* ARM per-thread stack protector
@ 2013-07-19 16:34 Will Newton
2013-07-19 21:41 ` Roland McGrath
0 siblings, 1 reply; 4+ messages in thread
From: Will Newton @ 2013-07-19 16:34 UTC (permalink / raw)
To: libc-ports
Hi all,
I've been looking into implementing pre-thread stack protector
canaries for ARM and I would be interested in people's opinions on
whether I have understood it correctly.
At the moment the global canary value is stored in __stack_chk_guard
which is exported by glibc and accesses to this variable are emitted
by gcc if it detects a capable glibc version at configure time.
glibc with per-thread stack canary values does not export
__stack_chk_guard but adds an element to the TCB to contain the
per-thread value and gcc emits TP-relative accesses to load this
value.
Implementing the per-thread scheme would therefore seem to me to break
ABI compatibility and cause problems when mixing gcc and glibc
versions:
Old gcc, old glibc: OK
Old gcc, new glibc: __stack_chk_guard is missing, link time failure.
New gcc, old glibc: stack canary value is loaded from uninitialized
TCB area, security issue.
New gcc, new glibc: OK.
It should be possible to add support for both schemes at the same time
in glibc, exporting __stack_chk_guard at the same time as supporting
per-thread canary values, which would fix the "old gcc, new glibc"
case.
I am not sure if there is a good fix for "new gcc, old glibc",
although gcc configure could be taught about glibc versions and do the
right thing for each.
Or is there a simpler way to handle this? Has any other architecture
implemented per-thread stack protector after already supporting the
simpler scheme?
Thanks,
--
Will Newton
Toolchain Working Group, Linaro
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: ARM per-thread stack protector
2013-07-19 16:34 ARM per-thread stack protector Will Newton
@ 2013-07-19 21:41 ` Roland McGrath
2013-07-22 9:15 ` Will Newton
0 siblings, 1 reply; 4+ messages in thread
From: Roland McGrath @ 2013-07-19 21:41 UTC (permalink / raw)
To: Will Newton; +Cc: libc-ports
No machine has a per-thread canary value. Using one would break
user-implemented stack-switching schemes.
Some machines store the global canary value in every thread's TCB for the
sole reason that it's cheaper to access a field in the TCB than to access a
global variable. I don't think that's the case on ARM.
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: ARM per-thread stack protector
2013-07-19 21:41 ` Roland McGrath
@ 2013-07-22 9:15 ` Will Newton
2013-07-22 22:01 ` Roland McGrath
0 siblings, 1 reply; 4+ messages in thread
From: Will Newton @ 2013-07-22 9:15 UTC (permalink / raw)
To: Roland McGrath; +Cc: libc-ports
On 19 July 2013 22:41, Roland McGrath <roland@hack.frob.com> wrote:
Hi Roland,
> No machine has a per-thread canary value. Using one would break
> user-implemented stack-switching schemes.
>
> Some machines store the global canary value in every thread's TCB for the
> sole reason that it's cheaper to access a field in the TCB than to access a
> global variable. I don't think that's the case on ARM.
Thanks for the clarification. So the two methods are functionally identical?
I looked at this problem initially after reading Carlos's mail to the
fedora devel list:
http://www.mail-archive.com/devel@lists.fedoraproject.org/msg62354.html
From what I understand then all that is missing on ARM from that list
is pointer mangling support?
Thanks,
--
Will Newton
Toolchain Working Group, Linaro
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: ARM per-thread stack protector
2013-07-22 9:15 ` Will Newton
@ 2013-07-22 22:01 ` Roland McGrath
0 siblings, 0 replies; 4+ messages in thread
From: Roland McGrath @ 2013-07-22 22:01 UTC (permalink / raw)
To: Will Newton; +Cc: libc-ports
> Thanks for the clarification. So the two methods are functionally identical?
Yes. Where you keep the value is just a matter of optimal implementation
convenience for the particular machine.
> >From what I understand then all that is missing on ARM from that list
> is pointer mangling support?
I have no new information.
^ permalink raw reply [flat|nested] 4+ messages in thread
end of thread, other threads:[~2013-07-22 22:01 UTC | newest]
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2013-07-19 16:34 ARM per-thread stack protector Will Newton
2013-07-19 21:41 ` Roland McGrath
2013-07-22 9:15 ` Will Newton
2013-07-22 22:01 ` Roland McGrath
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