linux-kernel - Re: [PATCH v3] arm64: Enable vmalloc-huge with ptdump

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <4a90e7b1-7592-4dea-9225-40c8e3b597d8@redhat.com>
Date: Mon, 16 Jun 2025 17:00:14 +0200
From: David Hildenbrand <david@...hat.com>
To: Dev Jain <dev.jain@....com>, catalin.marinas@....com, will@...nel.org
Cc: anshuman.khandual@....com, quic_zhenhuah@...cinc.com,
 ryan.roberts@....com, kevin.brodsky@....com, yangyicong@...ilicon.com,
 joey.gouly@....com, linux-arm-kernel@...ts.infradead.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] arm64: Enable vmalloc-huge with ptdump

On 16.06.25 12:33, Dev Jain wrote:
> arm64 disables vmalloc-huge when kernel page table dumping is enabled,
> because an intermediate table may be removed, potentially causing the
> ptdump code to dereference an invalid address. We want to be able to
> analyze block vs page mappings for kernel mappings with ptdump, so to
> enable vmalloc-huge with ptdump, synchronize between page table removal in
> pmd_free_pte_page()/pud_free_pmd_page() and ptdump pagetable walking. We
> use mmap_read_lock and not write lock because we don't need to synchronize
> between two different vm_structs; two vmalloc objects running this same
> code path will point to different page tables, hence there is no race.
> 
> For pud_free_pmd_page(), we isolate the PMD table to avoid taking the lock
> 512 times again via pmd_free_pte_page().
> 
> We implement the locking mechanism using static keys, since the chance
> of a race is very small. Observe that the synchronization is needed
> to avoid the following race:
> 
> CPU1							CPU2
> 						take reference of PMD table
> pud_clear()
> pte_free_kernel()
> 						walk freed PMD table
> 
> and similar race between pmd_free_pte_page and ptdump_walk_pgd.
> 
> Therefore, there are two cases: if ptdump sees the cleared PUD, then
> we are safe. If not, then the patched-in read and write locks help us
> avoid the race.
> 
> To implement the mechanism, we need the static key access from mmu.c and
> ptdump.c. Note that in case !CONFIG_PTDUMP_DEBUGFS, ptdump.o won't be a
> target in the Makefile, therefore we cannot initialize the key there, as
> is being done, for example, in the static key implementation of
> hugetlb-vmemmap. Therefore, include asm/cpufeature.h, which includes
> the jump_label mechanism. Declare the key there and define the key to false
> in mmu.c.
> 
> No issues were observed with mm-selftests. No issues were observed while
> parallelly running test_vmalloc.sh and dumping the kernel pagetable through
> sysfs in a loop.
> 
> v2->v3:
>   - Use static key mechanism
> 
> v1->v2:
>   - Take lock only when CONFIG_PTDUMP_DEBUGFS is on
>   - In case of pud_free_pmd_page(), isolate the PMD table to avoid taking
>     the lock 512 times again via pmd_free_pte_page()
> 
> Signed-off-by: Dev Jain <dev.jain@....com>
> ---
>   arch/arm64/include/asm/cpufeature.h |  1 +
>   arch/arm64/mm/mmu.c                 | 51 ++++++++++++++++++++++++++---
>   arch/arm64/mm/ptdump.c              |  5 +++
>   3 files changed, 53 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
> index c4326f1cb917..3e386563b587 100644
> --- a/arch/arm64/include/asm/cpufeature.h
> +++ b/arch/arm64/include/asm/cpufeature.h
> @@ -26,6 +26,7 @@
>   #include <linux/kernel.h>
>   #include <linux/cpumask.h>
>   
> +DECLARE_STATIC_KEY_FALSE(ptdump_lock_key);
>   /*
>    * CPU feature register tracking
>    *
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 8fcf59ba39db..e242ba428820 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -41,11 +41,14 @@
>   #include <asm/tlbflush.h>
>   #include <asm/pgalloc.h>
>   #include <asm/kfence.h>
> +#include <asm/cpufeature.h>
>   
>   #define NO_BLOCK_MAPPINGS	BIT(0)
>   #define NO_CONT_MAPPINGS	BIT(1)
>   #define NO_EXEC_MAPPINGS	BIT(2)	/* assumes FEAT_HPDS is not used */
>   
> +DEFINE_STATIC_KEY_FALSE(ptdump_lock_key);
> +
>   enum pgtable_type {
>   	TABLE_PTE,
>   	TABLE_PMD,
> @@ -1267,8 +1270,9 @@ int pmd_clear_huge(pmd_t *pmdp)
>   	return 1;
>   }
>   
> -int pmd_free_pte_page(pmd_t *pmdp, unsigned long addr)
> +static int __pmd_free_pte_page(pmd_t *pmdp, unsigned long addr, bool lock)
>   {
> +	bool lock_taken = false;
>   	pte_t *table;
>   	pmd_t pmd;
>   
> @@ -1279,15 +1283,29 @@ int pmd_free_pte_page(pmd_t *pmdp, unsigned long addr)
>   		return 1;
>   	}
>   
> +	/* See comment in pud_free_pmd_page for static key logic */
>   	table = pte_offset_kernel(pmdp, addr);
>   	pmd_clear(pmdp);
>   	__flush_tlb_kernel_pgtable(addr);
> +	if (static_branch_unlikely(&ptdump_lock_key) && lock) {
> +		mmap_read_lock(&init_mm);
> +		lock_taken = true;
> +	}
> +	if (unlikely(lock_taken))
> +		mmap_read_unlock(&init_mm);
> +

I'm missing something important: why not

if (static_branch_unlikely(&ptdump_lock_key) && lock) {
	mmap_read_lock(&init_mm);
	mmap_read_unlock(&init_mm);
}

-- 
Cheers,

David / dhildenb