Re: [PATCH] mm/arm: pgtable: remove young bit check for pte_valid_user

[Will Deacon]

On Thu, Apr 09, 2026 at 03:54:45PM +0300, Brian Ruley wrote:
> Fixes cache desync, which can cause undefined instruction,
> translation and permission faults under heavy memory use.
> 
> This is an old bug introduced in commit 1971188aa196 ("ARM: 7985/1: mm:
> implement pte_accessible for faulting mappings"), which included a check
> for the young bit of a PTE. The underlying assumption was that old pages
> are not cached, therefore, `__sync_icache_dcache' could be skipped
> entirely.
> 
> However, under extreme memory pressure, page migrations happen
> frequently and the assumption of uncached "old" pages does not hold.
> Especially for systems that do not have swap, the migrated pages are
> unequivocally marked old. This presents a problem, as it is possible
> for the original page to be immediately mapped to another VA that
> happens to share the same cache index in VIPT I-cache (we found this
> bug on Cortex-A9). Without cache invalidation, the CPU will see the
> old mapping whose physical page can now be used for a different
> purpose, as illustrated below:
> 
>                 Core                      Physical Memory
>   +-------------------------------+     +------------------+
>   | TLB                           |     |                  |
>   |  VA_A 0xb6e6f -> pfn_q        |     | pfn_q: code      |
>   +-------------------------------+     +------------------+
>   | I-cache                       |
>   |  set[VA_A bits] | tag=pfn_q   |
>   +-------------------------------+
> 
> migrate (kcompactd):
>   1. copy pfn_q --> pfn_r
>   2. free pfn_q
>   3. pte: VA_a -> pfn_r
>   4. pte_mkold(pte) --> !young
>   5. ICIALLUIS skipped (because !young)
> 
> pfn_src reused (OOM pressure):
>   pte: VA_B -> pfn_q (different code)
> 
> bug:
>                 Core                      Physical Memory
>   +-------------------------------+     +------------------+
>   | TLB (empty)                   |     | pfn_r: old code  |
>   +-------------------------------+     | pfn_q: new code  |
>   | I-cache                       |     +------------------+
>   |  set[VA_A bits] | tag=pfn_q   |<--- wrong instructions
>   +-------------------------------+

(nit: Do you have pfn_r and pfn_q mixed up in the "Physical Memory" box?)

> This was verified on ba16-based board (i.MX6Quad/Dual, Cortex-A9) by
> instrumenting the migration code to track recently migrated pages in a
> ring buffer and then dumping them in the undefined instruction fault
> handler. The bug can be triggered with `stress-ng':
> 
>   stress-ng --vm 4 --vm-bytes 2G --vm-method zero-one --verify
> 
> Note that the system we tested on has only 2G of memory, so the test
> triggered the OOM-killer in our case.
> 
> Fixes: 1971188aa196 ("ARM: 7985/1: mm: implement pte_accessible for faulting mappings")
> Signed-off-by: Brian Ruley <brian.ruley@xxxxxxxxxxxxxxxx>
> ---
>  arch/arm/include/asm/pgtable.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm/include/asm/pgtable.h b/arch/arm/include/asm/pgtable.h
> index 6fa9acd6a7f5..e3a5b4a9a65f 100644
> --- a/arch/arm/include/asm/pgtable.h
> +++ b/arch/arm/include/asm/pgtable.h
> @@ -185,7 +185,7 @@ static inline pte_t *pmd_page_vaddr(pmd_t pmd)
>  #define pte_exec(pte)		(pte_isclear((pte), L_PTE_XN))
>  
>  #define pte_valid_user(pte)	\
> -	(pte_valid(pte) && pte_isset((pte), L_PTE_USER) && pte_young(pte))
> +	(pte_valid(pte) && pte_isset((pte), L_PTE_USER))

This patch is from twelve years ago, so please forgive me for having
forgotten all of the details. However, my recollection is that when using
the classic/!lpae format (as you will be on Cortex-A9), page aging is
implemented by using invalid (translation faulting) ptes for 'old'
mappings.

So in the case you describe, we may well elide the I-cache maintenance,
but won't we also put down an invalid pte? If we later take a fault
on that, we should then perform the cache maintenance when installing
the young entry (via ptep_set_access_flags()). The more interesting part
is probably when the mapping for 'VA_B' is installed to map 'pfn_q' but,
again, I would've expected the cache maintenance to happen just prior to
installing the valid (young) mapping.

Please can you help me to understand the problem better?

Will