Re: Review request 0/1: fs/proc: Fix NULL pointer dereference in

[Eric W. Biederman]

Yahu Gao <yahu.gao@xxxxxxxxxxxxx> writes:

> There is a kernel NULL pointer dereference was found in Linux system.
> The details of kernel NULL is shown at bellow.
>
> Currently, we do not have a way to provoke this fault on purpose, but
> the reproduction rate in out CI loops is high enough that we could go
> for a trace patch in black or white UP and get a reproduction in few
> weeks.
>
> Our kernel version is 4.1.21, but via analyzing the source code of the
> call trace. The upstream version should be affected. Really sorry for
> havn't reproduced this in upstream version. But it's easier to be safe
> than to prove it can't happen, right?

Except I there are strong invariants that suggests that it takes
a memory stomp to get a NULL pointer deference here.

For the life of a proc inode PROC_I(inode)->pid should be non-NULL.

For a non-NULL pid pointer ->tasks[PIDTYPE_PID].first simply reads
an entry out of the struct pid.  Only pid needs to be non-NULL.

So I don't see how you are getting a NULL pointer derference.

Have you decoded the oops, looked at the assembly and seen which field
is NULL?  I expec that will help you track down what is wrong.


> Details of kernel crash:
> ----------------------------------------------------------------------
> [1446.285834] Unable to handle kernel NULL pointer dereference at
> virtual address 00000008
> [ 1446.293943] pgd = e4af0880
> [ 1446.296656] [00000008] *pgd=10cc3003, *pmd=04153003, *pte=00000000
> [ 1446.302898] Internal error: Oops: 207 1 PREEMPT SMP ARM
> [ 1446.302950] Modules linked in: adkNetD ncp
> lttng_ring_buffer_client_mmap_overwrite(C)
> lttng_ring_buffer_client_mmap_discard(C)
> lttng_ring_buffer_client_discard(C)
> lttng_ring_buffer_metadata_mmap_client(C) lttng_probe_printk(C)
> lttng_probe_irq(C) lttng_ring_buffer_metadata_client(C)
> lttng_ring_buffer_client_overwrite(C) lttng_probe_signal(C)
> lttng_probe_sched(C) lttng_tracer(C) lttng_statedump(C)
> lttng_lib_ring_buffer(C) lttng_clock_plugin_arm_cntpct(C) lttng_clock(C)
> [ 1446.302963] CPU: 0 PID: 12086 Comm: netstat Tainted: G C
> 4.1.21-rt13-* #1
> [ 1446.302967] Hardware name: Ericsson CPM1
> [ 1446.302972] task: cbd75480 ti: c4a68000 task.ti: c4a68000
> [ 1446.302984] PC is at pid_delete_dentry+0x8/0x18
> [ 1446.302992] LR is at dput+0x1a8/0x2b4
> [ 1446.303003] pc : [] lr : [] psr: 20070013
> [ 1446.303003] sp : c4a69e88 ip : 00000000 fp : 00000000
> [ 1446.303007] r10: 000218cc r9 : cd228000 r8 : e5f44320
> [ 1446.303011] r7 : 00000001 r6 : 00080040 r5 : c4aa97d0 r4 : c4aa9780
> [ 1446.303015] r3 : 00000000 r2 : cbd75480 r1 : 00000000 r0 : c4aa9780
> [ 1446.303020] Flags: nzCv IRQs on FIQs on Mode SVC_32 ISA ARM Segment
> user
> [ 1446.303026] Control: 30c5387d Table: 24af0880 DAC: 000000fd
> [ 1446.303033] Process netstat (pid: 12086, stack limit = 0xc4a68218)
> [ 1446.303039] Stack: (0xc4a69e88 to 0xc4a6a000)
> [ 1446.303052] 9e80: c4a69f70 0000a1c0 c4a69f13 00000002 e5f44320
> cd228000
> [ 1446.303059] 9ea0: 000218cc c0571604 c0a60bcc 00000000 00000000
> 00000000 c4a69f20 c4a69f15
> [ 1446.303065] 9ec0: 00003133 00000002 c4a69f13 00000000 0000001f
> c4a69f70 c35de800 0000007c
> [ 1446.303072] 9ee0: ce2b1c00 cd228000 00000001 c05747b8 c05745cc
> c35de800 0000001f 00000000
> [ 1446.303078] 9f00: 00000004 cd228008 00020000 c05745cc 33000004
> c0400031 c4a68000 00000400
> [ 1446.303086] 9f20: beb78c2c cd228000 c4a69f70 00000000 cd228008
> c0ffca90 c4a68000 00000400
> [ 1446.303103] 9f40: beb78c2c c052cd0c bf08a774 00000400 01480080
> 00008000 cd228000 cd228000
> [ 1446.303114] 9f60: c040f7c8 c4a68000 00000400 c052d22c c052cd8c
> 00000000 00000021 00000000
> [ 1446.303127] 9f80: 01480290 01480280 00007df0 ffffffea 01480060
> 01480060 01480064 b6e424c0
> [ 1446.303143] 9fa0: 0000008d c040f794 01480060 01480064 00000004
> 01480080 00008000 00000000
> [ 1446.303150] 9fc0: 01480060 01480064 b6e424c0 0000008d 01480080
> 01480060 00035440 beb78c2c
> [ 1446.303156] 9fe0: 01480080 beb78160 b6ede59c b6edea3c 60070010
> 00000004 00000000 00000000
> [ 1446.303167] [] (pid_delete_dentry) from [] (dput+0x1a8/0x2b4)
> [ 1446.303176] [] (dput) from [] (proc_fill_cache+0x54/0x10c)
> [ 1446.303189] [] (proc_fill_cache) from []
> (proc_readfd_common+0xd8/0x238)
> [ 1446.303203] [] (proc_readfd_common) from [] (iterate_dir+0x98/0x118)
> [ 1446.303217] [] (iterate_dir) from [] (SyS_getdents+0x7c/0xf0)
> [ 1446.303233] [] (SyS_getdents) from [] (__sys_trace_return+0x0/0x2c)
> [ 1446.303243] Code: e8bd0030 e12fff1e e5903028 e5133020 (e5930008) 

Eric