linux-kernel - preventing executable stack with file

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Date: Tue, 16 Jan 2024 15:37:45 +0300
From: Dmitry Mastykin <dmastykin@...ralinux.ru>
To: linux-kernel@...r.kernel.org, linux-security-module@...r.kernel.org,
 linux-mm@...ck.org
Cc: stephen.smalley.work@...il.com, aaw@...gle.com
Subject: preventing executable stack with file_mprotect hook

Hello all,

I use the file_mprotect hook to prevent executable stack. It's called 
from mprotect syscall and prevents linkage with execstack-flagged 
libraries. But I don't see it called when I execute a simple 
execstack-flagged binary: int main() { char shell[100] = "\xb0\x01" // 
mov al, 1 "\x31\xdb" // xor ebx, ebx "\xcd\x80" ; // int 0x80 
((void(*)())shell)(); return 0; } I'm thinking about a patch like one in 
the end of this message. I would be glad to have a feedback, if someone 
find this reasonable. Thank you! Kind regards Dmitry Mastykin

diff --git a/fs/exec.c b/fs/exec.c
index cebfe15bbad8..0288f14f11b2 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -50,6 +50,7 @@
  #include <linux/module.h>
  #include <linux/namei.h>
  #include <linux/mount.h>
+#include <linux/mman.h>
  #include <linux/security.h>
  #include <linux/syscalls.h>
  #include <linux/tsacct_kern.h>
@@ -759,6 +760,7 @@ int setup_arg_pages(struct linux_binprm *bprm,
  	struct vm_area_struct *vma = bprm->vma;
  	struct vm_area_struct *prev = NULL;
  	unsigned long vm_flags;
+	unsigned long prot = 0;
  	unsigned long stack_base;
  	unsigned long stack_size;
  	unsigned long stack_expand;
@@ -811,16 +813,19 @@ int setup_arg_pages(struct linux_binprm *bprm,
  	 * EXSTACK_ENABLE_X, disable for EXSTACK_DISABLE_X and leave alone
  	 * (arch default) otherwise.
  	 */
-	if (unlikely(executable_stack == EXSTACK_ENABLE_X))
+	if (unlikely(executable_stack == EXSTACK_ENABLE_X)) {
+		prot |= PROT_EXEC;
  		vm_flags |= VM_EXEC;
-	else if (executable_stack == EXSTACK_DISABLE_X)
+	} else if (executable_stack == EXSTACK_DISABLE_X)
  		vm_flags &= ~VM_EXEC;
  	vm_flags |= mm->def_flags;
  	vm_flags |= VM_STACK_INCOMPLETE_SETUP;
  
  	tlb_gather_mmu(&tlb, mm);
-	ret = mprotect_fixup(&tlb, vma, &prev, vma->vm_start, vma->vm_end,
-			vm_flags);
+	ret = security_file_mprotect(vma, prot, prot);
+	if (!ret)
+		ret = mprotect_fixup(&tlb, vma, &prev,
+				     vma->vm_start, vma->vm_end, vm_flags);
  	tlb_finish_mmu(&tlb);
  
  	if (ret)