Re: [PATCH v2 6/9] s390/module: Use s390_kernel_write() for late relocations

Joe Lawrence
2020-04-23T14:21:54Z

On Thu, Apr 23, 2020 at 03:22:06PM +0200, Miroslav Benes wrote:
> On Thu, 23 Apr 2020, Gerald Schaefer wrote:
>
> > On Wed, 22 Apr 2020 14:46:05 -0500
> > Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> >
> > > On Wed, Apr 22, 2020 at 05:21:26PM +0200, Gerald Schaefer wrote:
> > > > > Sorry, just noticed this. Heiko will return next month, and I'm not
> > > > > really familiar with s390 livepatching. Adding Vasily, he might
> > > > > have some more insight.
> > > > >
> > > > > So, I might be completely wrong here, but using s390_kernel_write()
> > > > > for writing to anything other than 1:1 mapped kernel, should go
> > > > > horribly wrong, as that runs w/o DAT. It would allow to bypass
> > > > > DAT write protection, which I assume is why you want to use it,
> > > > > but it should not work on module text section, as that would be
> > > > > in vmalloc space and not 1:1 mapped kernel memory.
> > > > >
> > > > > Not quite sure how to test / trigger this, did this really work for
> > > > > you on s390?
> > > >
> > > > OK, using s390_kernel_write() as default write function for module
> > > > relocation seems to work fine for me, so apparently I am missing /
> > > > mixing up something. Sorry for the noise, please ignore my concern.
> > >
> > > Hi Gerald,
> > >
> > > I think you were right. Joe found the below panic with his klp-convert
> > > tests.
> > >
> > > Your test was probably the early module loading case (normal relocations
> > > before write protection), rather than the late case. Not sure why that
> > > would work, but calling s390_kernel_write() late definitely seems to be
> > > broken.
> > >
> > > Is there some other way to write vmalloc'ed s390 text without using
> > > module_disable_ro()?
> > >
> > > [ 50.294476] Unable to handle kernel pointer dereference in virtual kernel address space
> > > [ 50.294479] Failing address: 000003ff8015b000 TEID: 000003ff8015b407
> > > [ 50.294480] Fault in home space mode while using kernel ASCE.
> > > [ 50.294483] AS:000000006cef0007 R3:000000007e2c4007 S:0000000003ccb800 P:0000 00000257321d
> > > [ 50.294557] Oops: 0004 ilc:3 [#1] SMP
> > > [ 50.294561] Modules linked in: test_klp_convert1(K+) test_klp_convert_mod ghash_s390 prng xts aes_s390 des_s390 libdes sha512_s390 vmur zcrypt_cex4 ip_tables xfs libcrc32c dasd_fba_mod qeth_l2 dasd_eckd_mod dasd_mod qeth lcs ctcm qdio cc
> > > wgroup fsm dm_mirror dm_region_hash dm_log dm_mod pkey zcrypt [last unloaded: test_klp_atomic_replace]
> > > [ 50.294576] CPU: 0 PID: 1743 Comm: modprobe Tainted: G K 5.6.0 + #2
> > > [ 50.294579] Hardware name: IBM 2964 N96 400 (z/VM 6.4.0)
> > > [ 50.294583] Krnl PSW : 0704e00180000000 000000006bf6be0a (apply_rela+0x2ba/0x 4e0)
> > > [ 50.294589] R:0 T:1 IO:1 EX:1 Key:0 M:1 W:0 P:0 AS:3 CC:2 PM:0 RI: 0 EA:3
> > > [ 50.294684] Krnl GPRS: 000003ff80147010 000003e0001b9588 000003ff8015c168 000 003ff8015b19a
> > > [ 50.294686] 000003ff8015b07c 0d10e310100a0004 000003ff80147010 000 00000000000a0
> > > [ 50.294687] 000003ff8015e588 000003ff8015e5e8 000003ff8015d300 000 0003b00000014
> > > [ 50.294698] 000000007a663000 000000006c6bbb80 000003e0009a7918 000 003e0009a78b8
> > > [ 50.294707] Krnl Code: 000000006bf6bdf8: e350d0080004 lg %r5,8(%r 13)
> > > [ 50.294707] 000000006bf6bdfe: e34010080008 ag %r4,8(%r 1)
> > > [ 50.294707] #000000006bf6be04: e340a2000008 ag %r4,512( %r10)
> > > [ 50.294707] >000000006bf6be0a: e35040000024 stg %r5,0(%r 4)
> > > [ 50.294707] 000000006bf6be10: c050007c6136 larl %r5,0000 00006cef807c
> > > [ 50.294707] 000000006bf6be16: e35050000012 lt %r5,0(%r 5)
> > > [ 50.294707] 000000006bf6be1c: a78400a6 brc 8,000000 006bf6bf68
> > > [ 50.294707] 000000006bf6be20: a55e07f1 llilh %r5,2033
> > > 01: HCPGSP2629I The virtual machine is placed in CP mode due to a SIGP stop from CPU 01.
> > > 01: HCPGSP2629I The virtual machine is placed in CP mode due to a SIGP stop from CPU 00.
> > > [ 50.295369] Call Trace:
> > > [ 50.295372] [<000000006bf6be0a>] apply_rela+0x2ba/0x4e0
> > > [ 50.295376] [<000000006bf6c5c8>] apply_relocate_add+0xe0/0x138
> > > [ 50.295378] [<000000006c0229a0>] klp_apply_section_relocs+0xe8/0x128
> > > [ 50.295380] [<000000006c022b4c>] klp_apply_object_relocs+0x9c/0xd0
> > > [ 50.295382] [<000000006c022bb0>] klp_init_object_loaded+0x30/0x138
> > > [ 50.295384] [<000000006c023052>] klp_enable_patch+0x39a/0x870
> > > [ 50.295387] [<000003ff8015b0da>] test_klp_convert_init+0x22/0x50 [test_klp_convert1]
> > > [ 50.295389] [<000000006bf54838>] do_one_initcall+0x40/0x1f0
> > > [ 50.295391] [<000000006c04d610>] do_init_module+0x70/0x280
> > > [ 50.295392] [<000000006c05002a>] load_module+0x1aba/0x1d10
> > > [ 50.295394] [<000000006c0504c4>] __do_sys_finit_module+0xa4/0xe8
> > > [ 50.295416] [<000000006c6b5742>] system_call+0x2aa/0x2c8
> > > [ 50.295416] Last Breaking-Event-Address:
> > > [ 50.295418] [<000000006c6b6aa0>] __s390_indirect_jump_r4+0x0/0xc
> > > [ 50.295421] Kernel panic - not syncing: Fatal exception: panic_on_oops
> > >
> >
> > Hi Josh,
> >
> > this is strange. While I would have expected an exception similar to
> > this, it really should have happened on the "sturg" instruction which
> > does the DAT-off store in s390_kernel_write(), and certainly not with
> > an ID of 0004 (protection). However, in your case, it happens on a
> > normal store instruction, with 0004 indicating a protection exception.
> >
> > This is more like what I would expect e.g. in the case where you do
> > _not_ use the s390_kernel_write() function for RO module text patching,
> > but rather normal memory access. So I am pretty sure that this is not
> > related to the s390_kernel_write(), but some other issue, maybe some
> > place left where you still use normal memory access?
>
> The call trace above also suggests that it is not a late relocation, no?

