x86/pkeys: Do not special case protection key 0

mm_pkey_is_allocated() treats pkey 0 as unallocated.  That is
inconsistent with the manpages, and also inconsistent with
mm->context.pkey_allocation_map.  Stop special casing it and only
disallow values that are actually bad (< 0).

The end-user visible effect of this is that you can now use
mprotect_pkey() to set pkey=0.

This is a bit nicer than what Ram proposed[1] because it is simpler
and removes special-casing for pkey 0.  On the other hand, it does
allow applications to pkey_free() pkey-0, but that's just a silly
thing to do, so we are not going to protect against it.

The scenario that could happen is similar to what happens if you free
any other pkey that is in use: it might get reallocated later and used
to protect some other data.  The most likely scenario is that pkey-0
comes back from pkey_alloc(), an access-disable or write-disable bit
is set in PKRU for it, and the next stack access will SIGSEGV.  It's
not horribly different from if you mprotect()'d your stack or heap to
be unreadable or unwritable, which is generally very foolish, but also
not explicitly prevented by the kernel.

1. http://lkml.kernel.org/r/1522112702-27853-1-git-send-email-linuxram@us.ibm.com

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>p
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Michael Ellermen <mpe@ellerman.id.au>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ram Pai <linuxram@us.ibm.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-mm@kvack.org
Cc: stable@vger.kernel.org
Fixes: 58ab9a088dda ("x86/pkeys: Check against max pkey to avoid overflows")
Link: http://lkml.kernel.org/r/20180509171358.47FD785E@viggo.jf.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
index 57e3785d0d26e19f9134485399d1b3aac01b3a2b..cf9911b5a53cb1de017a27cf20f866e42d2199fd 100644 (file)
--- a/arch/x86/include/asm/mmu_context.h
+++ b/arch/x86/include/asm/mmu_context.h
@@ -193,7 +193,7 @@ static inline int init_new_context(struct task_struct *tsk,
 
 #ifdef CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS
        if (cpu_feature_enabled(X86_FEATURE_OSPKE)) {
-               /* pkey 0 is the default and always allocated */
+               /* pkey 0 is the default and allocated implicitly */
                mm->context.pkey_allocation_map = 0x1;
                /* -1 means unallocated or invalid */
                mm->context.execute_only_pkey = -1;

diff --git a/arch/x86/include/asm/pkeys.h b/arch/x86/include/asm/pkeys.h
index 39cd292805a900a7d15c7c6680e1ef45dad23e21..851c04b7a0922cd3ea79f3a574246b6a94262d40 100644 (file)
--- a/arch/x86/include/asm/pkeys.h
+++ b/arch/x86/include/asm/pkeys.h
@@ -51,10 +51,10 @@ 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().
+        * from pkey_alloc() or pkey 0 which is allocated
+        * implicitly when the mm is created.
         */
-       if (pkey <= 0)
+       if (pkey < 0)
                return false;
        if (pkey >= arch_max_pkey())
                return false;