[PATCH V3] scsi_debugfs: fix crash in scsi_show_rq()

[Ming Lei <ming.lei@redhat.com>]

cmd->cmnd can be allocated/freed dynamically in case of T10_PI_TYPE2_PROTECTION,
so we should check it in scsi_show_rq() because this request may have
been freed in scsi_show_rq().

This patch fixs the following kernel crash when dumping request via block's
debugfs interface:

[  252.962045] BUG: unable to handle kernel NULL pointer dereference at           (null)
[  252.963007] IP: scsi_format_opcode_name+0x1a/0x1c0
[  252.963007] PGD 25e75a067 P4D 25e75a067 PUD 25e75b067 PMD 0
[  252.963007] Oops: 0000 [#1] PREEMPT SMP
[  252.963007] Dumping ftrace buffer:
[  252.963007]    (ftrace buffer empty)
[  252.963007] Modules linked in: scsi_debug ebtable_filter ebtables ip6table_filter ip6_tables xt_CHECKSUM iptable_mangle ipt_MASQUERADE nf_nat_masquerade_ipv4 iptable_nat nf_conntrack_ipv4 nf_defrag_ipv4 nf_nat_ipv4 nf_nat nf_conntrack libcrc32c bridge stp llc iptable_filter fuse ip_tables sd_mod sg mptsas mptscsih mptbase crc32c_intel ahci libahci nvme serio_raw scsi_transport_sas libata lpc_ich nvme_core virtio_scsi binfmt_misc dm_mod iscsi_tcp libiscsi_tcp libiscsi scsi_transport_iscsi null_blk configs
[  252.963007] CPU: 1 PID: 1881 Comm: cat Not tainted 4.14.0-rc2.blk_mq_io_hang+ #516
[  252.963007] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.9.3-1.fc25 04/01/2014
[  252.963007] task: ffff88025e6f6000 task.stack: ffffc90001bd0000
[  252.963007] RIP: 0010:scsi_format_opcode_name+0x1a/0x1c0
[  252.963007] RSP: 0018:ffffc90001bd3c50 EFLAGS: 00010286
[  252.963007] RAX: 00000000ffff4843 RBX: 0000000000000050 RCX: 0000000000000000
[  252.963007] RDX: 0000000000000000 RSI: 0000000000000050 RDI: ffffc90001bd3cd8
[  252.963007] RBP: ffffc90001bd3c88 R08: 0000000000001000 R09: 0000000000000000
[  252.963007] R10: ffff880275134000 R11: ffff88027513406c R12: 0000000000000050
[  252.963007] R13: ffffc90001bd3cd8 R14: 0000000000000000 R15: 0000000000000000
[  252.963007] FS:  00007f4d11762700(0000) GS:ffff88027fc40000(0000) knlGS:0000000000000000
[  252.963007] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  252.963007] CR2: 0000000000000000 CR3: 000000025e789003 CR4: 00000000003606e0
[  252.963007] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  252.963007] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[  252.963007] Call Trace:
[  252.963007]  __scsi_format_command+0x27/0xc0
[  252.963007]  scsi_show_rq+0x5c/0xc0
[  252.963007]  ? seq_printf+0x4e/0x70
[  252.963007]  ? blk_flags_show+0x5b/0xf0
[  252.963007]  __blk_mq_debugfs_rq_show+0x116/0x130
[  252.963007]  blk_mq_debugfs_rq_show+0xe/0x10
[  252.963007]  seq_read+0xfe/0x3b0
[  252.963007]  ? __handle_mm_fault+0x631/0x1150
[  252.963007]  full_proxy_read+0x54/0x90
[  252.963007]  __vfs_read+0x37/0x160
[  252.963007]  ? security_file_permission+0x9b/0xc0
[  252.963007]  vfs_read+0x96/0x130
[  252.963007]  SyS_read+0x55/0xc0
[  252.963007]  entry_SYSCALL_64_fastpath+0x1a/0xa5
[  252.963007] RIP: 0033:0x7f4d1127e9b0
[  252.963007] RSP: 002b:00007ffd27082568 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
[  252.963007] RAX: ffffffffffffffda RBX: 00007f4d1154bb20 RCX: 00007f4d1127e9b0
[  252.963007] RDX: 0000000000020000 RSI: 00007f4d115a7000 RDI: 0000000000000003
[  252.963007] RBP: 0000000000021010 R08: ffffffffffffffff R09: 0000000000000000
[  252.963007] R10: 000000000000037b R11: 0000000000000246 R12: 0000000000022000
[  252.963007] R13: 00007f4d1154bb78 R14: 0000000000001000 R15: 0000000000020000
[  252.963007] Code: c6 e8 1b ca 24 00 eb 8c e8 74 2c ae ff 0f 1f 40 00 0f 1f 44 00 00 55 48 89 e5 41 56 41 55 41 54 53 49 89 fd 49 89 f4 48 83 ec 18 <44> 0f b6 32 48 c7 45 c8 00 00 00 00 65 48 8b 04 25 28 00 00 00
[  252.963007] RIP: scsi_format_opcode_name+0x1a/0x1c0 RSP: ffffc90001bd3c50
[  252.963007] CR2: 0000000000000000
[  252.963007] ---[ end trace 83c5bddfbaa6573c ]---
[  252.963007] Kernel panic - not syncing: Fatal exception
[  252.963007] Dumping ftrace buffer:
[  252.963007]    (ftrace buffer empty)
[  252.963007] Kernel Offset: disabled
[  252.963007] ---[ end Kernel panic - not syncing: Fatal exception

Fixes: 0eebd005dd07(scsi: Implement blk_mq_ops.show_rq())
Cc: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: Omar Sandoval <osandov@fb.com>
Cc: Martin K. Petersen <martin.petersen@oracle.com>
Cc: James Bottomley <James.Bottomley@HansenPartnership.com>
Cc: Hannes Reinecke <hare@suse.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
V2:
	- fix typo
V3:
	- prefer to dump data and accept read-after-free
	- add some comment

James, since read-after-free is inevitable and we prefer to dump data,
we needn't to move clearing the buffer pointer before freeing the buffer
in sd_uninit_command(), otherwise we may miss the dump a bit

 drivers/scsi/scsi_debugfs.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/drivers/scsi/scsi_debugfs.c b/drivers/scsi/scsi_debugfs.c
index 5e9755008aed..2320d8f569b8 100644
--- a/drivers/scsi/scsi_debugfs.c
+++ b/drivers/scsi/scsi_debugfs.c
@@ -9,7 +9,17 @@ void scsi_show_rq(struct seq_file *m, struct request *rq)
 	int msecs = jiffies_to_msecs(jiffies - cmd->jiffies_at_alloc);
 	char buf[80];
 
-	__scsi_format_command(buf, sizeof(buf), cmd->cmnd, cmd->cmd_len);
+	/*
+	 * This rq may have been freed, so don't be surprised if
+	 * read-after-free is reported.
+	 *
+	 * __scsi_format_command() should be written as not being broken
+	 * with gargabe 'cdb' input.
+	 */
+	if (cmd->cmnd && cmd->cmd_len)
+		__scsi_format_command(buf, sizeof(buf), cmd->cmnd, cmd->cmd_len);
+	else
+		strcpy(buf, "");
 	seq_printf(m, ", .cmd=%s, .retries=%d, allocated %d.%03d s ago", buf,
 		   cmd->retries, msecs / 1000, msecs % 1000);
 }
-- 
2.9.5