Bug Report: Dereferencing a bad pointer

[David Chandler]

Bug Report

Summary:
Dereferencing a bad pointer in user space hangs rather than causing a
segmentation fault in 2.4.x kernels.

Keywords:
memory protection address dereference segmentation fault SIGSEGV

Full Description:

The following one-line C program, when compiled by gcc 2.96 without
optimization, should produce a SIGSEGV segmentation fault (on a machine
with 3 or less gigabytes of virtual memory, at least):

        int main() { int k  = *(int *)0xc0000000; }

However, it does not do so under 2.4.x -- it does cause a seg fault
under
2.2.x kernels.

Specifically, no seg fault occurs under kernels 2.4.2-2 (Red Hat build),

2.4.13, 2.4.13UML, 2.4.9UML, or 2.4.8UML.  This one-liner does cause a
seg fault on 2.2.5-15 (Red Hat build) and 2.2.14-5.0 (Red Hat build).
 All these were run on Pentium II, Pentium III, and Pentium 4 chips.
The "UML" kernels are Linus's official releases patched with the
user-mode linux patches and run on a Red Hat 7.1 2.4.2-2 Pentium 4 host;

Tom's rtbt was the UML file system.

Note that UML uses arch/um rather than arch/i386; this seems to remove
some suspicion from 'arch/i386/mm/fault.c', which has changed
considerably from 2.2.x to 2.4.x.

Rather than seg faulting, the 2.4.x kernels just sit at the offensive
dereference until you interrupt the process.  Interruption works
flawlessly; you can use 'kill -INT', 'kill -SEGV' or 'kill -BUS' to
interrupt the process.

Please Cc: me on any responses -- the linux-kernel traffic is too much
for me.

David Chandler

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David L. Chandler.                              GrammaTech, Inc.
mailto:[e-mail]         http://www.grammatech.com