mmap: Add an exception to the stack gap for Hotspot JVM compatibility

The Hotspot JVM can easily exhaust the default stack, and has a
SIGSEGV handler to cope with this by switching to a new stack segment.

However, on i386 it creates a single writable and executable page just
under the stack limit as a workaround for a bug in Exec Shield.  That
together with the enlarged stack gap causes the SIGSEGV handler to be
triggered when the stack pointer is further away from the stack limit,
and it doesn't recognise this as being a stack overflow.

This specifically affects programs that use JNI.  Hotspot doesn't
normally run Java code on the initial thread.

Reduce the effective stack guard gap on x86 if the previous vma is
a single page allocated as MAP_FIXED.

References: https://bugs.debian.org/865303
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Gbp-Pq: Topic bugfix/x86
Gbp-Pq: Name mmap-add-an-exception-to-the-stack-gap-for-hotspot-jvm.patch

diff --git a/mm/mmap.c b/mm/mmap.c
index 02ee60622ec5b2e2ff2074bebec05d1ce151a755..508afe5ebf05c20e81c1df20441fd321ff8686c6 100644 (file)
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2402,6 +2402,16 @@ int expand_downwards(struct vm_area_struct *vma,
        /* Check that both stack segments have the same anon_vma? */
        if (prev && !(prev->vm_flags & VM_GROWSDOWN) &&
                        (prev->vm_flags & (VM_WRITE|VM_READ|VM_EXEC))) {
+               /*
+                * bwh: Reduce the stack guard gap if this looks like
+                * Hotspot JVM craziness - see Debian bug #865303
+                */
+               if (IS_ENABLED(CONFIG_X86) && (prev->vm_flags & VM_FIXED) &&
+                   prev->vm_end - prev->vm_start == PAGE_SIZE) {
+                       if (address - prev->vm_end <
+                           min(stack_guard_gap, 4UL << PAGE_SHIFT))
+                               return -ENOMEM;
+               } else
                if (address - prev->vm_end < stack_guard_gap)
                        return -ENOMEM;
        }