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acd547b298
PKRU is the register that lets you disallow writes or all access to a given protection key. The XSAVE hardware defines an "init state" of 0 for PKRU: its most permissive state, allowing access/writes to everything. Since we start off all new processes with the init state, we start all processes off with the most permissive possible PKRU. This is unfortunate. If a thread is clone()'d [1] before a program has time to set PKRU to a restrictive value, that thread will be able to write to all data, no matter what pkey is set on it. This weakens any integrity guarantees that we want pkeys to provide. To fix this, we define a very restrictive PKRU to override the XSAVE-provided value when we create a new FPU context. We choose a value that only allows access to pkey 0, which is as restrictive as we can practically make it. This does not cause any practical problems with applications using protection keys because we require them to specify initial permissions for each key when it is allocated, which override the restrictive default. In the end, this ensures that threads which do not know how to manage their own pkey rights can not do damage to data which is pkey-protected. I would have thought this was a pretty contrived scenario, except that I heard a bug report from an MPX user who was creating threads in some very early code before main(). It may be crazy, but folks evidently _do_ it. Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com> Cc: linux-arch@vger.kernel.org Cc: Dave Hansen <dave@sr71.net> Cc: mgorman@techsingularity.net Cc: arnd@arndb.de Cc: linux-api@vger.kernel.org Cc: linux-mm@kvack.org Cc: luto@kernel.org Cc: akpm@linux-foundation.org Cc: torvalds@linux-foundation.org Link: http://lkml.kernel.org/r/20160729163021.F3C25D4A@viggo.jf.intel.com Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
44 lines
932 B
C
44 lines
932 B
C
#ifndef _LINUX_PKEYS_H
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#define _LINUX_PKEYS_H
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#include <linux/mm_types.h>
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#include <asm/mmu_context.h>
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#ifdef CONFIG_ARCH_HAS_PKEYS
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#include <asm/pkeys.h>
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#else /* ! CONFIG_ARCH_HAS_PKEYS */
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#define arch_max_pkey() (1)
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#define execute_only_pkey(mm) (0)
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#define arch_override_mprotect_pkey(vma, prot, pkey) (0)
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#define PKEY_DEDICATED_EXECUTE_ONLY 0
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#define ARCH_VM_PKEY_FLAGS 0
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static inline bool mm_pkey_is_allocated(struct mm_struct *mm, int pkey)
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{
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return (pkey == 0);
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}
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static inline int mm_pkey_alloc(struct mm_struct *mm)
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{
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return -1;
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}
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static inline int mm_pkey_free(struct mm_struct *mm, int pkey)
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{
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WARN_ONCE(1, "free of protection key when disabled");
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return -EINVAL;
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}
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static inline int arch_set_user_pkey_access(struct task_struct *tsk, int pkey,
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unsigned long init_val)
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{
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return 0;
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}
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static inline void copy_init_pkru_to_fpregs(void)
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{
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}
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#endif /* ! CONFIG_ARCH_HAS_PKEYS */
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#endif /* _LINUX_PKEYS_H */
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