From: Dave Hansen <dave@sr71.net>
To: Mel Gorman <mgorman@techsingularity.net>
Cc: linux-kernel@vger.kernel.org, x86@kernel.org,
linux-api@vger.kernel.org, linux-arch@vger.kernel.org,
linux-mm@kvack.org, torvalds@linux-foundation.org,
akpm@linux-foundation.org, dave.hansen@linux.intel.com,
arnd@arndb.de, hughd@google.com, viro@zeniv.linux.org.uk
Subject: Re: [PATCH 2/9] mm: implement new pkey_mprotect() system call
Date: Thu, 7 Jul 2016 09:51:52 -0700 [thread overview]
Message-ID: <577E88A8.8030909@sr71.net> (raw)
In-Reply-To: <20160707144031.GY11498@techsingularity.net>
On 07/07/2016 07:40 AM, Mel Gorman wrote:
> On Thu, Jul 07, 2016 at 05:47:22AM -0700, Dave Hansen wrote:
>> +#ifdef CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS
>> static inline int vma_pkey(struct vm_area_struct *vma)
>> {
>> - u16 pkey = 0;
>> -#ifdef CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS
>> unsigned long vma_pkey_mask = VM_PKEY_BIT0 | VM_PKEY_BIT1 |
>> VM_PKEY_BIT2 | VM_PKEY_BIT3;
>> - pkey = (vma->vm_flags & vma_pkey_mask) >> VM_PKEY_SHIFT;
>> -#endif
>> - return pkey;
>> +
>> + return (vma->vm_flags & vma_pkey_mask) >> VM_PKEY_SHIFT;
>> +}
>> +#else
>> +static inline int vma_pkey(struct vm_area_struct *vma)
>> +{
>> + return 0;
>> }
>> +#endif
>>
>> static inline bool __pkru_allows_pkey(u16 pkey, bool write)
>> {
>
> Looks like MASK could have been statically defined and be a simple shift
> and mask known at compile time. Minor though.
The VM_PKEY_BIT*'s are only ever defined as masks and not bit numbers.
So, if you want to use a mask, you end up doing something like:
unsigned long mask = (NR_PKEYS-1) << ffz(~VM_PKEY_BIT0);
Which ends up with the same thing, but I think ends up being pretty on
par for ugliness.
...
>> +/*
>> + * When setting a userspace-provided value, we need to ensure
>> + * that it is valid. The __ version can get used by
>> + * kernel-internal uses like the execute-only support.
>> + */
>> +int arch_set_user_pkey_access(struct task_struct *tsk, int pkey,
>> + unsigned long init_val)
>> +{
>> + if (!validate_pkey(pkey))
>> + return -EINVAL;
>> + return __arch_set_user_pkey_access(tsk, pkey, init_val);
>> +}
>
> There appears to be a subtle bug fixed for validate_key. It appears
> there wasn't protection of the dedicated key before but nothing could
> reach it.
Right. There was no user interface that took a key and we trusted that
the kernel knew what it was doing.
> The arch_max_pkey and PKEY_DEDICATE_EXECUTE_ONLY interaction is subtle
> but I can't find a problem with it either.
>
> That aside, the validate_pkey check looks weak. It might be a number
> that works but no guarantee it's an allocated key or initialised
> properly. At this point, garbage can be handed into the system call
> potentially but maybe that gets fixed later.
It's called in three paths:
1. by the kernel when setting up execute-only support
2. by pkey_alloc() on the pkey we just allocated
3. by pkey_set() on a pkey we just checked was allocated
So, it isn't broken, but it's also not clear at all why it is safe and
what validate_pkey() is actually validating.
But, that said, this does make me realize that with
pkey_alloc()/pkey_free(), this is probably redundant. We verify that
the key is allocated, and we only allow valid keys to be allocated.
IOW, I think I can remove validate_pkey(), but only if we keep pkey_alloc().
...
>> - newflags = calc_vm_prot_bits(prot, pkey);
>> + new_vma_pkey = arch_override_mprotect_pkey(vma, prot, pkey);
>> + newflags = calc_vm_prot_bits(prot, new_vma_pkey);
>> newflags |= (vma->vm_flags & ~(VM_READ | VM_WRITE | VM_EXEC));
>>
>
> On CPUs that do not support the feature, arch_override_mprotect_pkey
> returns 0 and the normal protections are used. It's not clear how an
> application is meant to detect if the operation succeeded or not. What
> if the application relies on pkeys to be working?
