[PATCH v2 1/1] binfmt_elf: mseal address zero

[jeffxu@chromium.org]

From: Jeff Xu <jeffxu@chromium.org>

Some legacy SVr4 apps might depend on page on address zero
to be readable, however I can't find a reason that the page
ever becomes writeable, so seal it.

If there is a compain, we can make this configurable.

Signed-off-by: Jeff Xu <jeffxu@chromium.org>
---
 fs/binfmt_elf.c    |  5 +++++
 include/linux/mm.h | 10 ++++++++++
 mm/mseal.c         |  2 +-
 3 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 19fa49cd9907..f839fa228509 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -1314,6 +1314,11 @@ static int load_elf_binary(struct linux_binprm *bprm)
 		   emulate the SVr4 behavior. Sigh. */
 		error = vm_mmap(NULL, 0, PAGE_SIZE, PROT_READ | PROT_EXEC,
 				MAP_FIXED | MAP_PRIVATE, 0);
+
+		retval = do_mseal(0, PAGE_SIZE, 0);
+		if (retval)
+			pr_warn("pid=%d, couldn't seal address 0, ret=%d.\n",
+					task_pid_nr(current), retval);
 	}
 
 	regs = current_pt_regs();
diff --git a/include/linux/mm.h b/include/linux/mm.h
index c4b238a20b76..a178c15812eb 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -4201,4 +4201,14 @@ void vma_pgtable_walk_end(struct vm_area_struct *vma);
 
 int reserve_mem_find_by_name(const char *name, phys_addr_t *start, phys_addr_t *size);
 
+#ifdef CONFIG_64BIT
+int do_mseal(unsigned long start, size_t len_in, unsigned long flags);
+#else
+static inline int do_mseal(unsigned long start, size_t len_in, unsigned long flags)
+{
+	/* noop on 32 bit */
+	return 0;
+}
+#endif
+
 #endif /* _LINUX_MM_H */
diff --git a/mm/mseal.c b/mm/mseal.c
index bf783bba8ed0..7a40a84569c8 100644
--- a/mm/mseal.c
+++ b/mm/mseal.c
@@ -248,7 +248,7 @@ static int apply_mm_seal(unsigned long start, unsigned long end)
  *
  *  unseal() is not supported.
  */
-static int do_mseal(unsigned long start, size_t len_in, unsigned long flags)
+int do_mseal(unsigned long start, size_t len_in, unsigned long flags)
 {
 	size_t len;
 	int ret = 0;
-- 
2.46.0.rc2.264.g509ed76dc8-goog