On Wed, Nov 08, 2017 at 08:01:08AM -0800, James Bottomley wrote:
> On Wed, 2017-11-08 at 21:21 +0800, Ming Lei wrote:
> > 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@xxxxxxxxxxx>
> > Cc: Omar Sandoval <osandov@xxxxxx>
> > Cc: Martin K. Petersen <martin.petersen@xxxxxxxxxx>
> > Cc: James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx>
> > Cc: Hannes Reinecke <hare@xxxxxxxx>
> > Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
> > ---
> > 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.
>
> This can be fixed, see below.
>
> > + *
> > + * __scsi_format_command() should be written as not being
> > broken
> > + * with gargabe 'cdb' input.
> > + */
> > + if (cmd->cmnd && cmd->cmd_len)
>
> Why is it necessary to check cmd_len as well? It's not wrong, it just
> doesn't seem to be useful
If cmd->cmd_len is 0, the 'buf' won't be initialized by __scsi_format_command().
>
> > + __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);
> > }
>
>
> This is what you need as well:
>
> diff --git a/drivers/scsi/sd.c b/drivers/scsi/sd.c
> index d175c5c5ccf8..2d5e918c3e75 100644
> --- a/drivers/scsi/sd.c
> +++ b/drivers/scsi/sd.c
> @@ -1284,6 +1284,7 @@ static int sd_init_command(struct scsi_cmnd *cmd)
> static void sd_uninit_command(struct scsi_cmnd *SCpnt)
> {
> struct request *rq = SCpnt->request;
> + unsigned char *cmnd = SCpnt->cmnd;
>
> if (SCpnt->flags & SCMD_ZONE_WRITE_LOCK)
> sd_zbc_write_unlock_zone(SCpnt);
> @@ -1292,9 +1293,9 @@ static void sd_uninit_command(struct scsi_cmnd *SCpnt)
> __free_page(rq->special_vec.bv_page);
>
> if (SCpnt->cmnd != scsi_req(rq)->cmd) {
> - mempool_free(SCpnt->cmnd, sd_cdb_pool);
> SCpnt->cmnd = NULL;
> SCpnt->cmd_len = 0;
> + mempool_free(cmnd, sd_cdb_pool);
> }
> }
>
>
> The reason it works is that function calls are an absolute barrier, so
> setting the cmnd to NULL is seen everywhere before the mempool_free()
> starts and the execution path of mempool_free() is much longer
> than _scsi_format_command() so you can never get a use after free in
> practice. To make this completely bullet proof, scsi_show_rq() should
Yeah, that is true, but still can't avoid use-after-free since preemption
or interrupt may change the timing a lot. Especially sd_uninit_command()
is run in interrupt context.
> take a copy of the cmnd pointer before testing and using it, otherwise
> there's a minute window where it could go to NULL between the check and
> __scsi_format_command(). I've forgotten if we can rely on no-
> preemption for RT or we need to add it explicitly, but I'm sure you can
> figure it out.
If we set SCpnt->cmnd as NULL earlier, we may miss the command data with
more possibility, that seems what we want to avoid too from debug function
view.
--
Ming
