FDPIC: Respect PT_GNU_STACK exec protection markings when creating NOMMU stack

The current code will load the stack size and protection markings, but
then only use the markings in the MMU code path.  The NOMMU code path
always passes PROT_EXEC to the mmap() call.  While this doesn't matter
to most people whilst the code is running, it will cause a pointless
icache flush when starting every FDPIC application.  Typically this
icache flush will be of a region on the order of 128KB in size, or may
be the entire icache, depending on the facilities available on the CPU.

In the case where the arch default behaviour seems to be desired
(EXSTACK_DEFAULT), we probe VM_STACK_FLAGS for VM_EXEC to determine
whether we should be setting PROT_EXEC or not.

For arches that support an MPU (Memory Protection Unit - an MMU without
the virtual mapping capability), setting PROT_EXEC or not will make an
important difference.

It should be noted that this change also affects the executability of
the brk region, since ELF-FDPIC has that share with the stack.  However,
this is probably irrelevant as NOMMU programs aren't likely to use the
brk region, preferring instead allocation via mmap().

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 11 insertions, 2 deletions

diff --git a/fs/binfmt_elf_fdpic.c b/fs/binfmt_elf_fdpic.c
index 7dc85997e96c..c57d9ce5ff7e 100644
--- a/fs/binfmt_elf_fdpic.c
+++ b/fs/binfmt_elf_fdpic.c
@@ -171,6 +171,9 @@ static int load_elf_fdpic_binary(struct linux_binprm *bprm,
 #ifdef ELF_FDPIC_PLAT_INIT
         unsigned long dynaddr;
 #endif
+#ifndef CONFIG_MMU
+        unsigned long stack_prot;
+#endif
         struct file *interpreter = NULL; /* to shut gcc up */
         char *interpreter_name = NULL;
         int executable_stack;
@@ -316,6 +319,8 @@ static int load_elf_fdpic_binary(struct linux_binprm *bprm,
          * defunct, deceased, etc. after this point we have to exit via
          * error_kill */
         set_personality(PER_LINUX_FDPIC);
+        if (elf_read_implies_exec(&exec_params.hdr, executable_stack))
+                current->personality |= READ_IMPLIES_EXEC;
         set_binfmt(&elf_fdpic_format);
 
         current->mm->start_code = 0;
@@ -377,9 +382,13 @@ static int load_elf_fdpic_binary(struct linux_binprm *bprm,
         if (stack_size < PAGE_SIZE * 2)
                 stack_size = PAGE_SIZE * 2;
 
+        stack_prot = PROT_READ | PROT_WRITE;
+        if (executable_stack == EXSTACK_ENABLE_X ||
+            (executable_stack == EXSTACK_DEFAULT && VM_STACK_FLAGS & VM_EXEC))
+                stack_prot |= PROT_EXEC;
+
         down_write(&current->mm->mmap_sem);
-        current->mm->start_brk = do_mmap(NULL, 0, stack_size,
-                                         PROT_READ | PROT_WRITE | PROT_EXEC,
+        current->mm->start_brk = do_mmap(NULL, 0, stack_size, stack_prot,
                                          MAP_PRIVATE | MAP_ANONYMOUS |
                                          MAP_UNINITIALIZED | MAP_GROWSDOWN,
                                          0);