On Mon, Oct 21, 2024 at 03:45:31PM +0200, Vlastimil Babka wrote:
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.
Wasn't even aware that existed!!
I'm not sure a process can do anything particularly useful with this information though? Hitting a guard page would indicate a programming error rather than something that might allow meaningful feedback to a user like memory protection keys.
Do you think there's enough value int his to warrant digging in? And indeed I imagine bits are in short supply for this and would need a strong argument to get... so yeah I don't think too worthwhile most likely!
Thanks for the suggestion though!
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 lorenzo.stoakes@oracle.com
Acked-by: Vlastimil Babka vbabka@suse.cz
A nit below:
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?
} }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;@@ -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);