x86: Implement array_index_mask_nospec
array_index_nospec() uses a mask to sanitize user controllable array indexes, i.e. generate a 0 mask if 'index' >= 'size', and a ~0 mask otherwise. While the default array_index_mask_nospec() handles the carry-bit from the (index - size) result in software. The x86 array_index_mask_nospec() does the same, but the carry-bit is handled in the processor CF flag without conditional instructions in the control flow. Suggested-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Dan Williams <dan.j.williams@intel.com> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Cc: linux-arch@vger.kernel.org Cc: kernel-hardening@lists.openwall.com Cc: gregkh@linuxfoundation.org Cc: alan@linux.intel.com Link: https://lkml.kernel.org/r/151727414808.33451.1873237130672785331.stgit@dwillia2-desk3.amr.corp.intel.com
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@ -24,6 +24,30 @@
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#define wmb() asm volatile("sfence" ::: "memory")
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#endif
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/**
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* array_index_mask_nospec() - generate a mask that is ~0UL when the
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* bounds check succeeds and 0 otherwise
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* @index: array element index
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* @size: number of elements in array
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*
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* Returns:
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* 0 - (index < size)
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*/
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static inline unsigned long array_index_mask_nospec(unsigned long index,
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unsigned long size)
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{
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unsigned long mask;
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asm ("cmp %1,%2; sbb %0,%0;"
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:"=r" (mask)
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:"r"(size),"r" (index)
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:"cc");
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return mask;
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}
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/* Override the default implementation from linux/nospec.h. */
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#define array_index_mask_nospec array_index_mask_nospec
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#ifdef CONFIG_X86_PPRO_FENCE
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#define dma_rmb() rmb()
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#else
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