Re: [PATCH] arm64: mm: refresh stale pmd snapshot after split_contpmd()

[Yang Shi]

On 6/25/26 4:39 AM, lirongqing wrote:
> From: Li RongQing <lirongqing@xxxxxxxxx>
>
> split_contpmd() modifies the pmd entries in-place by clearing the CONT
> bit, but the local 'pmd' variable still holds the old snapshot with CONT
> set. The subsequent split_pmd() call uses this stale value to derive the
> pgprot for the new PTE entries via pmd_pgprot(), causing the resulting
> PTEs to be populated with incorrect protection bits.

If I read the code correctly, CONT bit is cleared by split_pmd(), then 
the bit may be set again for PTEs if we want to have cont ptes. So I 
don't see any problem, did I miss something?

Thanks,
Yang

> Fix this by re-reading the pmd from memory after split_contpmd() returns
> in both call sites: split_kernel_leaf_mapping_locked() and
> split_to_ptes_pmd_entry().
>
> Fixes: a166563e7ec3 ("arm64: mm: support large block mapping when rodata=full")
> Signed-off-by: Li RongQing <lirongqing@xxxxxxxxx>
> ---
>   arch/arm64/mm/mmu.c | 8 ++++++--
>   1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 12e862c..e510336 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -746,8 +746,10 @@ static int split_kernel_leaf_mapping_locked(unsigned long addr)
>   	if (!pmd_present(pmd))
>   		goto out;
>   	if (pmd_leaf(pmd)) {
> -		if (pmd_cont(pmd))
> +		if (pmd_cont(pmd)) {
>   			split_contpmd(pmdp);
> +			pmd = pmdp_get(pmdp);
> +		}
>   		/*
>   		 * PMD: If addr is PMD aligned then addr already describes a
>   		 * leaf boundary. Otherwise, split to contpte.
> @@ -891,8 +893,10 @@ static int split_to_ptes_pmd_entry(pmd_t *pmdp, unsigned long addr,
>   	int ret = 0;
>   
>   	if (pmd_leaf(pmd)) {
> -		if (pmd_cont(pmd))
> +		if (pmd_cont(pmd)) {
>   			split_contpmd(pmdp);
> +			pmd = pmdp_get(pmdp);
> +		}
>   		ret = split_pmd(pmdp, pmd, gfp, false);
>   
>   		/*