From: Dave Hansen dave.hansen@linux.intel.com
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.c...
Signed-off-by: Dave Hansen dave.hansen@linux.intel.com Fixes: 58ab9a088dda ("x86/pkeys: Check against max pkey to avoid overflows") Cc: stable@vger.kernel.org Cc: Ram Pai linuxram@us.ibm.com Cc: Thomas Gleixner tglx@linutronix.de Cc: Dave Hansen dave.hansen@intel.com Cc: Michael Ellermen mpe@ellerman.id.au Cc: Ingo Molnar mingo@kernel.org Cc: Andrew Morton akpm@linux-foundation.orgp Cc: Shuah Khan shuah@kernel.org ---
b/arch/x86/include/asm/mmu_context.h | 2 +- b/arch/x86/include/asm/pkeys.h | 6 +++--- 2 files changed, 4 insertions(+), 4 deletions(-)
diff -puN arch/x86/include/asm/mmu_context.h~x86-pkey-0-default-allocated arch/x86/include/asm/mmu_context.h --- a/arch/x86/include/asm/mmu_context.h~x86-pkey-0-default-allocated 2018-05-09 09:20:24.362698393 -0700 +++ b/arch/x86/include/asm/mmu_context.h 2018-05-09 09:20:24.367698393 -0700 @@ -193,7 +193,7 @@ static inline int init_new_context(struc
#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 -puN arch/x86/include/asm/pkeys.h~x86-pkey-0-default-allocated arch/x86/include/asm/pkeys.h --- a/arch/x86/include/asm/pkeys.h~x86-pkey-0-default-allocated 2018-05-09 09:20:24.364698393 -0700 +++ b/arch/x86/include/asm/pkeys.h 2018-05-09 09:20:24.367698393 -0700 @@ -51,10 +51,10 @@ bool mm_pkey_is_allocated(struct mm_stru { /* * "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; _