4.4-stable review patch. If anyone has any objections, please let me know.
------------------
[ Upstream commit 6d332747fa5f0a6843b56b5b129168ba909336d1 ]
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: 66dbd6e61a52 ("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 Signed-off-by: Sasha Levin sashal@kernel.org --- arch/arm64/mm/fault.c | 15 ++++++++------- 1 file changed, 8 insertions(+), 7 deletions(-)
diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c index 86485415c5f0..be7f8416809f 100644 --- a/arch/arm64/mm/fault.c +++ b/arch/arm64/mm/fault.c @@ -107,26 +107,27 @@ 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;