Date: Thu, 20 Jun 2024 19:05:50 +0200
From: Ilya Leoshkevich <iii@...ux.ibm.com>
To: Alexander Potapenko <glider@...gle.com>
Cc: Alexander Gordeev <agordeev@...ux.ibm.com>,
        Andrew Morton
 <akpm@...ux-foundation.org>,
        Christoph Lameter <cl@...ux.com>,
        David
 Rientjes <rientjes@...gle.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Joonsoo
 Kim <iamjoonsoo.kim@....com>, Marco Elver <elver@...gle.com>,
        Masami
 Hiramatsu <mhiramat@...nel.org>,
        Pekka Enberg <penberg@...nel.org>,
        Steven
 Rostedt <rostedt@...dmis.org>,
        Vasily Gorbik <gor@...ux.ibm.com>, Vlastimil
 Babka <vbabka@...e.cz>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>, kasan-dev@...glegroups.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-s390@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
        Mark Rutland <mark.rutland@....com>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Sven Schnelle
 <svens@...ux.ibm.com>
Subject: Re: [PATCH v5 33/37] s390/uaccess: Add KMSAN support to put_user()
 and get_user()

On Thu, 2024-06-20 at 13:19 +0200, Ilya Leoshkevich wrote:
> On Thu, 2024-06-20 at 10:36 +0200, Alexander Potapenko wrote:
> > On Wed, Jun 19, 2024 at 5:45 PM Ilya Leoshkevich
> > <iii@...ux.ibm.com>
> > wrote:
> > > 
> > > put_user() uses inline assembly with precise constraints, so
> > > Clang
> > > is
> > > in principle capable of instrumenting it automatically.
> > > Unfortunately,
> > > one of the constraints contains a dereferenced user pointer, and
> > > Clang
> > > does not currently distinguish user and kernel pointers.
> > > Therefore
> > > KMSAN attempts to access shadow for user pointers, which is not a
> > > right
> > > thing to do.
> > 
> > By the way, how does this problem manifest?
> > I was expecting KMSAN to generate dummy shadow accesses in this
> > case,
> > and reading/writing 1-8 bytes from dummy shadow shouldn't be a
> > problem.
> > 
> > (On the other hand, not inlining the get_user/put_user functions is
> > probably still faster than retrieving the dummy shadow, so I'm fine
> > either way)
> 
> We have two problems here: not only clang can't distinguish user and
> kernel pointers, the KMSAN runtime - which is supposed to clean that
> up - can't do that either due to overlapping kernel and user address
> spaces on s390. So the instrumentation ultimately tries to access the
> real shadow.
> 
> I forgot what the consequences of that were exactly, so I reverted
> the
> patch and now I get:
> 
> Unable to handle kernel pointer dereference in virtual kernel address
> space
> Failing address: 000003fed25fa000 TEID: 000003fed25fa403
> Fault in home space mode while using kernel ASCE.
> AS:0000000005a70007 R3:00000000824d8007 S:0000000000000020 
> Oops: 0010 ilc:2 [#1] SMP 
> Modules linked in:
> CPU: 3 PID: 1 Comm: init Tainted: G    B            N 6.10.0-rc4-
> g8aadb00f495e #11
> Hardware name: IBM 3931 A01 704 (KVM/Linux)
> Krnl PSW : 0704c00180000000 000003ffe288975a (memset+0x3a/0xa0)
>            R:0 T:1 IO:1 EX:1 Key:0 M:1 W:0 P:0 AS:3 CC:0 PM:0 RI:0
> EA:3
> Krnl GPRS: 0000000000000000 000003fed25fa180 000003fed25fa180
> 000003ffe28897a6
>            0000000000000007 000003ffe0000000 0000000000000000
> 000002ee06e68190
>            000002ee06f19000 000003fed25fa180 000003ffd25fa180
> 000003ffd25fa180
>            0000000000000008 0000000000000000 000003ffe17262e0
> 0000037ee000f730
> Krnl Code: 000003ffe288974c: 41101100           la      %r1,256(%r1)
>            000003ffe2889750: a737fffb           brctg  
> %r3,000003ffe2889746
>           #000003ffe2889754: c03000000029       larl   
> %r3,000003ffe28897a6
>           >000003ffe288975a: 44403000           ex      %r4,0(%r3)
>            000003ffe288975e: 07fe               bcr     15,%r14
>            000003ffe2889760: a74f0001           cghi    %r4,1
>            000003ffe2889764: b9040012           lgr     %r1,%r2
>            000003ffe2889768: a784001c           brc    
> 8,000003ffe28897a0
> Call Trace:
>  [<000003ffe288975a>] memset+0x3a/0xa0 
> ([<000003ffe17262bc>] kmsan_internal_set_shadow_origin+0x21c/0x3a0)
>  [<000003ffe1725fb6>] kmsan_internal_unpoison_memory+0x26/0x30 
>  [<000003ffe1c1c646>] create_elf_tables+0x13c6/0x2620 
>  [<000003ffe1c0ebaa>] load_elf_binary+0x50da/0x68f0  
>  [<000003ffe18c41fc>] bprm_execve+0x201c/0x2f40 
>  [<000003ffe18bff9a>] kernel_execve+0x2cda/0x2d00 
>  [<000003ffe49b745a>] kernel_init+0x9ba/0x1630 
>  [<000003ffe000cd5c>] __ret_from_fork+0xbc/0x180 
>  [<000003ffe4a1907a>] ret_from_fork+0xa/0x30 
> Last Breaking-Event-Address:
>  [<000003ffe2889742>] memset+0x22/0xa0
> Kernel panic - not syncing: Fatal exception: panic_on_oops
> 
> So is_bad_asm_addr() returned false for a userspace address.
> Why? Because it happened to collide with the kernel modules area:
> precisely the effect of overlapping.
> 
> VMALLOC_START: 0x37ee0000000
> VMALLOC_END:   0x3a960000000
> MODULES_VADDR: 0x3ff60000000
> Address:       0x3ffd157a580
> MODULES_END:   0x3ffe0000000
> 
> Now the question is, why do we crash when accessing shadow for
> modules?

So, Alexander G. and I have figured it out. KMSAN maps vmalloc/modules
metadata lazily - when the corresponding memory is allocated. Here we
have a completely random address that did not come from a prior
vmalloc()/execmem_alloc(), so the corresponding metadata pages are
missing.

We could probably detect this situation and perform the lazy
initialization in this case as well, but I don't know if it's worth the
effort.

> I'll need to investigate, this does not look normal. But even if that
> worked, we clearly wouldn't want userspace accesses to pollute module
> shadow, so I think we need this patch in its current form.
> 
> [...]

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