[PATCH v2] mm/mprotect.c: don't imply PROT_EXEC on non-exec fs

[Piotr Kwapulinski]

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@xxxxxxxxx>
---
The difference between v1 is that the prot variable is reset to
 reqprot for each loop iteration (thanks to Konstantin Khlebnikov for
pointing this out).
rier means "(current->personality & [R]EAD_[I]MPLIES_[E]XEC) &&
(prot & PROT_[R]EAD)".

 mm/mprotect.c | 18 +++++++++---------
 1 file changed, 9 insertions(+), 9 deletions(-)

diff --git a/mm/mprotect.c b/mm/mprotect.c
index 8eb7bb4..1b9597f 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -352,10 +352,12 @@ fail:
 SYSCALL_DEFINE3(mprotect, unsigned long, start, size_t, len,
 		unsigned long, prot)
 {
-	unsigned long vm_flags, nstart, end, tmp, reqprot;
+	unsigned long nstart, end, tmp, reqprot;
 	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;
@@ -372,13 +374,6 @@ 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;
-
-	vm_flags = calc_vm_prot_bits(prot);
 
 	down_write(&current->mm->mmap_sem);
 
@@ -412,7 +407,11 @@ SYSCALL_DEFINE3(mprotect, unsigned long, start, size_t, len,
 
 		/* Here we know that vma->vm_start <= nstart < vma->vm_end. */
 
-		newflags = vm_flags;
+		/* 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);
 		newflags |= (vma->vm_flags & ~(VM_READ | VM_WRITE | VM_EXEC));
 
 		/* newflags >> 4 shift VM_MAY% in place of VM_% */
@@ -443,6 +442,7 @@ SYSCALL_DEFINE3(mprotect, unsigned long, start, size_t, len,
 			error = -ENOMEM;
 			goto out;
 		}
+		prot = reqprot;
 	}
 out:
 	up_write(&current->mm->mmap_sem);
-- 
2.7.0