On Fri, Dec 06, 2019 at 12:27:39PM +0000, fdmanana@xxxxxxxxxx wrote:
> From: Filipe Manana <fdmanana@xxxxxxxx>
>
> When a tree mod log user no longer needs to use the tree it calls
> btrfs_put_tree_mod_seq() to remove itself from the list of users and
> delete all no longer used elements of the tree's red black tree, which
> should be all elements with a sequence number less then our equals to
> the caller's sequence number. However the logic is broken because it
> can delete and free elements from the red black tree that have a
> sequence number greater then the caller's sequence number:
>
> 1) At a point in time we have sequence numbers 1, 2, 3 and 4 in the
> tree mod log;
>
> 2) The task which got assigned the sequence number 1 calls
> btrfs_put_tree_mod_seq();
>
> 3) Sequence number 1 is deleted from the list of sequence numbers;
>
> 4) The current minimum sequence number is computed to be the sequence
> number 2;
>
> 5) A task using sequence number 2 is at tree_mod_log_rewind() and gets
> a pointer to one of its elements from the red black tree through
> a call to tree_mod_log_search();
>
> 6) The task with sequence number 1 iterates the red black tree of tree
> modification elements and deletes (and frees) all elements with a
> sequence number less then or equals to 2 (the computed minimum sequence
> number) - it ends up only leaving elements with sequence numbers of 3
> and 4;
>
> 7) The task with sequence number 2 now uses the pointer to its element,
> already freed by the other task, at __tree_mod_log_rewind(), resulting
> in a use-after-free issue. When CONFIG_DEBUG_PAGEALLOC=y it produces
> a trace like the following:
>
> [16804.546854] general protection fault: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC PTI
> [16804.547451] CPU: 0 PID: 28257 Comm: pool Tainted: G W 5.4.0-rc8-btrfs-next-51 #1
> [16804.548059] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.12.0-0-ga698c8995f-prebuilt.qemu.org 04/01/2014
> [16804.548666] RIP: 0010:rb_next+0x16/0x50
> (...)
> [16804.550581] RSP: 0018:ffffb948418ef9b0 EFLAGS: 00010202
> [16804.551227] RAX: 6b6b6b6b6b6b6b6b RBX: ffff90e0247f6600 RCX: 6b6b6b6b6b6b6b6b
> [16804.551873] RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff90e0247f6600
> [16804.552504] RBP: ffff90dffe0d4688 R08: 0000000000000001 R09: 0000000000000000
> [16804.553136] R10: ffff90dffa4a0040 R11: 0000000000000000 R12: 000000000000002e
> [16804.553768] R13: ffff90e0247f6600 R14: 0000000000001663 R15: ffff90dff77862b8
> [16804.554399] FS: 00007f4b197ae700(0000) GS:ffff90e036a00000(0000) knlGS:0000000000000000
> [16804.555039] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [16804.555683] CR2: 00007f4b10022000 CR3: 00000002060e2004 CR4: 00000000003606f0
> [16804.556336] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> [16804.556968] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> [16804.557583] Call Trace:
> [16804.558207] __tree_mod_log_rewind+0xbf/0x280 [btrfs]
> [16804.558835] btrfs_search_old_slot+0x105/0xd00 [btrfs]
> [16804.559468] resolve_indirect_refs+0x1eb/0xc70 [btrfs]
> [16804.560087] ? free_extent_buffer.part.19+0x5a/0xc0 [btrfs]
> [16804.560700] find_parent_nodes+0x388/0x1120 [btrfs]
> [16804.561310] btrfs_check_shared+0x115/0x1c0 [btrfs]
> [16804.561916] ? extent_fiemap+0x59d/0x6d0 [btrfs]
> [16804.562518] extent_fiemap+0x59d/0x6d0 [btrfs]
> [16804.563112] ? __might_fault+0x11/0x90
> [16804.563706] do_vfs_ioctl+0x45a/0x700
> [16804.564299] ksys_ioctl+0x70/0x80
> [16804.564885] ? trace_hardirqs_off_thunk+0x1a/0x20
> [16804.565461] __x64_sys_ioctl+0x16/0x20
> [16804.566020] do_syscall_64+0x5c/0x250
> [16804.566580] entry_SYSCALL_64_after_hwframe+0x49/0xbe
> [16804.567153] RIP: 0033:0x7f4b1ba2add7
> (...)
> [16804.568907] RSP: 002b:00007f4b197adc88 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
> [16804.569513] RAX: ffffffffffffffda RBX: 00007f4b100210d8 RCX: 00007f4b1ba2add7
> [16804.570133] RDX: 00007f4b100210d8 RSI: 00000000c020660b RDI: 0000000000000003
> [16804.570726] RBP: 000055de05a6cfe0 R08: 0000000000000000 R09: 00007f4b197add44
> [16804.571314] R10: 0000000000000000 R11: 0000000000000246 R12: 00007f4b197add48
> [16804.571905] R13: 00007f4b197add40 R14: 00007f4b100210d0 R15: 00007f4b197add50
> (...)
> [16804.575623] ---[ end trace 87317359aad4ba50 ]---
>
> Fix this by making btrfs_put_tree_mod_seq() skip deletion of elements that
> have a sequence number equals to the computed minimum sequence number, and
> not just elements with a sequence number greater then that minimum.
>
> Fixes: bd989ba359f2ac ("Btrfs: add tree modification log functions")
> Signed-off-by: Filipe Manana <fdmanana@xxxxxxxx>
Reviewed-by: Josef Bacik <josef@xxxxxxxxxxxxxx>
Thanks,
Josef
