Re: [PATCH 4/4] fuzz: delay IO until they can't trigger the crash

[Alexander Bulekov]

On 201222 1922, Qiuhao Li wrote:
> On Mon, 2020-12-21 at 16:17 -0500, Alexander Bulekov wrote:
> > On 201220 0256, Qiuhao Li wrote:
> > > Since programmers usually trigger an IO just before they need it.
> > > Try to
> > > delay some IO instructions may help us better understanding the
> > > timing
> > > context when debug.
> > > 
> > > Tested with Bug 1908062. Refined vs. Original result:
> > > 
> > > outl 0xcf8 0x8000081c            outl 0xcf8 0x0
> > > outb 0xcfc 0xc3                | outl 0xcf8 0x8000081c
> > > outl 0xcf8 0x80000804          | outb 0xcfc 0xc3
> > > outl 0xcfc 0x10000006          | outl 0xcf8 0x80000804
> > > write 0xc300001028 0x1 0x5a    | outl 0xcfc 0x10000006
> > > write 0xc300001024 0x2 0x10    | write 0xc300001028 0x1 0x5a
> > > write 0xc30000101c 0x1 0x01    | writel 0xc30000100c 0x2a6f6c63
> > > write 0xc300003002 0x1 0x0     v write 0xc300001024 0x2 0x10
> > > write 0x5c 0x1 0x10              write 0xc30000101c 0x1 0x01
> > > writel 0xc30000100c 0x2a6f6c63   write 0xc300001018 0x1 0x80
> > > write 0xc300001018 0x1 0x80      write 0x5c 0x1 0x10
> > > outl 0xcf8 0x0                   write 0xc300003002 0x1 0x0
> > > 
> > 
> > In this example, I can remove the outl 0xcf8 0x0, and I still see the
> > crash, so maybe the 1st step in the minimizer is failing somewhere.. 
> 
> I think it might because of our one-time scan and remove strategy,
> which is not suitable for timing dependent instructions.
> 
> For example, instruction A will indicate an address where the config
> chunk locates, and instruction B will make the configuration active. If
> we have the following instruction sequence:
> 
> ...
> A1
> B1
> A2
> B2
> ...
> 
> A2 and B2 are the actual instructions that trigger the bug.
> 
> If we scan from top to bottom, after we remove A1, the behavior of B1
> might be unknowable, including not to crash the program. But we will
> successfully remove B1 later cause A2 and B2 will crash the process
> anyway:
> 
> ...
> A1
> A2
> B2
> ...
> 
> Now one more trimming will remove A1.
> 
> As for the example I gave, the instructions before the delaying
> minimizer are like this:
> 
> outl 0xcf8 0x8000081c
> outb 0xcfc 0xc3
> outl 0xcf8 0x0                <--- The A instruction, didn't be removed
> (outl 0xcfc 0x0)              <--- The B instruction, removed
> outl 0xcf8 0x80000804
> outl 0xcfc 0x10000006
> write 0xc300001024 0x2 0x10
> write 0xc300001028 0x1 0x5a
> write 0xc30000101c 0x1 0x01
> writel 0xc30000100c 0x2a6f6c63
> write 0xc300001018 0x1 0x80
> write 0x5c 0x1 0x10
> write 0xc300003002 0x1 0x0
> 
> If we run the remove minimizer again, The A instruction outl 0xcf8 0x0
> will be removed.
> 
> Since we only remove instructions, this iterative algorithm is
> converging. Maybe we can keep removing the trace until the
> len(newtrace) become unchanged.
> 

I found a bunch of work related to this "test-case minimization". There
are algorithms such as "ddmin" that try to tackle this. There might be
some interesting ideas there.

I think in the perfect case, we would need to be able to remove A and B
at the same time. You described the situation where B1 might lead to a
bad state without A1, but there is also the possibility that A1 might
leave bad state around, without B1. And both of these might be true at
the same time :) Probably not something we encounter very often, though.

