Patch "x86/pkeys: Check against max pkey to avoid overflows" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

x86/pkeys: Check against max pkey to avoid overflows

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
x86-pkeys-check-against-max-pkey-to-avoid-overflows.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Dave Hansen <[email protected]>
Date: Thu, 23 Feb 2017 14:26:03 -0800
Subject: x86/pkeys: Check against max pkey to avoid overflows

From: Dave Hansen <[email protected]>

commit 58ab9a088ddac4efe823471275859d64f735577e upstream.

Kirill reported a warning from UBSAN about undefined behavior when using protection keys. He is running on hardware that actually has support for it, which is not widely available.

The warning triggers because of very large shifts of integers when doing a pkey_free() of a large, invalid value. This happens because we never check that the pkey "fits" into the mm_pkey_allocation_map().

I do not believe there is any danger here of anything bad happening other than some aliasing issues where somebody could do:

	pkey_free(35);

and the kernel would effectively execute:

	pkey_free(8);

While this might be confusing to an app that was doing something stupid, it has to do something stupid and the effects are limited to the app shooting itself in the foot.

---
 arch/x86/include/asm/pkeys.h |   15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

@@ -46,6 +46,15 @@ extern int __arch_set_user_pkey_access(s
 static inline
 bool mm_pkey_is_allocated(struct mm_struct *mm, int pkey)
 {
+	/*
+	 * "Allocated" pkeys are those that have been returned
+	 * from pkey_alloc().  pkey 0 is special, and never
+	 * returned from pkey_alloc().
+	 */
+	if (pkey <= 0)
+		return false;
+	if (pkey >= arch_max_pkey())
+		return false;
 	return mm_pkey_allocation_map(mm) & (1U << pkey);
 }
 

@@ -82,12 +91,6 @@ int mm_pkey_alloc(struct mm_struct *mm)
 static inline
 int mm_pkey_free(struct mm_struct *mm, int pkey)
 {
-	/*
-	 * pkey 0 is special, always allocated and can never
-	 * be freed.
-	 */
-	if (!pkey)
-		return -EINVAL;
 	if (!mm_pkey_is_allocated(mm, pkey))
 		return -EINVAL;
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/sigaltstack-support-ss_autodisarm-for-config_compat.patch queue-4.9/x86-pkeys-check-against-max-pkey-to-avoid-overflows.patch