mm/mprotect.c: don't imply PROT_EXEC on non-exec fs

The mprotect(PROT_READ) fails when called by the READ_IMPLIES_EXEC
binary on a memory mapped file located on non-exec fs.  The mprotect
does not check whether fs is _executable_ or not.  The PROT_EXEC flag is
set automatically even if a memory mapped file is located on non-exec
fs.  Fix it by checking whether a memory mapped file is located on a
non-exec fs.  If so the PROT_EXEC is not implied by the PROT_READ.  The
implementation uses the VM_MAYEXEC flag set properly in mmap.  Now it is
consistent with mmap.

I did the isolated tests (PT_GNU_STACK X/NX, multiple VMAs, X/NX fs).  I
also patched the official 3.19.0-47-generic Ubuntu 14.04 kernel and it
seems to work.
Signed-off-by: Piotr Kwapulinski <kwapulinski.piotr@gmail.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Konstantin Khlebnikov <koct9i@gmail.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/mprotect.c

@@ -359,6 +359,9 @@ SYSCALL_DEFINE3(mprotect, unsigned long, start, size_t, len,
 	struct vm_area_struct *vma, *prev;
 	int error = -EINVAL;
 	const int grows = prot & (PROT_GROWSDOWN|PROT_GROWSUP);
+	const bool rier = (current->personality & READ_IMPLIES_EXEC) &&
+				(prot & PROT_READ);
+
 	prot &= ~(PROT_GROWSDOWN|PROT_GROWSUP);
 	if (grows == (PROT_GROWSDOWN|PROT_GROWSUP)) /* can't be both */
 		return -EINVAL;
@@ -375,11 +378,6 @@ SYSCALL_DEFINE3(mprotect, unsigned long, start, size_t, len,
 		return -EINVAL;
 
 	reqprot = prot;
-	/*
-	 * Does the application expect PROT_READ to imply PROT_EXEC:
-	 */
-	if ((prot & PROT_READ) && (current->personality & READ_IMPLIES_EXEC))
-		prot |= PROT_EXEC;
 
 	down_write(&current->mm->mmap_sem);
 
@@ -414,6 +412,10 @@ SYSCALL_DEFINE3(mprotect, unsigned long, start, size_t, len,
 
 		/* Here we know that vma->vm_start <= nstart < vma->vm_end. */
 
+		/* Does the application expect PROT_READ to imply PROT_EXEC */
+		if (rier && (vma->vm_flags & VM_MAYEXEC))
+			prot |= PROT_EXEC;
+
 		newflags = calc_vm_prot_bits(prot, pkey);
 		newflags |= (vma->vm_flags & ~(VM_READ | VM_WRITE | VM_EXEC));
 
@@ -445,6 +447,7 @@ SYSCALL_DEFINE3(mprotect, unsigned long, start, size_t, len,
 			error = -ENOMEM;
 			goto out;
 		}
+		prot = reqprot;
 	}
 out:
 	up_write(&current->mm->mmap_sem);