[Bug nptl/14609] New: Stack clobbering in pthread cancellation cleanup handlers

["bugdal at aerifal dot cx" <sourceware-bugzilla at sourceware dot org>]

http://sourceware.org/bugzilla/show_bug.cgi?id=14609

             Bug #: 14609
           Summary: Stack clobbering in pthread cancellation cleanup
                    handlers
           Product: glibc
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: nptl
        AssignedTo: unassigned@sourceware.org
        ReportedBy: bugdal@aerifal.cx
                CC: drepper.fsp@gmail.com
    Classification: Unclassified


Created attachment 6650
  --> http://sourceware.org/bugzilla/attachment.cgi?id=6650
test case demonstrating crash due to this bug

As they are specified in POSIX, cancellation cleanup handlers are simply
"invoked" when cancellation is acted upon, but nowhere is it specified that the
block scope of the function from which the cancellation point is called
terminates prematurely any time during the cleanup process, or that the block
scope of one cleanup push/pop is terminates before the next cleanup handler is
run. However, glibc's implementation of cleanup in terms of C++-style
exceptions results in cleanup handlers running as exceptions in the block where
they were installed, rather than merely being invoked. This means that objects
used by the cleanup handler may no longer exist at the time the cleanup handler
runs, and attempts to write to them will clobber the new stack frame in which
the cleanup handler is running.

I'm attaching a working proof-of-concept that demonstrates the issue by
clobbering its own stack and crashing, even though it has not done anything
that the POSIX standard disallows. It's rather ugly and unnatural since I wrote
it to ensure that the crash happens, not to mimic code that might appear in the
real world. A more real-world example is something like:

char small_bug[SMALL_SIZE];
struct context ctx = { .buf = small_buf };
pthread_cleanup_push(cleanup_func, &ctx);
...
if (...) {
    char large_buf[LARGE_SIZE];
    ctx->buf = large_buf;
    ...
    ctx->buf = small_buf;
}
...
pthread_cleanup_pop(0);

I'm aware that this bug is inherent in the cancellation design glibc chose to
use, and is not easy to fix, but please do not mark this bug as invalid. If the
glibc team wants to be able to keep this behavior, a defect report process with
the Austin Group based on this glibc bug report should be started, with the
goal of making a rigorous definition of the interaction of cancellation cleanup
handlers and automatic object lifetimes, so that programs like my test case
become non-conforming in the next edition (or TC?) of POSIX.

Short of an amendment to the standard, I believe this bug report is valid.

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