special_mapping_fault() is broken

Hello,

special_mapping_fault() is absolutely broken. It seems it was always wrong, but this didn't matter until vdso/vvar started to use more than one page.

The patches are the same, just 1/3 was re-diffed on top of the recent 6b7339f4c31ad "mm: avoid setting up anonymous pages into file mapping" from Kirill.

And after this change vma_is_anonymous() becomes really trivial, it simply checks vm_ops == NULL. However, I do think the helper makes sense. There are a lot of ->vm_ops != NULL checks, the helper makes the caller's code more understandable (self-documented) and this is more grep-friendly.

Oleg.

 include/linux/mm.h |    5 +++++
 mm/memory.c        |    8 ++++----
 mm/mmap.c          |   14 +++-----------
 3 files changed, 12 insertions(+), 15 deletions(-)

Preparation. Add the new simple helper, vma_is_anonymous(vma), and change handle_pte_fault() to use it. It will have more users.

The name is not accurate, say a hpet_mmap()'ed vma is not anonymous. Perhaps it should be named vma_has_fault() instead. But it matches the logic in mmap.c/memory.c (see next changes). "True" just means that a page fault will use do_anonymous_page().

---
 include/linux/mm.h |    5 +++++
 mm/memory.c        |    8 ++++----
 2 files changed, 9 insertions(+), 4 deletions(-)

index 0755b9f..0207ffa 100644
@@ -1225,6 +1225,11 @@ static inline int vma_growsdown(struct vm_area_struct *vma, unsigned long addr)
 	return vma && (vma->vm_end == addr) && (vma->vm_flags & VM_GROWSDOWN);
 }
 
+static inline bool vma_is_anonymous(struct vm_area_struct *vma)
+{
+	return !vma->vm_ops;
+}
+
 static inline int stack_guard_page_start(struct vm_area_struct *vma,
 					     unsigned long addr)
 {

index 2a9e098..87c9285 100644
@@ -3249,12 +3249,12 @@ static int handle_pte_fault(struct mm_struct *mm,
 	barrier();
 	if (!pte_present(entry)) {
 		if (pte_none(entry)) {
-			if (vma->vm_ops)
+			if (vma_is_anonymous(vma))
+				return do_anonymous_page(mm, vma, address,
+							 pte, pmd, flags);
+			else
 				return do_fault(mm, vma, address, pte, pmd,
 						flags, entry);
-
-			return do_anonymous_page(mm, vma, address, pte, pmd,
-					flags);
 		}
 		return do_swap_page(mm, vma, address,
 					pte, pmd, flags, entry);

1.5.5.1

Test-case:

	#define _GNU_SOURCE
	#include <stdio.h>
	#include <unistd.h>
	#include <stdlib.h>
	#include <string.h>
	#include <sys/mman.h>
	#include <assert.h>

	void *find_vdso_vaddr(void)
	{
		FILE *perl;
		char buf[32] = {};

		perl = popen("perl -e 'open STDIN,qq|/proc/@{[getppid]}/maps|;"
				"/^(.*?)-.*vdso/ && print hex $1 while <>'", "r");
		fread(buf, sizeof(buf), 1, perl);
		fclose(perl);

		return (void *)atol(buf);
	}

	#define PAGE_SIZE	4096

	void *get_unmapped_area(void)
	{
		void *p = mmap(0, PAGE_SIZE, PROT_NONE,
				MAP_PRIVATE|MAP_ANONYMOUS, -1,0);
		assert(p != MAP_FAILED);
		munmap(p, PAGE_SIZE);
		return p;
	}

	char save[2][PAGE_SIZE];

	int main(void)
	{
		void *vdso = find_vdso_vaddr();
		void *page[2];

		assert(vdso);
		memcpy(save, vdso, sizeof (save));
		// force another fault on the next check
		assert(madvise(vdso, 2 * PAGE_SIZE, MADV_DONTNEED) == 0);

		page[0] = mremap(vdso,
				PAGE_SIZE, PAGE_SIZE, MREMAP_FIXED | MREMAP_MAYMOVE,
				get_unmapped_area());
		page[1] = mremap(vdso + PAGE_SIZE,
				PAGE_SIZE, PAGE_SIZE, MREMAP_FIXED | MREMAP_MAYMOVE,
				get_unmapped_area());

		assert(page[0] != MAP_FAILED && page[1] != MAP_FAILED);
		printf("match: %d %d\n",
			!memcmp(save[0], page[0], PAGE_SIZE),
			!memcmp(save[1], page[1], PAGE_SIZE));

		return 0;
	}

fails without this patch. Before the previous commit it gets the wrong page, now it segfaults (which is imho better).

This is because copy_vma() wrongly assumes that if vma->vm_file == NULL is irrelevant until the first fault which will use do_anonymous_page(). This is obviously wrong for the special mapping.