> > 
> > Is the Refined one better? To me the original one read as:
> > "Do a bunch of PCI configuration to map an MMIO BAR, then interact
> > with
> > the MMIO range and trigger some DMA activity". I also know exactly
> > the
> > line that will trigger the DMA activity and access 0x5c. With the
> > refined one, I'm not so sure. Which line now causes the DMA read from
> > 0x5c? writel 0xc30000100c? write 0xc300001018?
> > Is there another example where this type of reordering makes the
> > result
> > easier to read?
> > 
> > > Signed-off-by: Qiuhao Li <Qiuhao.Li@outlook.com>
> > > ---
> > >  scripts/oss-fuzz/minimize_qtest_trace.py | 21
> > > +++++++++++++++++++++
> > >  1 file changed, 21 insertions(+)
> > > 
> > > diff --git a/scripts/oss-fuzz/minimize_qtest_trace.py
> > > b/scripts/oss-fuzz/minimize_qtest_trace.py
> > > index f3e88064c4..da7aa73b3c 100755
> > > --- a/scripts/oss-fuzz/minimize_qtest_trace.py
> > > +++ b/scripts/oss-fuzz/minimize_qtest_trace.py
> > > @@ -214,6 +214,27 @@ def minimize_trace(inpath, outpath):
> > >  
> > >      assert(check_if_trace_crashes(newtrace, outpath))
> > >  
> > > +    # delay IO instructions until they can't trigger the crash
> > > +    # Note: O(n^2) and many timeouts, kinda slow
> > 
> > Maybe do a binary search instead of a linear scan for the optimal
> > position
> > to save some time?
> > Also, if you re-run this multiple times, you can end up with
> > different
> > results, since some lines might not really be tied to a position
> > (e.g.
> > the outl cf8 0x0 in your example). Maybe it's not a problem, but i'm
> > still not sure that this is making the result easier to read.
> > -Alex
> 
> I'm not familiar with the PCI configuration and DMA mechanism in QEMU.
> This patch is just random thinking based on empiricism. Maybe I should
> add the "RFC" tag :). In version 2, I won't post this patch.
> 
> BTW, may I ask where I can learn about these IO emulations? How do you
> know the address corresponding to the PCI bar and DMA?

On PCs, the PCI configuration space is accessed using two I/O ports:
0xcfc and 0xcf8. To interact further with a  PCI device, you have to
configure its BARs (i.e. the Port IO and memory ranges that will map to
device registers).
https://en.wikipedia.org/wiki/PCI_configuration_space#Bus_enumeration

So we can look at the trace again. First there are no virtio-vga MMIO/PIO
ranges accessible, so the only thing the fuzzer can do is interact with
its PCI configuration space using 0xCF8/CFC:

outl 0xcf8 0x8000081c
outb 0xcfc 0xc3
^^^ The above two lines write the value 0xc3 to PCI config address 0x1c
for the vga device. You can confirm this by running the testcase with
-trace pci\*. 0x1c is the location of the PCI register that represents
BAR #3 for the device. 
outl 0xcf8 0x80000804
outb 0xcfc 0x06
^^^ These two lines write to the PCI command register (0x04) to allow
the device to respond to memory accesses.

write 0xc300001024 0x2 0x0040
write 0xc300001028 0x1 0x5a
write 0xc30000101c 0x1 0x01
writel 0xc30000100c 0x20000000
write 0xc300001016 0x3 0x80a080
write 0xc300003002 0x1 0x80
^^^ Now we start to see what looks like MMIO accesses. And if we look at
the output of -trace pci\* we will find that the 0xc3 value we wrote
above, configured an MMIO range at 0xc300000000. That is why the MMIO
accesses are close to that address.

write 0x5c 0x1 0x10
^^^ This I am guessing is a DMA command. Usually I know this simply by
looking at the [DMA] annotations in the input file to
reorder_fuzzer_qtest_trace.py. This just means that the device tried to
read from this location in memory, so the fuzzer placed some data there.

Beyond just broadly seeing that there are some initial PCI
configurations on registers 0xCF8/0xCFC, some accesses to addresses that
look like an MMIO range, and one line that probably puts one byte at
address 0x5c in ram, I can't really tell anything else just by looking
at the trace. To write the descriptions above, I had to look at
PCI-related resources. Im not convinced that reordering the accesses
will really help much with this. Probably the best aid I found for
understanding traces are good trace events (when they exist).

-Alex

> 
> > 
> > > +    i = len(newtrace) - 1
> > > +    while i >= 0:
> > > +        tmp_i = newtrace[i]
> > > +        if len(tmp_i) < 2:
> > > +            i -= 1
> > > +            continue
> > > +        print("Delaying ", newtrace[i])
> > > +        for j in reversed(range(i+1, len(newtrace)+1)):
> > > +            newtrace.insert(j, tmp_i)
> > > +            del newtrace[i]
> > > +            if check_if_trace_crashes(newtrace, outpath):
> > > +                break
> > > +            newtrace.insert(i, tmp_i)
> > > +            del newtrace[j]
> > > +        i -= 1
> > > +
> > > +    assert(check_if_trace_crashes(newtrace, outpath))
> > > +    # maybe another removing round
> > > +
> > >  
> > >  if __name__ == '__main__':
> > >      if len(sys.argv) < 3:
> > > -- 
> > > 2.25.1
> > > 
>