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([2620:10d:c091:480::7e55]) by smtp.gmail.com with ESMTPSA id n25sm2442203qkh.88.2020.01.22.12.29.15 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 22 Jan 2020 12:29:15 -0800 (PST) Subject: Re: [PATCH 1/2] Btrfs: fix race between adding and putting tree mod seq elements and nodes To: fdmanana@kernel.org, linux-btrfs@vger.kernel.org References: <20200122122320.30073-1-fdmanana@kernel.org> From: Josef Bacik Message-ID: <0991d646-30f7-ae4e-3bf3-d262747c1eff@toxicpanda.com> Date: Wed, 22 Jan 2020 15:29:14 -0500 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.14; rv:68.0) Gecko/20100101 Thunderbird/68.4.1 MIME-Version: 1.0 In-Reply-To: <20200122122320.30073-1-fdmanana@kernel.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-btrfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org On 1/22/20 7:23 AM, fdmanana@kernel.org wrote: > From: Filipe Manana > > There is a race between adding and removing elements to the tree mod log > list and rbtree that can lead to use-after-free problems. > > Consider the following example that explains how/why the problems happens: > > 1) Task A has mod log element with sequence number 200. It currently is > the only element in the mod log list; > > 2) Task A calls btrfs_put_tree_mod_seq() because it no longer needs to > access the tree mod log. When it enters the function, it initializes > 'min_seq' to (u64)-1. Then it acquires the lock 'tree_mod_seq_lock' > before checking if there are other elements in the mod seq list. > Since the list it empty, 'min_seq' remains set to (u64)-1. Then it > unlocks the lock 'tree_mod_seq_lock'; > > 3) Before task A acquires the lock 'tree_mod_log_lock', task B adds > itself to the mod seq list through btrfs_get_tree_mod_seq() and gets a > sequence number of 201; > > 4) Some other task, name it task C, modifies a btree and because there > elements in the mod seq list, it adds a tree mod elem to the tree > mod log rbtree. That node added to the mod log rbtree is assigned > a sequence number of 202; > > 5) Task B, which is doing fiemap and resolving indirect back references, > calls btrfs get_old_root(), with 'time_seq' == 201, which in turn > calls tree_mod_log_search() - the search returns the mod log node > from the rbtree with sequence number 202, created by task C; > > 6) Task A now acquires the lock 'tree_mod_log_lock', starts iterating > the mod log rbtree and finds the node with sequence number 202. Since > 202 is less than the previously computed 'min_seq', (u64)-1, it > removes the node and frees it; > > 7) Task B still has a pointer to the node with sequence number 202, and > it dereferences the pointer itself and through the call to > __tree_mod_log_rewind(), resulting in a use-after-free problem._ > > This issue can be triggered sporadically with the test case generic/561 > from fstests, and it happens more frequently with a higher number of > duperemove processes. When it happens to me, it either freezes the vm or > it produces a trace like the following before crashing: > > [ 1245.321140] general protection fault: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC PTI > [ 1245.321200] CPU: 1 PID: 26997 Comm: pool Not tainted 5.5.0-rc6-btrfs-next-52 #1 > [ 1245.321235] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.12.0-0-ga698c8995f-prebuilt.qemu.org 04/01/2014 > [ 1245.321287] RIP: 0010:rb_next+0x16/0x50 > [ 1245.321307] Code: .... > [ 1245.321372] RSP: 0018:ffffa151c4d039b0 EFLAGS: 00010202 > [ 1245.321388] RAX: 6b6b6b6b6b6b6b6b RBX: ffff8ae221363c80 RCX: 6b6b6b6b6b6b6b6b > [ 1245.321409] RDX: 0000000000000001 RSI: 0000000000000000 RDI: ffff8ae221363c80 > [ 1245.321439] RBP: ffff8ae20fcc4688 R08: 0000000000000002 R09: 0000000000000000 > [ 1245.321475] R10: ffff8ae20b120910 R11: 00000000243f8bb1 R12: 0000000000000038 > [ 1245.321506] R13: ffff8ae221363c80 R14: 000000000000075f R15: ffff8ae223f762b8 > [ 1245.321539] FS: 00007fdee1ec7700(0000) GS:ffff8ae236c80000(0000) knlGS:0000000000000000 > [ 1245.321591] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 > [ 1245.321614] CR2: 00007fded4030c48 CR3: 000000021da16003 CR4: 00000000003606e0 > [ 1245.321642] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 > [ 1245.321668] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 > [ 1245.321706] Call Trace: > [ 1245.321798] __tree_mod_log_rewind+0xbf/0x280 [btrfs] > [ 1245.321841] btrfs_search_old_slot+0x105/0xd00 [btrfs] > [ 1245.321877] resolve_indirect_refs+0x1eb/0xc60 [btrfs] > [ 1245.321912] find_parent_nodes+0x3dc/0x11b0 [btrfs] > [ 1245.321947] btrfs_check_shared+0x115/0x1c0 [btrfs] > [ 1245.321980] ? extent_fiemap+0x59d/0x6d0 [btrfs] > [ 1245.322029] extent_fiemap+0x59d/0x6d0 [btrfs] > [ 1245.322066] do_vfs_ioctl+0x45a/0x750 > [ 1245.322081] ksys_ioctl+0x70/0x80 > [ 1245.322092] ? trace_hardirqs_off_thunk+0x1a/0x1c > [ 1245.322113] __x64_sys_ioctl+0x16/0x20 > [ 1245.322126] do_syscall_64+0x5c/0x280 > [ 1245.322139] entry_SYSCALL_64_after_hwframe+0x49/0xbe > [ 1245.322155] RIP: 0033:0x7fdee3942dd7 > [ 1245.322177] Code: .... > [ 1245.322258] RSP: 002b:00007fdee1ec6c88 EFLAGS: 00000246 ORIG_RAX: 0000000000000010 > [ 1245.322294] RAX: ffffffffffffffda RBX: 00007fded40210d8 RCX: 00007fdee3942dd7 > [ 1245.322314] RDX: 00007fded40210d8 RSI: 00000000c020660b RDI: 0000000000000004 > [ 1245.322337] RBP: 0000562aa89e7510 R08: 0000000000000000 R09: 00007fdee1ec6d44 > [ 1245.322369] R10: 0000000000000073 R11: 0000000000000246 R12: 00007fdee1ec6d48 > [ 1245.322390] R13: 00007fdee1ec6d40 R14: 00007fded40210d0 R15: 00007fdee1ec6d50 > [ 1245.322423] Modules linked in: .... > [ 1245.323443] ---[ end trace 01de1e9ec5dff3cd ]--- > > Fix this by ensuring that btrfs_put_tree_mod_seq() computes the minimum > sequence number and iterates the rbtree while holding the lock > 'tree_mod_log_lock' in write mode. Also get rid of the 'tree_mod_seq_lock' > lock, since it is now redundant. > > Fixes: bd989ba359f2ac ("Btrfs: add tree modification log functions") > Fixes: 097b8a7c9e48e2 ("Btrfs: join tree mod log code with the code holding back delayed refs") > CC: stable@vger.kernel.org > Signed-off-by: Filipe Manana Ugh what a pain in the ass, thanks Reviewed-by: Josef Bacik Thanks, Josef