---
 mm/mmap.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

index 992417f..2185cd9 100644
@@ -2905,7 +2905,7 @@ struct vm_area_struct *copy_vma(struct vm_area_struct **vmap,
 	 * If anonymous vma has not yet been faulted, update new pgoff
 	 * to match new location, to increase its chance of merging.
 	 */
-	if (unlikely(!vma->vm_file && !vma->anon_vma)) {
+	if (unlikely(vma_is_anonymous(vma) && !vma->anon_vma)) {
 		pgoff = addr >> PAGE_SHIFT;
 		faulted_in_anon_vma = false;
 	}

1.5.5.1

Test-case:

	#include <stdio.h>
	#include <unistd.h>
	#include <stdlib.h>
	#include <string.h>
	#include <sys/mman.h>
	#include <assert.h>

	void *find_vdso_vaddr(void)
	{
		FILE *perl;
		char buf[32] = {};

		perl = popen("perl -e 'open STDIN,qq|/proc/@{[getppid]}/maps|;"
				"/^(.*?)-.*vdso/ && print hex $1 while <>'", "r");
		fread(buf, sizeof(buf), 1, perl);
		fclose(perl);

		return (void *)atol(buf);
	}

	#define PAGE_SIZE	4096

	int main(void)
	{
		void *vdso = find_vdso_vaddr();
		assert(vdso);

		// of course they should differ, and they do so far
		printf("vdso pages differ: %d\n",
			!!memcmp(vdso, vdso + PAGE_SIZE, PAGE_SIZE));

		// split into 2 vma's
		assert(mprotect(vdso, PAGE_SIZE, PROT_READ) == 0);

		// force another fault on the next check
		assert(madvise(vdso, 2 * PAGE_SIZE, MADV_DONTNEED) == 0);

		// now they no longer differ, the 2nd vm_pgoff is wrong
		printf("vdso pages differ: %d\n",
			!!memcmp(vdso, vdso + PAGE_SIZE, PAGE_SIZE));

		return 0;
	}

Output:

vdso pages differ: 1
vdso pages differ: 0

This is because split_vma() correctly updates ->vm_pgoff, but the logic in insert_vm_struct() and special_mapping_fault() is absolutely broken, so the fault at vdso + PAGE_SIZE return the 1st page. The same happens if you simply unmap the 1st page.

special_mapping_fault() does:

	pgoff = vmf->pgoff - vma->vm_pgoff;

and this is _only_ correct if vma->vm_start mmaps the first page from ->vm_private_data array.

vdso or any other user of install_special_mapping() is not anonymous, it has the "backing storage" even if it is just the array of pages. So we actually need to make vm_pgoff work as an offset in this array.

Note: this also allows to fix another problem: currently gdb can't access "[vvar]" memory because in this case special_mapping_fault() doesn't work. Now that we can use ->vm_pgoff we can implement ->access() and fix this.

---
 mm/mmap.c |   12 ++----------
 1 files changed, 2 insertions(+), 10 deletions(-)

index bb50cac..992417f 100644
@@ -2871,7 +2871,7 @@ int insert_vm_struct(struct mm_struct *mm, struct vm_area_struct *vma)
 	 * using the existing file pgoff checks and manipulations.
 	 * Similarly in do_mmap_pgoff and in do_brk.
 	 */
-	if (!vma->vm_file) {
+	if (vma_is_anonymous(vma)) {
 		BUG_ON(vma->anon_vma);
 		vma->vm_pgoff = vma->vm_start >> PAGE_SHIFT;
 	}

@@ -3013,21 +3013,13 @@ static int special_mapping_fault(struct vm_area_struct *vma,
 	pgoff_t pgoff;
 	struct page **pages;
 
-	/*
-	 * special mappings have no vm_file, and in that case, the mm
-	 * uses vm_pgoff internally. So we have to subtract it from here.
-	 * We are allowed to do this because we are the mm; do not copy
-	 * this code into drivers!
-	 */
-	pgoff = vmf->pgoff - vma->vm_pgoff;
-
 	if (vma->vm_ops == &legacy_special_mapping_vmops)
 		pages = vma->vm_private_data;
 	else
 		pages = ((struct vm_special_mapping *)vma->vm_private_data)->
 			pages;
 
-	for (; pgoff && *pages; ++pages)
+	for (pgoff = vmf->pgoff; pgoff && *pages; ++pages)
 		pgoff--;
 
 	if (*pages) {

1.5.5.1

For all three:

 Kirill A. Shutemov

Yes, I for one, very much welcome this helper.

I'm trying to work out which kernel version(s) this should go into, without a lot of success.

What do we think the worst-case effects of the bug?

Ah, I should have mentioned this. And when I re-read my messages I see that "absolutely broken" looks like "should be fixed asap". Sorry for confusion.

No, this bug is not serious. Nothing bad can happen from the kernel perspective. And I doubt that some application will ever unmap/remap the part of vdso or any other install_special_mapping() user. So this is just correctness fix. In fact, to me the main problem is that I was totally confused when I tried to read/understand this code ;)

Oleg.