[PATCHv2 1/2] mm/memory: Do not populate page table entries beyond i_size

[Kiryl Shutsemau <kirill@shutemov.name>]

From: Kiryl Shutsemau <kas@kernel.org>

Accesses within VMA, but beyond i_size rounded up to PAGE_SIZE are
supposed to generate SIGBUS.

Recent changes attempted to fault in full folio where possible. They did
not respect i_size, which led to populating PTEs beyond i_size and
breaking SIGBUS semantics.

Darrick reported generic/749 breakage because of this.

However, the problem existed before the recent changes. With huge=always
tmpfs, any write to a file leads to PMD-size allocation. Following the
fault-in of the folio will install PMD mapping regardless of i_size.

Fix filemap_map_pages() and finish_fault() to not install:
  - PTEs beyond i_size;
  - PMD mappings across i_size;

Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
Fixes: 19773df031bc ("mm/fault: try to map the entire file folio in finish_fault()")
Fixes: 357b92761d94 ("mm/filemap: map entire large folio faultaround")
Fixes: 800d8c63b2e9 ("shmem: add huge pages support")
Reported-by: "Darrick J. Wong" <djwong@kernel.org>
---
 mm/filemap.c | 18 ++++++++++--------
 mm/memory.c  | 13 +++++++++++--
 2 files changed, 21 insertions(+), 10 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 13f0259d993c..0d251f6ab480 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -3681,7 +3681,8 @@ static struct folio *next_uptodate_folio(struct xa_state *xas,
 static vm_fault_t filemap_map_folio_range(struct vm_fault *vmf,
 			struct folio *folio, unsigned long start,
 			unsigned long addr, unsigned int nr_pages,
-			unsigned long *rss, unsigned short *mmap_miss)
+			unsigned long *rss, unsigned short *mmap_miss,
+			pgoff_t file_end)
 {
 	unsigned int ref_from_caller = 1;
 	vm_fault_t ret = 0;
@@ -3697,7 +3698,8 @@ static vm_fault_t filemap_map_folio_range(struct vm_fault *vmf,
 	 */
 	addr0 = addr - start * PAGE_SIZE;
 	if (folio_within_vma(folio, vmf->vma) &&
-	    (addr0 & PMD_MASK) == ((addr0 + folio_size(folio) - 1) & PMD_MASK)) {
+	    (addr0 & PMD_MASK) == ((addr0 + folio_size(folio) - 1) & PMD_MASK) &&
+	    file_end >= folio_next_index(folio)) {
 		vmf->pte -= start;
 		page -= start;
 		addr = addr0;
@@ -3817,7 +3819,11 @@ vm_fault_t filemap_map_pages(struct vm_fault *vmf,
 	if (!folio)
 		goto out;
 
-	if (filemap_map_pmd(vmf, folio, start_pgoff)) {
+	file_end = DIV_ROUND_UP(i_size_read(mapping->host), PAGE_SIZE) - 1;
+	end_pgoff = min(end_pgoff, file_end);
+
+	if (file_end >= folio_next_index(folio) &&
+	    filemap_map_pmd(vmf, folio, start_pgoff)) {
 		ret = VM_FAULT_NOPAGE;
 		goto out;
 	}
@@ -3830,10 +3836,6 @@ vm_fault_t filemap_map_pages(struct vm_fault *vmf,
 		goto out;
 	}
 
-	file_end = DIV_ROUND_UP(i_size_read(mapping->host), PAGE_SIZE) - 1;
-	if (end_pgoff > file_end)
-		end_pgoff = file_end;
-
 	folio_type = mm_counter_file(folio);
 	do {
 		unsigned long end;
@@ -3850,7 +3852,7 @@ vm_fault_t filemap_map_pages(struct vm_fault *vmf,
 		else
 			ret |= filemap_map_folio_range(vmf, folio,
 					xas.xa_index - folio->index, addr,
-					nr_pages, &rss, &mmap_miss);
+					nr_pages, &rss, &mmap_miss, file_end);
 
 		folio_unlock(folio);
 	} while ((folio = next_uptodate_folio(&xas, mapping, end_pgoff)) != NULL);
diff --git a/mm/memory.c b/mm/memory.c
index 74b45e258323..9bbe59e6922f 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -5480,6 +5480,7 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
 	int type, nr_pages;
 	unsigned long addr;
 	bool needs_fallback = false;
+	pgoff_t file_end = -1UL;
 
 fallback:
 	addr = vmf->address;
@@ -5501,8 +5502,15 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
 			return ret;
 	}
 
+	if (vma->vm_file) {
+		struct inode *inode = vma->vm_file->f_mapping->host;
+
+		file_end = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
+	}
+
 	if (pmd_none(*vmf->pmd)) {
-		if (folio_test_pmd_mappable(folio)) {
+		if (folio_test_pmd_mappable(folio) &&
+		    file_end >= folio_next_index(folio)) {
 			ret = do_set_pmd(vmf, folio, page);
 			if (ret != VM_FAULT_FALLBACK)
 				return ret;
@@ -5533,7 +5541,8 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
 		if (unlikely(vma_off < idx ||
 			    vma_off + (nr_pages - idx) > vma_pages(vma) ||
 			    pte_off < idx ||
-			    pte_off + (nr_pages - idx)  > PTRS_PER_PTE)) {
+			    pte_off + (nr_pages - idx) > PTRS_PER_PTE ||
+			    file_end < folio_next_index(folio))) {
 			nr_pages = 1;
 		} else {
 			/* Now we can set mappings for the whole large folio. */
-- 
2.50.1