Re: [PATCH v2 2/5] mm: add PTE_MARKER_GUARD PTE marker
From: Vlastimil Babka <hidden>
Date: 2024-10-21 13:45:33
Also in:
linux-alpha, linux-arch, linux-kselftest, linux-mips, linux-mm, lkml
On 10/20/24 18:20, Lorenzo Stoakes wrote:
Add a new PTE marker that results in any access causing the accessing process to segfault.
Should we distinguish it from other segfaults? Is there a way? I can see memory protection keys use SEGV_PKUERR, but no idea if we have any free values.
This is preferable to PTE_MARKER_POISONED, which results in the same handling as hardware poisoned memory, and is thus undesirable for cases where we simply wish to 'soft' poison a range. This is in preparation for implementing the ability to specify guard pages at the page table level, i.e. ranges that, when accessed, should cause process termination. Additionally, rename zap_drop_file_uffd_wp() to zap_drop_markers() - the function checks the ZAP_FLAG_DROP_MARKER flag so naming it for this single purpose was simply incorrect. We then reuse the same logic to determine whether a zap should clear a guard entry - this should only be performed on teardown and never on MADV_DONTNEED or the like. Signed-off-by: Lorenzo Stoakes <redacted>
Acked-by: Vlastimil Babka <redacted> A nit below:
quoted hunk ↗ jump to hunk
diff --git a/mm/hugetlb.c b/mm/hugetlb.c index 906294ac85dc..50e3f6ed73ac 100644 --- a/mm/hugetlb.c +++ b/mm/hugetlb.c@@ -6353,6 +6353,9 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma, ret = VM_FAULT_HWPOISON_LARGE | VM_FAULT_SET_HINDEX(hstate_index(h)); goto out_mutex; + } else if (marker & PTE_MARKER_GUARD) { + ret = VM_FAULT_SIGSEGV; + goto out_mutex;
Given we don't support hugetlb, should we WARN_ON_ONCE() if such unexpected marker appears there?
quoted hunk ↗ jump to hunk
} }diff --git a/mm/memory.c b/mm/memory.c index 0f614523b9f4..551455cd453f 100644 --- a/mm/memory.c +++ b/mm/memory.c@@ -1455,7 +1455,7 @@ static inline bool should_zap_folio(struct zap_details *details, return !folio_test_anon(folio); } -static inline bool zap_drop_file_uffd_wp(struct zap_details *details) +static inline bool zap_drop_markers(struct zap_details *details) { if (!details) return false;@@ -1476,7 +1476,7 @@ zap_install_uffd_wp_if_needed(struct vm_area_struct *vma, if (vma_is_anonymous(vma)) return; - if (zap_drop_file_uffd_wp(details)) + if (zap_drop_markers(details)) return; for (;;) {@@ -1671,7 +1671,15 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb, * drop the marker if explicitly requested. */ if (!vma_is_anonymous(vma) && - !zap_drop_file_uffd_wp(details)) + !zap_drop_markers(details)) + continue; + } else if (is_guard_swp_entry(entry)) { + /* + * Ordinary zapping should not remove guard PTE + * markers. Only do so if we should remove PTE markers + * in general. + */ + if (!zap_drop_markers(details)) continue; } else if (is_hwpoison_entry(entry) || is_poisoned_swp_entry(entry)) {@@ -4003,6 +4011,10 @@ static vm_fault_t handle_pte_marker(struct vm_fault *vmf) if (marker & PTE_MARKER_POISONED) return VM_FAULT_HWPOISON; + /* Hitting a guard page is always a fatal condition. */ + if (marker & PTE_MARKER_GUARD) + return VM_FAULT_SIGSEGV; + if (pte_marker_entry_uffd_wp(entry)) return pte_marker_handle_uffd_wp(vmf);