKIE_STATE_WITHDRAWING (in "case FSCACHE_COOKIE_STATE_FAILED"), but not for FSCACHE_COOKIES_TATE_LRU_DISCARDING ("case FSCACHE_COOKIE_STATE_ACTIVE"). This patch adds the missing check. With a non-zero access counter, the function returns and the next fscache_end_cookie_access() call will queue another fscache_cookie_state_machine() call to handle the still-pending FSCACHE_COOKIE_DO_LRU_DISCARD. Cc: Signed-off-by: Max Kellermann --- fs/netfs/fscache_cookie.c | 4 ++++ 1 file changed, 4 insertions(+) diff --git a/fs/netfs/fscache_cookie.c b/fs/netfs/fscache_cookie.c index bce2492186d0..d4d4b3a8b106 100644 --- a/fs/netfs/fscache_cookie.c +++ b/fs/netfs/fscache_cookie.c @@ -741,6 +741,10 @@ static void fscache_cookie_state_machine(struct fscache_cookie *cookie) spin_lock(&cookie->lock); } if (test_bit(FSCACHE_COOKIE_DO_LRU_DISCARD, &cookie->flags)) { + if (atomic_read(&cookie->n_accesses) != 0) + /* still being accessed: postpone it */ + break; + __fscache_set_cookie_state(cookie, FSCACHE_COOKIE_STATE_LRU_DISCARDING); wake = true; -- 2.39.2