You are correct, details below...

> The path is from KLP module init function through klp_enable_patch. It should
> mean that the to-be-patched object is loaded (it must be a module thanks
> to a check klp_init_object_loaded(), vmlinux relocations were processed
> earlier in apply_relocations()).
>
> However, the KLP module state here must be COMING, so s390_kernel_write()
> should be used. What are we missing?
>
> Joe, could you debug this a bit, please?
>

Here is the combined branch that I tested yesterday:
https://github.com/joe-lawrence/linux/tree/jp-v2-klp-convert

That combined WIP klp-convert changes merged on top of Josh's v2 patches
(and follow-ups) posted here. Before I merged with Josh's changes, the
klp-convert code + tests ran without incident. There was a slight merge
conflict, but I hope that's not related to this crash. Perhaps the test
case is shaky and Josh's changes expose an underlying issue?

More info on the test case:

# TEST: klp-convert symbols
https://github.com/joe-lawrence/linux/blob/jp-v2-klp-convert/tools/testing/selftests/livepatch/test-livepatch.sh#L172

which loads an ordinary kernel module (test_klp_convert_mod) and then
this livepatch module, which contains references to both vmlinux
symbols and a few found in the first module:
https://github.com/joe-lawrence/linux/blob/jp-v2-klp-convert/lib/livepatch/test_klp_convert1.c

The livepatch's init function calls klp_enable_patch() as usual, and
then for testing purposes calls a few functions that required
klp-relocations. I don't know if real livepatch modules would ever need
to do this from init code, but I did so just to make the test code
simpler.

This is what klp-convert created for klp-relocations:

% readelf --wide --relocs test_klp_convert1.ko
...
Relocation section '.klp.rela.vmlinux..text.unlikely' at offset 0x52000 contains 1 entry:
Offset Info Type Symbol's Value Symbol's Name + Addend
0000000000000002 000000340000001a R_390_GOTENT 0000000000000000 .klp.sym.vmlinux.saved_command_line,0 + 2

Relocation section '.klp.rela.test_klp_convert_mod..text.unlikely' at offset 0x52018 contains 4 entries:
Offset Info Type Symbol's Value Symbol's Name + Addend
000000000000008a 0000003b00000014 R_390_PLT32DBL 0000000000000000 .klp.sym.test_klp_convert_mod.get_homonym_string,1 + 2
000000000000006c 000000350000001a R_390_GOTENT 0000000000000000 .klp.sym.test_klp_convert_mod.homonym_string,1 + 2
0000000000000042 0000003e00000014 R_390_PLT32DBL 0000000000000000 .klp.sym.test_klp_convert_mod.test_klp_get_driver_name,0 + 2
0000000000000024 000000380000001a R_390_GOTENT 0000000000000000 .klp.sym.test_klp_convert_mod.driver_name,0 + 2

I can rework the test case to simplify and debug few things. Let me
know if you have any specific ideas in mind.

-- Joe