if the corruption is caused by a context switch the problem can be
caused by the kernel.
try the following and disable "CONFIG_KERNEL_MODE_NEON"
in the kernel config. this will disable some kernel crypto assembly code
Am 24.03.2020 um 16:11 schrieb Matt Johnston:
Good work narrowing down a test case there.
That's an interesting finding - I guess it might be worth posting on
OpenWRT lists/forum to try find other testers.
Could it be power related if the tight multiplication loop is
stressing it somehow? It doesn't seem to be using the Neon instruction
for anything apart from loads/stores though - is there something that
the compiler should be doing mixing Neon and non-Neon operations?
Cheers,
Matt
(Your emails got held up being over 100kB, I've trimmed the reply
below and let them through. Apologies to everyone for the stale old
one that got let through with them just now, I wasn't looking closely)
On Tue 24/3/2020, at 11:23 am, Horshack <horsh...@live.com
<mailto:horsh...@live.com>> wrote:
I was able to isolate the issue to just a handful of assembly
instructions within fast_s_mp_sqr(), related to the squaring loop. I
broke that code out into a separate utility that reproduces the issue
within a few seconds. The failure is somewhat sensitive to the data
pattern and very sensitive to timing, indicating a likely memory/data
path issue within my particular router. I'm guessing it's the IPQ8065
and not the SDRAM because I can get it to fail with a tiny data set
easily fits within DCACHE. I can alter the frequency of the failure
with a single ARM memory barrier instruction, which at first implied
a superscalar data ordering condition but the memory barrier also
alters the timing through the DCACHE so that is likely the effect
it's having. I was able to exclude the VFP/Neon register corruption
as the cause with some test code. I also excluded any context
switch-speciifc issue by measuring the # of context switches in
/proc/<pid>/status and catching a failure where no switches had
occurred. I also modified the affinity so the utility runs on just
one processor to rule out a specific core having the issue.
I put the source and binary of my utility on github - if anyone on
this mailing list has this model router can you give it a try if
possible? You only need the ipq8065-sqrbug (binary) and
run-ipq8065-sqrbug.sh (script). Here's the link to the
repository:https://github.com/horshack-dpreview/ipq8065-sqrbug
<https://github.com/horshack-dpreview/ipq8065-sqrbug>
------------------------------------------------------------------------
*From:*Horshack <horsh...@live.com <mailto:horsh...@live.com>>
*Sent:*Saturday, March 21, 2020 7:54 AM
*To:*dropbear@ucc.asn.au
<mailto:dropbear@ucc.asn.au><dropbear@ucc.asn.au
<mailto:dropbear@ucc.asn.au>>
*Subject:*SSH key exchange fails 30-70% of the time on Netgear X4S R7800
Including mailing list for my last two messages below...
Begin forwarded message:
*From:*Horshack <horsh...@live.com <mailto:horsh...@live.com>>
*Date:*March 21, 2020 at 7:35:18 AM PDT
*To:*Matt Johnston <m...@ucc.asn.au <mailto:m...@ucc.asn.au>>
*Cc:*"dropbear@ucc.asn.au <mailto:dropbear@ucc.asn.au>"
<dropbear@ucc.asn.au <mailto:dropbear@ucc.asn.au>>
*Subject:**Re: SSH key exchange fails 30-70% of the time on Netgear
X4S R7800*
Disassembly of fast_s_mp_sqr() and other libtommath functions
reveals gcc is utilizing the arm NEON SIMD instructions and
registers for calculations involved with libtommath's mp_word
scalar. Based on the 64-bit word corruption I see I'm guessing the
SIMD registers aren't being preserved/restored properly somewhere,
probably during a context switch, specifically s16–s31 (d8–d15,
q4–q7), which AAPCS says must be preserved and which I see being
used in the disassembly of fast_s_mp_sqr(). I'lll write some test
code later today to see if this is the case, and if so, try to track
down where and why the registers aren't being preserved.
------------------------------------------------------------------------
*From:*Horshack <horsh...@live.com <mailto:horsh...@live.com>>
*Sent:*Saturday, March 21, 2020 1:11 AM
*To:*Matt Johnston <m...@ucc.asn.au <mailto:m...@ucc.asn.au>>
*Cc:*dropbear@ucc.asn.au <mailto:dropbear@ucc.asn.au>
<dropbear@ucc.asn.au <mailto:dropbear@ucc.asn.au>>
*Subject:*Re: SSH key exchange fails 30-70% of the time on Netgear
X4S R7800
I have one of the failure paths isolated down to a single corrupt
64-bit word in memory, which required a significant amount of code
instrumentation to achieve. I implemented a code execution history
buffer that gets filled at various checkpoints within s_mp_exptmod()
and some of the modules called by it. To facilitate this history
mechanism I packaged all of s_mp_exptmod()'s local variables inside
a structure , which consists of saving the local scalar vars in
addition to crc32's of all the mp_int data structures with a
separate crc32 of the mp_int.dp payload (data). When a failure
occurs, ie one or more of the three back-to-back debug invocations
of s_mp_exptmod yields a mismatching signed key result, I dump out
the history elements for each of the invocations to determine the
first code checkpoint where failing invocation departed from the
known correct invocation.
*snipped*