make pthread stacks non-executable

this change is necessary or pthread_create will always fail on
security-hardened kernels. i considered first trying to make the stack
executable and simply retrying without execute permissions when the
first try fails, but (1) this would incur a serious performance
penalty on hardened systems, and (2) having the stack be executable is
just a bad idea from a security standpoint.

if there is real-world "GNU C" code that uses nested functions with
threads, and it can't be fixed, we'll have to consider other ways of
solving the problem, but for now this seems like the best fix.

diff --git a/src/thread/pthread_create.c b/src/thread/pthread_create.c
index c3b65ae955ab2b3757a32cf4f1597b87fb669361..917be54f2fb6d4c06bf3ebf9fffcd877af511cdd 100644 (file)
--- a/src/thread/pthread_create.c
+++ b/src/thread/pthread_create.c
@@ -104,7 +104,7 @@ int pthread_create(pthread_t *res, const pthread_attr_t *attr, void *(*entry)(vo
                size = guard + ROUND(attr->_a_stacksize + DEFAULT_STACK_SIZE);
        }
        size += __pthread_tsd_size;
-       map = mmap(0, size, PROT_READ|PROT_WRITE|PROT_EXEC, MAP_PRIVATE|MAP_ANON, -1, 0);
+       map = mmap(0, size, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANON, -1, 0);
        if (map == MAP_FAILED) return EAGAIN;
        if (guard) mprotect(map, guard, PROT_NONE);