[PATCH 6/6] mm: Forbid splitting special mappings

[Dmitry Safonov]

Don't allow splitting of vm_special_mapping's.
It affects vdso/vvar areas. Uprobes have only one page in xol_area so
they aren't affected.

Those restrictions were enforced by checks in .mremap() callbacks.
Restrict resizing with generic .split() callback.

Signed-off-by: Dmitry Safonov <dima@xxxxxxxxxx>
---
 arch/arm/kernel/vdso.c    |  9 ---------
 arch/arm64/kernel/vdso.c  | 41 +++------------------------------------
 arch/mips/vdso/genvdso.c  |  4 ----
 arch/s390/kernel/vdso.c   | 11 +----------
 arch/x86/entry/vdso/vma.c | 17 ----------------
 mm/mmap.c                 | 12 ++++++++++++
 6 files changed, 16 insertions(+), 78 deletions(-)

diff --git a/arch/arm/kernel/vdso.c b/arch/arm/kernel/vdso.c
index fddd08a6e063..3408269d19c7 100644
--- a/arch/arm/kernel/vdso.c
+++ b/arch/arm/kernel/vdso.c
@@ -50,15 +50,6 @@ static const struct vm_special_mapping vdso_data_mapping = {
 static int vdso_mremap(const struct vm_special_mapping *sm,
 		struct vm_area_struct *new_vma)
 {
-	unsigned long new_size = new_vma->vm_end - new_vma->vm_start;
-	unsigned long vdso_size;
-
-	/* without VVAR page */
-	vdso_size = (vdso_total_pages - 1) << PAGE_SHIFT;
-
-	if (vdso_size != new_size)
-		return -EINVAL;
-
 	current->mm->context.vdso = new_vma->vm_start;
 
 	return 0;
diff --git a/arch/arm64/kernel/vdso.c b/arch/arm64/kernel/vdso.c
index d4202a32abc9..a1a4220a405b 100644
--- a/arch/arm64/kernel/vdso.c
+++ b/arch/arm64/kernel/vdso.c
@@ -82,17 +82,9 @@ static union {
 } vdso_data_store __page_aligned_data;
 struct vdso_data *vdso_data = vdso_data_store.data;
 
-static int __vdso_remap(enum vdso_abi abi,
-			const struct vm_special_mapping *sm,
-			struct vm_area_struct *new_vma)
+static int vdso_mremap(const struct vm_special_mapping *sm,
+		struct vm_area_struct *new_vma)
 {
-	unsigned long new_size = new_vma->vm_end - new_vma->vm_start;
-	unsigned long vdso_size = vdso_info[abi].vdso_code_end -
-				  vdso_info[abi].vdso_code_start;
-
-	if (vdso_size != new_size)
-		return -EINVAL;
-
 	current->mm->context.vdso = (void *)new_vma->vm_start;
 
 	return 0;
@@ -223,17 +215,6 @@ static vm_fault_t vvar_fault(const struct vm_special_mapping *sm,
 	return vmf_insert_pfn(vma, vmf->address, pfn);
 }
 
-static int vvar_mremap(const struct vm_special_mapping *sm,
-		       struct vm_area_struct *new_vma)
-{
-	unsigned long new_size = new_vma->vm_end - new_vma->vm_start;
-
-	if (new_size != VVAR_NR_PAGES * PAGE_SIZE)
-		return -EINVAL;
-
-	return 0;
-}
-
 static int __setup_additional_pages(enum vdso_abi abi,
 				    struct mm_struct *mm,
 				    struct linux_binprm *bprm,
@@ -284,14 +265,6 @@ static int __setup_additional_pages(enum vdso_abi abi,
 /*
  * Create and map the vectors page for AArch32 tasks.
  */
-#ifdef CONFIG_COMPAT_VDSO
-static int aarch32_vdso_mremap(const struct vm_special_mapping *sm,
-		struct vm_area_struct *new_vma)
-{
-	return __vdso_remap(VDSO_ABI_AA32, sm, new_vma);
-}
-#endif /* CONFIG_COMPAT_VDSO */
-
 enum aarch32_map {
 	AA32_MAP_VECTORS, /* kuser helpers */
 #ifdef CONFIG_COMPAT_VDSO
@@ -313,11 +286,10 @@ static struct vm_special_mapping aarch32_vdso_maps[] = {
 	[AA32_MAP_VVAR] = {
 		.name = "[vvar]",
 		.fault = vvar_fault,
-		.mremap = vvar_mremap,
 	},
 	[AA32_MAP_VDSO] = {
 		.name = "[vdso]",
-		.mremap = aarch32_vdso_mremap,
+		.mremap = vdso_mremap,
 	},
 #endif /* CONFIG_COMPAT_VDSO */
 	[AA32_MAP_SIGPAGE] = {
@@ -465,12 +437,6 @@ int aarch32_setup_additional_pages(struct linux_binprm *bprm, int uses_interp)
 }
 #endif /* CONFIG_COMPAT */
 
