x86/kprobes: Set instruction page as executable

Set the page as executable after allocation.  This patch is a
preparatory patch for a following patch that makes module allocated
pages non-executable.

While at it, do some small cleanup of what appears to be unnecessary
masking.

Signed-off-by: Nadav Amit <namit@vmware.com>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: <akpm@linux-foundation.org>
Cc: <ard.biesheuvel@linaro.org>
Cc: <deneen.t.dock@intel.com>
Cc: <kernel-hardening@lists.openwall.com>
Cc: <kristen@linux.intel.com>
Cc: <linux_dti@icloud.com>
Cc: <will.deacon@arm.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Rik van Riel <riel@surriel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20190426001143.4983-11-namit@vmware.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
index fed46dd..06058c4 100644 (file)
--- a/arch/x86/kernel/kprobes/core.c
+++ b/arch/x86/kernel/kprobes/core.c
@@ -431,8 +431,20 @@ void *alloc_insn_page(void)
        void *page;
 
        page = module_alloc(PAGE_SIZE);
-       if (page)
-               set_memory_ro((unsigned long)page & PAGE_MASK, 1);
+       if (!page)
+               return NULL;
+
+       /*
+        * First make the page read-only, and only then make it executable to
+        * prevent it from being W+X in between.
+        */
+       set_memory_ro((unsigned long)page, 1);
+
+       /*
+        * TODO: Once additional kernel code protection mechanisms are set, ensure
+        * that the page was not maliciously altered and it is still zeroed.
+        */
+       set_memory_x((unsigned long)page, 1);
 
        return page;
 }
@@ -440,8 +452,12 @@ void *alloc_insn_page(void)
 /* Recover page to RW mode before releasing it */
 void free_insn_page(void *page)
 {
-       set_memory_nx((unsigned long)page & PAGE_MASK, 1);
-       set_memory_rw((unsigned long)page & PAGE_MASK, 1);
+       /*
+        * First make the page non-executable, and only then make it writable to
+        * prevent it from being W+X in between.
+        */
+       set_memory_nx((unsigned long)page, 1);
+       set_memory_rw((unsigned long)page, 1);
        module_memfree(page);
 }