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X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 30 May 2025 09:58:35.4383 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 941602bd-5aec-4bbe-4359-08dd9f608b12 X-MS-Exchange-CrossTenant-Id: f34e5979-57d9-4aaa-ad4d-b122a662184d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=f34e5979-57d9-4aaa-ad4d-b122a662184d;Ip=[4.158.2.129];Helo=[outbound-uk1.az.dlp.m.darktrace.com] X-MS-Exchange-CrossTenant-AuthSource: AM3PEPF00009B9E.eurprd04.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: AS8PR08MB10071 On 30/05/25 3:17 pm, Ryan Roberts wrote: > On 30/05/2025 10:14, Dev Jain wrote: >> On 30/05/25 2:10 pm, Ryan Roberts wrote: >>> On 30/05/2025 09:20, 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. >> My "correction" from race->no problem was incorrect after all :) There will >> be no race too since the vm_struct object has exclusive access to whatever >> table it is clearing. >> >>>> Signed-off-by: Dev Jain >>>> --- >>>>   arch/arm64/include/asm/vmalloc.h | 6 ++---- >>>>   arch/arm64/mm/mmu.c              | 7 +++++++ >>>>   2 files changed, 9 insertions(+), 4 deletions(-) >>>> >>>> diff --git a/arch/arm64/include/asm/vmalloc.h b/arch/arm64/include/asm/vmalloc.h >>>> index 38fafffe699f..28b7173d8693 100644 >>>> --- a/arch/arm64/include/asm/vmalloc.h >>>> +++ b/arch/arm64/include/asm/vmalloc.h >>>> @@ -12,15 +12,13 @@ static inline bool arch_vmap_pud_supported(pgprot_t prot) >>>>       /* >>>>        * SW table walks can't handle removal of intermediate entries. >>>>        */ >>>> -    return pud_sect_supported() && >>>> -           !IS_ENABLED(CONFIG_PTDUMP_DEBUGFS); >>>> +    return pud_sect_supported(); >>>>   } >>>>     #define arch_vmap_pmd_supported arch_vmap_pmd_supported >>>>   static inline bool arch_vmap_pmd_supported(pgprot_t prot) >>>>   { >>>> -    /* See arch_vmap_pud_supported() */ >>>> -    return !IS_ENABLED(CONFIG_PTDUMP_DEBUGFS); >>>> +    return true; >>>>   } >>>>     #endif >>>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c >>>> index ea6695d53fb9..798cebd9e147 100644 >>>> --- a/arch/arm64/mm/mmu.c >>>> +++ b/arch/arm64/mm/mmu.c >>>> @@ -1261,7 +1261,11 @@ int pmd_free_pte_page(pmd_t *pmdp, unsigned long addr) >>>>       } >>>>         table = pte_offset_kernel(pmdp, addr); >>>> + >>>> +    /* Synchronize against ptdump_walk_pgd() */ >>>> +    mmap_read_lock(&init_mm); >>>>       pmd_clear(pmdp); >>>> +    mmap_read_unlock(&init_mm); >>> So this works because ptdump_walk_pgd() takes the write_lock (which is mutually >>> exclusive with any read_lock holders) for the duration of the table walk, so it >>> will either consistently see the pgtables before or after this removal. It will >>> never disappear during the walk, correct? >>> >>> I guess there is a risk of this showing up as contention with other init_mm >>> write_lock holders. But I expect that pmd_free_pte_page()/pud_free_pmd_page() >>> are called sufficiently rarely that the risk is very small. Let's fix any perf >>> problem if/when we see it. >> We can avoid all of that by my initial approach - to wrap the lock around >> CONFIG_PTDUMP_DEBUGFS. >> I don't have a strong opinion, just putting it out there. >> >>>>       __flush_tlb_kernel_pgtable(addr); >>> And the tlbi doesn't need to be serialized because there is no security issue. >>> The walker can be trusted to only dereference memory that it sees as it walks >>> the pgtable (obviously). >>> >>>>       pte_free_kernel(NULL, table); >>>>       return 1; >>>> @@ -1289,7 +1293,10 @@ int pud_free_pmd_page(pud_t *pudp, unsigned long addr) >>>>           pmd_free_pte_page(pmdp, next); >>>>       } while (pmdp++, next += PMD_SIZE, next != end); >>>>   +    /* Synchronize against ptdump_walk_pgd() */ >>>> +    mmap_read_lock(&init_mm); >>>>       pud_clear(pudp); >>>> +    mmap_read_unlock(&init_mm); >>> Hmm, so pud_free_pmd_page() is now going to cause us to acquire and release the >>> (upto) lock 513 times (for a 4K kernel). I wonder if there is an argument for >>> clearing the pud first (under the lock), then the pmds can all be cleared >>> without a lock, since the walker won't be able to see the pmds once the pud is >>> cleared. >> Yes, we can isolate the PMD table in case the caller of pmd_free_pte_page is >> pud_free_pmd_page. In this case, vm_struct_1 has exclusive access to the entire >> pmd page, hence no race will occur. But, in case of vmap_try_huge_pmd() being the >> caller, we cannot drop the locks around pmd_free_pte_page. So we can have something >> like >> >> #ifdef CONFIG_PTDUMP_DEBUGFS >> static inline void ptdump_synchronize_lock(bool flag) >> { >>     if (flag) >>         mmap_read_lock(&init_mm); >> } >> >> and pass false when the caller is pud_free_pmd_page. > of something like this? (completely untested): > > ---8<--- > diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c > index 8fcf59ba39db..1f3a922167e4 100644 > --- a/arch/arm64/mm/mmu.c > +++ b/arch/arm64/mm/mmu.c > @@ -1267,7 +1267,7 @@ 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) > { > pte_t *table; > pmd_t pmd; > @@ -1280,12 +1280,23 @@ int pmd_free_pte_page(pmd_t *pmdp, unsigned long addr) > } > > table = pte_offset_kernel(pmdp, addr); > + > + if (lock) > + mmap_read_lock(&init_mm); > pmd_clear(pmdp); > + if (lock) > + mmap_read_unlock(&init_mm); > + > __flush_tlb_kernel_pgtable(addr); > pte_free_kernel(NULL, table); > return 1; > } > > +int pmd_free_pte_page(pmd_t *pmdp, unsigned long addr) > +{ > + return __pmd_free_pte_page(pmdp, addr, true); > +} > + > int pud_free_pmd_page(pud_t *pudp, unsigned long addr) > { > pmd_t *table; > @@ -1300,15 +1311,19 @@ int pud_free_pmd_page(pud_t *pudp, unsigned long addr) > return 1; > } > > + /* Synchronize against ptdump_walk_pgd() */ > + mmap_read_lock(&init_mm); > + pud_clear(pudp); > + mmap_read_unlock(&init_mm); > + > table = pmd_offset(pudp, addr); > pmdp = table; > next = addr; > end = addr + PUD_SIZE; > do { > - pmd_free_pte_page(pmdp, next); > + __pmd_free_pte_page(pmdp, next, false); > } while (pmdp++, next += PMD_SIZE, next != end); > > - pud_clear(pudp); > __flush_tlb_kernel_pgtable(addr); > pmd_free(NULL, table); > return 1; > ---8<--- This looks good too! Although would like to first decide on whether we want to wrap around CONFIG_PTDUMP_DEBUGFS. > > >>> Thanks, >>> Ryan >>> >>>>       __flush_tlb_kernel_pgtable(addr); >>>>       pmd_free(NULL, table); >>>>       return 1;