selinux: revert our use of vma_is_initial_heap()

Unfortunately it appears that vma_is_initial_heap() is currently broken
for applications that do not currently have any heap allocated, e.g.
brk == start_brk.  The breakage is such that it will cause SELinux to
check for the process/execheap permission on memory regions that cross
brk/start_brk even when there is no heap.

The proper fix would be to correct vma_is_initial_heap(), but as there
are multiple callers I am hesitant to unilaterally modify the helper
out of concern that I would end up breaking some other subsystem.  The
mm developers have been made aware of the situation and hopefully they
will have a fix at some point in the future, but we need a fix soon so
we are simply going to revert our use of vma_is_initial_heap() in favor
of our old logic/code which works as expected, even in the face of a
zero size heap.  We can return to using vma_is_initial_heap() at some
point in the future when it is fixed.

Cc: stable@vger.kernel.org
Reported-by: Marc Reisner <reisner.marc@gmail.com>
Closes: https://lore.kernel.org/all/ZrPmoLKJEf1wiFmM@marcreisner.com
Fixes: 68df1baf158f ("selinux: use vma_is_initial_stack() and vma_is_initial_heap()")
Signed-off-by: Paul Moore <paul@paul-moore.com>

diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 55c78c318ccd78b8dcec24886c0ff5e919c67b85..bfa61e005aace203b4e66f799285f22c33e08902 100644 (file)
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -3852,7 +3852,17 @@ static int selinux_file_mprotect(struct vm_area_struct *vma,
        if (default_noexec &&
            (prot & PROT_EXEC) && !(vma->vm_flags & VM_EXEC)) {
                int rc = 0;
-               if (vma_is_initial_heap(vma)) {
+               /*
+                * We don't use the vma_is_initial_heap() helper as it has
+                * a history of problems and is currently broken on systems
+                * where there is no heap, e.g. brk == start_brk.  Before
+                * replacing the conditional below with vma_is_initial_heap(),
+                * or something similar, please ensure that the logic is the
+                * same as what we have below or you have tested every possible
+                * corner case you can think to test.
+                */
+               if (vma->vm_start >= vma->vm_mm->start_brk &&
+                   vma->vm_end <= vma->vm_mm->brk) {
                        rc = avc_has_perm(sid, sid, SECCLASS_PROCESS,
                                          PROCESS__EXECHEAP, NULL);
                } else if (!vma->vm_file && (vma_is_initial_stack(vma) ||