Commit 6d332747 authored Jul 25, 2017 by Catalin Marinas Committed by Will Deacon Aug 04, 2017

arm64: Fix potential race with hardware DBM in ptep_set_access_flags()



In a system with DBM (dirty bit management) capable agents there is a
possible race between a CPU executing ptep_set_access_flags() (maybe
non-DBM capable) and a hardware update of the dirty state (clearing of
PTE_RDONLY). The scenario:

a) the pte is writable (PTE_WRITE set), clean (PTE_RDONLY set) and old
   (PTE_AF clear)
b) ptep_set_access_flags() is called as a result of a read access and it
   needs to set the pte to writable, clean and young (PTE_AF set)
c) a DBM-capable agent, as a result of a different write access, is
   marking the entry as young (setting PTE_AF) and dirty (clearing
   PTE_RDONLY)

The current ptep_set_access_flags() implementation would set the
PTE_RDONLY bit in the resulting value overriding the DBM update and
losing the dirty state.

This patch fixes such race by setting PTE_RDONLY to the most permissive
(lowest value) of the current entry and the new one.

Fixes: 66dbd6e6 ("arm64: Implement ptep_set_access_flags() for hardware AF/DBM")
Cc: Will Deacon <will.deacon@arm.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Steve Capper <steve.capper@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

parent c6f97add

arch/arm64/mm/fault.c

+8 −7

Original line number	Diff line number	Diff line
		@@ -163,26 +163,27 @@ int ptep_set_access_flags(struct vm_area_struct *vma,
		/* only preserve the access flags and write permission */
		pte_val(entry) &= PTE_AF \| PTE_WRITE \| PTE_DIRTY;

		/*
		* PTE_RDONLY is cleared by default in the asm below, so set it in
		* back if necessary (read-only or clean PTE).
		*/
		/* set PTE_RDONLY if actual read-only or clean PTE */
		if (!pte_write(entry) \|\| !pte_sw_dirty(entry))
		pte_val(entry) \|= PTE_RDONLY;

		/*
		* Setting the flags must be done atomically to avoid racing with the
		* hardware update of the access/dirty state.
		* hardware update of the access/dirty state. The PTE_RDONLY bit must
		* be set to the most permissive (lowest value) of *ptep and entry
		* (calculated as: a & b == ~(~a \| ~b)).
		*/
		pte_val(entry) ^= PTE_RDONLY;
		asm volatile("// ptep_set_access_flags\n"
		" prfm pstl1strm, %2\n"
		"1: ldxr %0, %2\n"
		" and %0, %0, %3 // clear PTE_RDONLY\n"
		" eor %0, %0, %3 // negate PTE_RDONLY in *ptep\n"
		" orr %0, %0, %4 // set flags\n"
		" eor %0, %0, %3 // negate final PTE_RDONLY\n"
		" stxr %w1, %0, %2\n"
		" cbnz %w1, 1b\n"
		: "=&r" (old_pteval), "=&r" (tmp), "+Q" (pte_val(*ptep))
		: "L" (~PTE_RDONLY), "r" (pte_val(entry)));
		: "L" (PTE_RDONLY), "r" (pte_val(entry)));

		flush_tlb_fix_spurious_fault(vma, address);
		return 1;

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