Messages in this thread | | | Subject | Re: [PATCH v5 5/6] arm64/mm: Populate the swapper_pg_dir by fixmap. | From | James Morse <> | Date | Mon, 1 Oct 2018 11:41:37 +0100 |
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Hi Mark,
On 24/09/18 17:36, Mark Rutland wrote: > On Mon, Sep 17, 2018 at 12:43:32PM +0800, Jun Yao wrote: >> Since we will move the swapper_pg_dir to rodata section, we need a >> way to update it. The fixmap can handle it. When the swapper_pg_dir >> needs to be updated, we map it dynamically. The map will be >> canceled after the update is complete. In this way, we can defend >> against KSMA(Kernel Space Mirror Attack).
>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c >> index 71532bcd76c1..a8a60927f716 100644 >> --- a/arch/arm64/mm/mmu.c >> +++ b/arch/arm64/mm/mmu.c >> @@ -67,6 +67,24 @@ static pte_t bm_pte[PTRS_PER_PTE] __page_aligned_bss; >> static pmd_t bm_pmd[PTRS_PER_PMD] __page_aligned_bss __maybe_unused; >> static pud_t bm_pud[PTRS_PER_PUD] __page_aligned_bss __maybe_unused; >> >> +static DEFINE_SPINLOCK(swapper_pgdir_lock); >> + >> +void set_swapper_pgd(pgd_t *pgdp, pgd_t pgd) >> +{ >> + pgd_t *fixmap_pgdp; >> + >> + spin_lock(&swapper_pgdir_lock); >> + fixmap_pgdp = pgd_set_fixmap(__pa(pgdp)); >> + WRITE_ONCE(*fixmap_pgdp, pgd); >> + /* >> + * We need dsb(ishst) here to ensure the page-table-walker sees >> + * our new entry before set_p?d() returns. The fixmap's >> + * flush_tlb_kernel_range() via clear_fixmap() does this for us. >> + */ >> + pgd_clear_fixmap(); >> + spin_unlock(&swapper_pgdir_lock); >> +} > > I'm rather worried that we could deadlock here.
We can use the irqsave versions if you're worried, but I think any code doing this is already broken.
(I'd like to eventually depend on the init_mm.page_table_lock for this, but it isn't held when the vmemmap is being populated.)
> Are we certain we never poke the kernel page tables in IRQ context?
The RAS code was doing this, but was deemed unsafe, and changed to use the fixmap: https://lkml.org/lkml/2017/10/30/500 The fixmap only ever touches the last level, so can't hit this.
x86 can't do its IPI tlb-maintenance from IRQ context, so anything trying to unmap from irq context is already broken: https://lkml.org/lkml/2018/9/6/324
vunmap()/vfree() is allowed from irq context, but it defers its work.
I can't find any way to pass GFP_ATOMIC into ioremap(), I didn't think vmalloc() could either, ... but now I spot __vmalloc() does...
This __vmalloc() path is used by the percpu allocator, which starting from pcpu_alloc() can be passed something other than GFP_KERNEL, and uses spin_lock_irqsave(), so it is expecting to be called in irq context.
... so yes it looks like this can happen.
I'll post a fix
Thanks!
James
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