-static int vdso_mremap(const struct vm_special_mapping *sm,
-		struct vm_area_struct *new_vma)
-{
-	return __vdso_remap(VDSO_ABI_AA64, sm, new_vma);
-}
-
 enum aarch64_map {
 	AA64_MAP_VVAR,
 	AA64_MAP_VDSO,
@@ -480,7 +446,6 @@ static struct vm_special_mapping aarch64_vdso_maps[] __ro_after_init = {
 	[AA64_MAP_VVAR] = {
 		.name	= "[vvar]",
 		.fault = vvar_fault,
-		.mremap = vvar_mremap,
 	},
 	[AA64_MAP_VDSO] = {
 		.name	= "[vdso]",
diff --git a/arch/mips/vdso/genvdso.c b/arch/mips/vdso/genvdso.c
index abb06ae04b40..09e30eb4be86 100644
--- a/arch/mips/vdso/genvdso.c
+++ b/arch/mips/vdso/genvdso.c
@@ -263,10 +263,6 @@ int main(int argc, char **argv)
 	fprintf(out_file, "	const struct vm_special_mapping *sm,\n");
 	fprintf(out_file, "	struct vm_area_struct *new_vma)\n");
 	fprintf(out_file, "{\n");
-	fprintf(out_file, "	unsigned long new_size =\n");
-	fprintf(out_file, "	new_vma->vm_end - new_vma->vm_start;\n");
-	fprintf(out_file, "	if (vdso_image.size != new_size)\n");
-	fprintf(out_file, "		return -EINVAL;\n");
 	fprintf(out_file, "	current->mm->context.vdso =\n");
 	fprintf(out_file, "	(void *)(new_vma->vm_start);\n");
 	fprintf(out_file, "	return 0;\n");
diff --git a/arch/s390/kernel/vdso.c b/arch/s390/kernel/vdso.c
index c4baefaa6e34..291ead792d64 100644
--- a/arch/s390/kernel/vdso.c
+++ b/arch/s390/kernel/vdso.c
@@ -59,17 +59,8 @@ static vm_fault_t vdso_fault(const struct vm_special_mapping *sm,
 static int vdso_mremap(const struct vm_special_mapping *sm,
 		       struct vm_area_struct *vma)
 {
-	unsigned long vdso_pages;
-
-	vdso_pages = vdso64_pages;
-
-	if ((vdso_pages << PAGE_SHIFT) != vma->vm_end - vma->vm_start)
-		return -EINVAL;
-
-	if (WARN_ON_ONCE(current->mm != vma->vm_mm))
-		return -EFAULT;
-
 	current->mm->context.vdso_base = vma->vm_start;
+
 	return 0;
 }
 
diff --git a/arch/x86/entry/vdso/vma.c b/arch/x86/entry/vdso/vma.c
index 9185cb1d13b9..0c942b31825d 100644
--- a/arch/x86/entry/vdso/vma.c
+++ b/arch/x86/entry/vdso/vma.c
@@ -89,30 +89,14 @@ static void vdso_fix_landing(const struct vdso_image *image,
 static int vdso_mremap(const struct vm_special_mapping *sm,
 		struct vm_area_struct *new_vma)
 {
-	unsigned long new_size = new_vma->vm_end - new_vma->vm_start;
 	const struct vdso_image *image = current->mm->context.vdso_image;
 
-	if (image->size != new_size)
-		return -EINVAL;
-
 	vdso_fix_landing(image, new_vma);
 	current->mm->context.vdso = (void __user *)new_vma->vm_start;
 
 	return 0;
 }
 
-static int vvar_mremap(const struct vm_special_mapping *sm,
-		struct vm_area_struct *new_vma)
-{
-	const struct vdso_image *image = new_vma->vm_mm->context.vdso_image;
-	unsigned long new_size = new_vma->vm_end - new_vma->vm_start;
-
-	if (new_size != -image->sym_vvar_start)
-		return -EINVAL;
-
-	return 0;
-}
-
 #ifdef CONFIG_TIME_NS
 static struct page *find_timens_vvar_page(struct vm_area_struct *vma)
 {
@@ -252,7 +236,6 @@ static const struct vm_special_mapping vdso_mapping = {
 static const struct vm_special_mapping vvar_mapping = {
 	.name = "[vvar]",
 	.fault = vvar_fault,
-	.mremap = vvar_mremap,
 };
 
 /*
diff --git a/mm/mmap.c b/mm/mmap.c
index a62cb3ccafce..41100f6505ff 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -3389,6 +3389,17 @@ static int special_mapping_mremap(struct vm_area_struct *new_vma,
 	return 0;
 }
 
+static int special_mapping_split(struct vm_area_struct *vma, unsigned long addr)
+{
+	/*
+	 * Forbid splitting special mappings - kernel has expectations over
+	 * the number of pages in mapping. Together with VM_DONTEXPAND
+	 * the size of vma should stay the same over the special mapping's
+	 * lifetime.
+	 */
+	return -EINVAL;
+}
+
 static const struct vm_operations_struct special_mapping_vmops = {
 	.close = special_mapping_close,
 	.fault = special_mapping_fault,
@@ -3396,6 +3407,7 @@ static const struct vm_operations_struct special_mapping_vmops = {
 	.name = special_mapping_name,
 	/* vDSO code relies that VVAR can't be accessed remotely */
 	.access = NULL,
+	.may_split = special_mapping_split,
 };
 
 static const struct vm_operations_struct legacy_special_mapping_vmops = {
-- 
2.28.0