x86/kprobes: Set instruction page as executable [Linux 5.2]

x86/kprobes: Set instruction page as executable [Linux 5.2]

This Linux kernel change "x86/kprobes: Set instruction page as executable" is included in the Linux 5.2 release. This change is authored by Nadav Amit <namit [at] vmware.com> on Thu Apr 25 17:11:30 2019 -0700. The commit for this change in Linux stable tree is 7298e24 (patch).

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

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>

There are 24 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 arch/x86/kernel/kprobes/core.c | 24 ++++++++++++++++++++----
 1 file changed, 20 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
index fed46dd..06058c4 100644
--- 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);

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