It actually shows up as -ENOSPC from pkey_alloc(). This sounds goofy,
but it teaches programs something very important: they always have to
look for ENOSPC, and must always be prepared to function without
protection keys. A library might have stolen all the keys, or an
LD_PRELOAD, so an app can never be sure what is available.
If we teach them to check for ENOSPC from day one, they'll never be
surprised.
I've tried to spell this out a bit more clearly in the manpages. I'll
also add it to the changelog.
next prev parent reply other threads:[~2016-07-07 16:52 UTC|newest]
Thread overview: 49+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-07-07 12:47 [PATCH 0/9] [REVIEW-REQUEST] [v4] System Calls for Memory Protection Keys Dave Hansen
2016-07-07 12:47 ` [PATCH 1/9] x86, pkeys: add fault handling for PF_PK page fault bit Dave Hansen
2016-07-07 14:40 ` Mel Gorman
2016-07-07 15:42 ` Dave Hansen
2016-07-07 12:47 ` [PATCH 2/9] mm: implement new pkey_mprotect() system call Dave Hansen
2016-07-07 14:40 ` Mel Gorman
2016-07-07 16:51 ` Dave Hansen [this message]
2016-07-08 10:15 ` Mel Gorman
2016-07-07 12:47 ` [PATCH 3/9] x86, pkeys: make mprotect_key() mask off additional vm_flags Dave Hansen
2016-07-07 12:47 ` [PATCH 4/9] x86: wire up mprotect_key() system call Dave Hansen
2016-07-07 12:47 ` [PATCH 5/9] x86, pkeys: allocation/free syscalls Dave Hansen
2016-07-07 14:40 ` Mel Gorman
2016-07-07 15:38 ` Dave Hansen
2016-07-07 12:47 ` [PATCH 6/9] x86, pkeys: add pkey set/get syscalls Dave Hansen
2016-07-07 14:45 ` Mel Gorman
2016-07-07 17:33 ` Dave Hansen
2016-07-08 7:18 ` Ingo Molnar
2016-07-08 16:32 ` Dave Hansen
2016-07-09 8:37 ` Ingo Molnar
2016-07-11 4:25 ` Andy Lutomirski
2016-07-11 7:35 ` Ingo Molnar
2016-07-11 14:28 ` Dave Hansen
2016-07-12 7:13 ` Ingo Molnar
2016-07-12 15:39 ` Dave Hansen
2016-07-11 14:50 ` Andy Lutomirski
2016-07-11 14:34 ` Dave Hansen
2016-07-11 14:45 ` Andy Lutomirski
2016-07-11 15:48 ` Dave Hansen
2016-07-12 16:32 ` Andy Lutomirski
2016-07-12 17:12 ` Dave Hansen
2016-07-12 22:55 ` Andy Lutomirski
2016-07-13 7:56 ` Ingo Molnar
2016-07-13 18:43 ` Andy Lutomirski
2016-07-14 8:07 ` Ingo Molnar
2016-07-18 4:43 ` Andy Lutomirski
2016-07-18 9:56 ` Ingo Molnar
2016-07-18 18:02 ` Dave Hansen
2016-07-18 20:12 ` Dave Hansen
2016-07-08 19:26 ` Dave Hansen
2016-07-08 10:22 ` Mel Gorman
2016-07-07 12:47 ` [PATCH 7/9] generic syscalls: wire up memory protection keys syscalls Dave Hansen
2016-07-07 12:47 ` [PATCH 8/9] pkeys: add details of system call use to Documentation/ Dave Hansen
2016-07-07 12:47 ` [PATCH 9/9] x86, pkeys: add self-tests Dave Hansen
2016-07-07 14:47 ` [PATCH 0/9] [REVIEW-REQUEST] [v4] System Calls for Memory Protection Keys Mel Gorman
2016-07-08 18:38 ` Hugh Dickins
-- strict thread matches above, loose matches on Subject: below --
2016-06-09 0:01 [PATCH 0/9] [v3] " Dave Hansen
2016-06-09 0:01 ` [PATCH 2/9] mm: implement new pkey_mprotect() system call Dave Hansen
2016-06-11 9:47 ` Thomas Gleixner
2016-06-13 16:03 ` Dave Hansen
2016-06-07 20:47 [PATCH 0/9] [v2] System Calls for Memory Protection Keys Dave Hansen
2016-06-07 20:47 ` [PATCH 2/9] mm: implement new pkey_mprotect() system call Dave Hansen
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