From: Tejun Heo tj@kernel.org Subject: delayacct: fix crash in delayacct_blkio_end() after delayacct init failure

While forking, if delayacct init fails due to memory shortage, it continues expecting all delayacct users to check task->delays pointer against NULL before dereferencing it, which all of them used to do.

c96f5471ce7d ("delayacct: Account blkio completion on the correct task"), while updating delayacct_blkio_end() to take the target task instead of always using %current, made the function test NULL on %current->delays and then continue to operated on @p->delays. If %current succeeded init while @p didn't, it leads to the following crash.

BUG: unable to handle kernel NULL pointer dereference at 0000000000000004 IP: __delayacct_blkio_end+0xc/0x40 PGD 8000001fd07e1067 P4D 8000001fd07e1067 PUD 1fcffbb067 PMD 0 Oops: 0000 [#1] SMP PTI CPU: 4 PID: 25774 Comm: QIOThread0 Not tainted 4.16.0-9_fbk1_rc2_1180_g6b593215b4d7 #9 Hardware name: Quanta Leopard ORv2-DDR4/Leopard ORv2-DDR4, BIOS F06_3B12 08/17/2017 RIP: 0010:__delayacct_blkio_end+0xc/0x40 RSP: 0000:ffff881fff703bf8 EFLAGS: 00010086 RAX: ffff881f1ec8b800 RBX: ffff8804f735cd54 RCX: ffff881fff703cb0 RDX: 0000000000000002 RSI: 0000000000000003 RDI: 0000000000000000 RBP: 0000000000000000 R08: 0000000000000000 R09: ffff881fff703cc0 R10: 0000000000001000 R11: ffff881fd3f73d00 R12: ffff8804f735c600 R13: 0000000000000000 R14: 000000000000001d R15: ffff881fff703cb0 FS: 00007f5003f7d700(0000) GS:ffff881fff700000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 0000000000000004 CR3: 0000001f401a6006 CR4: 00000000003606e0 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 Call Trace: <IRQ> try_to_wake_up+0x2c0/0x600 autoremove_wake_function+0xe/0x30 __wake_up_common+0x74/0x120 wake_up_page_bit+0x9c/0xe0 mpage_end_io+0x27/0x70 blk_update_request+0x78/0x2c0 scsi_end_request+0x2c/0x1e0 scsi_io_completion+0x20b/0x5f0 blk_mq_complete_request+0xa2/0x100 ata_scsi_qc_complete+0x79/0x400 ata_qc_complete_multiple+0x86/0xd0 ahci_handle_port_interrupt+0xc9/0x5c0 ahci_handle_port_intr+0x54/0xb0 ahci_single_level_irq_intr+0x3b/0x60 __handle_irq_event_percpu+0x43/0x190 handle_irq_event_percpu+0x20/0x50 handle_irq_event+0x2a/0x50 handle_edge_irq+0x80/0x1c0 handle_irq+0xaf/0x120 do_IRQ+0x41/0xc0 common_interrupt+0xf/0xf </IRQ>

Fix it by updating delayacct_blkio_end() check @p->delays instead.

Link: http://lkml.kernel.org/r/20180724175542.GP1934745@devbig577.frc2.facebook.co... Fixes: c96f5471ce7d ("delayacct: Account blkio completion on the correct task") Signed-off-by: Tejun Heo tj@kernel.org Reported-by: Dave Jones dsj@fb.com Debugged-by: Dave Jones dsj@fb.com Reviewed-by: Andrew Morton akpm@linux-foundation.org Cc: Josh Snyder joshs@netflix.com Cc: stable@vger.kernel.org [4.15+] Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

include/linux/delayacct.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/include/linux/delayacct.h~delayacct-fix-crash-in-delayacct_blkio_end-after-delayacct-init-failure +++ a/include/linux/delayacct.h @@ -124,7 +124,7 @@ static inline void delayacct_blkio_start

static inline void delayacct_blkio_end(struct task_struct *p) { - if (current->delays) + if (p->delays) __delayacct_blkio_end(p); delayacct_clear_flag(DELAYACCT_PF_BLKIO); } _