uprobes: fix kernel info leak via "[uprobes]" vma

xol_add_vma() maps the uninitialized page allocated by __create_xol_area()
into userspace. On some architectures (x86) this memory is readable even
without VM_READ, VM_EXEC results in the same pgprot_t as VM_EXEC|VM_READ,
although this doesn't really matter, debugger can read this memory anyway.

Link: https://lore.kernel.org/all/[email protected]/

Reported-by: Will Deacon <[email protected]>
Fixes: d4b3b63 ("uprobes/core: Allocate XOL slots for uprobes use")
Cc: [email protected]
Acked-by: Masami Hiramatsu (Google) <[email protected]>
Signed-off-by: Oleg Nesterov <[email protected]>
Signed-off-by: Masami Hiramatsu (Google) <[email protected]>

Author: oleg-nesterov

kernel/events/uprobes.c

@@ -1545,7 +1545,7 @@ static struct xol_area *__create_xol_area(unsigned long vaddr)
 	if (!area->bitmap)
 		goto free_area;
 
-	area->page = alloc_page(GFP_HIGHUSER);
+	area->page = alloc_page(GFP_HIGHUSER | __GFP_ZERO);
 	if (!area->page)
 		goto free_bitmap;