linux-kernel - Re: [PATCH v2] riscv: Improve KASAN

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Date:   Tue, 09 Mar 2021 18:37:43 -0800 (PST)
From:   Palmer Dabbelt <palmer@...belt.com>
To:     alex@...ti.fr
CC:     Paul Walmsley <paul.walmsley@...ive.com>, aou@...s.berkeley.edu,
        nylon7@...estech.com, nickhu@...estech.com,
        aryabinin@...tuozzo.com, glider@...gle.com, dvyukov@...gle.com,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        kasan-dev@...glegroups.com, alex@...ti.fr
Subject:     Re: [PATCH v2] riscv: Improve KASAN_VMALLOC support

On Fri, 26 Feb 2021 10:01:54 PST (-0800), alex@...ti.fr wrote:
> When KASAN vmalloc region is populated, there is no userspace process and
> the page table in use is swapper_pg_dir, so there is no need to read
> SATP. Then we can use the same scheme used by kasan_populate_p*d
> functions to go through the page table, which harmonizes the code.
>
> In addition, make use of set_pgd that goes through all unused page table
> levels, contrary to p*d_populate functions, which makes this function work
> whatever the number of page table levels.
>
> And finally, make sure the writes to swapper_pg_dir are visible using
> an sfence.vma.

So I think this is actually a bug: without the fence we could get a 
kasan-related fault at any point (as the mappings might not be visible yet), 
and if we get one when inside do_page_fault() (or while holding a lock it 
wants) we'll end up deadlocking against ourselves.  That'll probably never 
happen in practice, but it'd still be good to get the fence onto fixes.  The 
rest are cleanups, they're for for-next (and should probably be part of your 
sv48 series, if you need to re-spin it -- I'll look at that next).

LMK if you want to split this up, or if you want me to do it.  Either way,

Reviewed-by: Palmer Dabbelt <palmerdabbelt@...gle.com>

Thanks!

> Signed-off-by: Alexandre Ghiti <alex@...ti.fr>
> ---
>
> Changes in v2:
> - Quiet kernel test robot warnings about missing prototypes by declaring
>   the introduced functions as static.
>
>  arch/riscv/mm/kasan_init.c | 61 +++++++++++++-------------------------
>  1 file changed, 20 insertions(+), 41 deletions(-)
>
> diff --git a/arch/riscv/mm/kasan_init.c b/arch/riscv/mm/kasan_init.c
> index e3d91f334b57..aaa3bdc0ffc0 100644
> --- a/arch/riscv/mm/kasan_init.c
> +++ b/arch/riscv/mm/kasan_init.c
> @@ -11,18 +11,6 @@
>  #include <asm/fixmap.h>
>  #include <asm/pgalloc.h>
>
> -static __init void *early_alloc(size_t size, int node)
> -{
> -	void *ptr = memblock_alloc_try_nid(size, size,
> -		__pa(MAX_DMA_ADDRESS), MEMBLOCK_ALLOC_ACCESSIBLE, node);
> -
> -	if (!ptr)
> -		panic("%pS: Failed to allocate %zu bytes align=%zx nid=%d from=%llx\n",
> -			__func__, size, size, node, (u64)__pa(MAX_DMA_ADDRESS));
> -
> -	return ptr;
> -}
> -
>  extern pgd_t early_pg_dir[PTRS_PER_PGD];
>  asmlinkage void __init kasan_early_init(void)
>  {
> @@ -155,38 +143,29 @@ static void __init kasan_populate(void *start, void *end)
>  	memset(start, KASAN_SHADOW_INIT, end - start);
>  }
>
> -void __init kasan_shallow_populate(void *start, void *end)
> +static void __init kasan_shallow_populate_pgd(unsigned long vaddr, unsigned long end)
>  {
> -	unsigned long vaddr = (unsigned long)start & PAGE_MASK;
> -	unsigned long vend = PAGE_ALIGN((unsigned long)end);
> -	unsigned long pfn;
> -	int index;
> +	unsigned long next;
>  	void *p;
> -	pud_t *pud_dir, *pud_k;
> -	pgd_t *pgd_dir, *pgd_k;
> -	p4d_t *p4d_dir, *p4d_k;
> -
> -	while (vaddr < vend) {
> -		index = pgd_index(vaddr);
> -		pfn = csr_read(CSR_SATP) & SATP_PPN;
> -		pgd_dir = (pgd_t *)pfn_to_virt(pfn) + index;
> -		pgd_k = init_mm.pgd + index;
> -		pgd_dir = pgd_offset_k(vaddr);
> -		set_pgd(pgd_dir, *pgd_k);
> -
> -		p4d_dir = p4d_offset(pgd_dir, vaddr);
> -		p4d_k  = p4d_offset(pgd_k, vaddr);
> -
> -		vaddr = (vaddr + PUD_SIZE) & PUD_MASK;
> -		pud_dir = pud_offset(p4d_dir, vaddr);
> -		pud_k = pud_offset(p4d_k, vaddr);
> -
> -		if (pud_present(*pud_dir)) {
> -			p = early_alloc(PAGE_SIZE, NUMA_NO_NODE);
> -			pud_populate(&init_mm, pud_dir, p);
> +	pgd_t *pgd_k = pgd_offset_k(vaddr);
> +
> +	do {
> +		next = pgd_addr_end(vaddr, end);
> +		if (pgd_page_vaddr(*pgd_k) == (unsigned long)lm_alias(kasan_early_shadow_pmd)) {
> +			p = memblock_alloc(PAGE_SIZE, PAGE_SIZE);
> +			set_pgd(pgd_k, pfn_pgd(PFN_DOWN(__pa(p)), PAGE_TABLE));
>  		}
> -		vaddr += PAGE_SIZE;
> -	}
> +	} while (pgd_k++, vaddr = next, vaddr != end);
> +}
> +
> +static void __init kasan_shallow_populate(void *start, void *end)
> +{
> +	unsigned long vaddr = (unsigned long)start & PAGE_MASK;
> +	unsigned long vend = PAGE_ALIGN((unsigned long)end);
> +
> +	kasan_shallow_populate_pgd(vaddr, vend);
> +
> +	local_flush_tlb_all();
>  }
>
>  void __init kasan_init(void)