Hi James, On Mon, Nov 13, 2017 at 10:55:52AM -0800, James Bottomley wrote: > On Sat, 2017-11-11 at 10:43 +0800, Ming Lei wrote: > > On Fri, Nov 10, 2017 at 08:51:58AM -0800, James Bottomley wrote: > > > > > > On Fri, 2017-11-10 at 17:01 +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 already here, and cmd- > > > > >cmnd has been set as null. > > > > > > > > We choose to accept read-after-free and dump request data as far > > > > as possible. > > > > > > > > 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 > > > > V4: > > > > - read the two variable into local variable first, for > > > > avoiding > > > > free between check and passing to __scsi_format_command(). > > > > > > > > > > > > drivers/scsi/scsi_debugfs.c | 14 +++++++++++++- > > > > 1 file changed, 13 insertions(+), 1 deletion(-) > > > > > > > > diff --git a/drivers/scsi/scsi_debugfs.c > > > > b/drivers/scsi/scsi_debugfs.c > > > > index 5e9755008aed..f8437c456458 100644 > > > > --- a/drivers/scsi/scsi_debugfs.c > > > > +++ b/drivers/scsi/scsi_debugfs.c > > > > @@ -8,8 +8,20 @@ void scsi_show_rq(struct seq_file *m, struct > > > > request > > > > *rq) > > > > struct scsi_cmnd *cmd = container_of(scsi_req(rq), > > > > typeof(*cmd), req); > > > > int msecs = jiffies_to_msecs(jiffies - cmd- > > > > > > > > > > jiffies_at_alloc); > > > > char buf[80]; > > > > + unsigned char *cdb = cmd->cmnd; > > > > + unsigned short cdb_len = cmd->cmd_len; > > > > > > > > > > > > - __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. > > > > > > I said fix this in sd.c:sd_uninit_command() by setting SCpnt->cmnd > > > to NULL before calling mempool_free(). > > > > Hi James, > > > > That can't avoid the use-after-free in both reality and theory. > > I believe I already gave you the theory of how to avoid it. > > > When 'cdb = cmd->cmnd' is executed in CPU1, 'cbd' is a valid pointer, > > just after this instruction is done, this request is completed in > > CPU2, and mempool_free() is run to complete this buffer. > > > > Firstly, both preemption and IRQ can happen inside > > __scsi_format_command(), > > when this function is scheduled to rerun, this buffer is freed since > > mempool_free() has completed already, which is run in irq context. > > so disable irq's before taking the cmnd pointer copy ... It isn't enough. > > > Secondly, the use-after-free can't be avoided too if we disable irq > > when checking and reading the variable. See following: > > > > mempool_free(cmd->cmnd) is often run much quick because we complete > > request on same CPU with the submission CPU(the CPU for allocation of > > cmd->cmnd). > > only in the add element case, which won't cause a use after free from I don't understand why that is only in add-element case, could you explain it a bit? For slub/slab..., I believe there is much optimization in case that the allocation/free is done on one local CPU, which should be quicker than memory read from a remote NUMA node. > any tool's point of view. If you want greater assurance, take the For example of KASAN[1], each byte of memory is covered by its shadow memory. Once pool->free() in mempool_free(cmd->cmnd) is run before __scsi_format_command() returns, the use-after-free is triggered. And there are several access to this buffer in __scsi_format_command(). KASAN checks if each access is safe before the actual access, and the check is added by GCC, see below words from [1]: Compile-time instrumentation used for checking memory accesses. Compiler inserts function calls (__asan_load*(addr), __asan_store*(addr)) before each memory access of size 1, 2, 4, 8 or 16. These functions check whether memory access is valid or not by checking corresponding shadow memory. [1] Documentation/dev-tools/kasan.rst > pool->lock (with encapsulation so we're not poking around in mempool > internals). The pool->lock isn't needed in fast path of 'pool->free()', and it is required only when 'if (unlikely(pool->curr_nr < pool->min_nr))' is true, so this way doesn't work too. > > > So from CPU1's review, cmd->cmnd is in a remote NUMA node, > > __scsi_format_command() is executed much slower than mempool_free(). > > So when mempool_free() returns, __scsi_format_command() may not > > fetched the buffer in L1 cache yet, then use-after-free > > is still triggered. > > > > That is why I say this use-after-free is inevitable no matter > > 'setting SCpnt->cmnd to NULL before calling mempool_free()' or not. > > The bottom line is that there are several creative ways around this but > the proposed code is currently broken and simply putting a comment in > saying so doesn't make it acceptable. As I explained above, I didn't see one really workable way. Or please correct it if I am wrong. Thanks, Ming
