[PATCH] x86_64: access of some bad address

x86_64 has a large sparse gate area between VSYSCALL_START and
VSYSCALL_END, not all of it presently backed by pmds.  Alexander Nyberg has
found that in some circumstances gdb may try to ptrace here, and hit
get_user_pages BUG_ON.  It seems odd that gdb should be accessing here, but
it certainly shouldn't crash in this way: relax BUG_ON to -EFAULT.  Fixes
kernel bugzilla #4801.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Cc: Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 6 insertions, 2 deletions

diff --git a/mm/memory.c b/mm/memory.c
index 4e1c673784db..2405289dfdf8 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -910,9 +910,13 @@ int get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
                         pud = pud_offset(pgd, pg);
                         BUG_ON(pud_none(*pud));
                         pmd = pmd_offset(pud, pg);
-                        BUG_ON(pmd_none(*pmd));
+                        if (pmd_none(*pmd))
+                                return i ? : -EFAULT;
                         pte = pte_offset_map(pmd, pg);
-                        BUG_ON(pte_none(*pte));
+                        if (pte_none(*pte)) {
+                                pte_unmap(pte);
+                                return i ? : -EFAULT;
+                        }
                         if (pages) {
                                 pages[i] = pte_page(*pte);
                                 get_page(pages[i]);