[PATCH 63/82] mm: Refactor intentional wrap-around test

[Kees Cook <keescook@chromium.org>]

In an effort to separate intentional arithmetic wrap-around from
unexpected wrap-around, we need to refactor places that depend on this
kind of math. One of the most common code patterns of this is:

	VAR + value < VAR

Notably, this is considered "undefined behavior" for signed and pointer
types, which the kernel works around by using the -fno-strict-overflow
option in the build[1] (which used to just be -fwrapv). Regardless, we
want to get the kernel source to the position where we can meaningfully
instrument arithmetic wrap-around conditions and catch them when they
are unexpected, regardless of whether they are signed[2], unsigned[3],
or pointer[4] types.

Refactor open-coded wrap-around addition test to use add_would_overflow().
This paves the way to enabling the wrap-around sanitizers in the future.

Link: https://git.kernel.org/linus/68df3755e383e6fecf2354a67b08f92f18536594 [1]
Link: https://github.com/KSPP/linux/issues/26 [2]
Link: https://github.com/KSPP/linux/issues/27 [3]
Link: https://github.com/KSPP/linux/issues/344 [4]
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: linux-mm@kvack.org
Cc: linux-kselftest@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 mm/memory.c | 4 ++--
 mm/mmap.c   | 2 +-
 mm/mremap.c | 2 +-
 mm/nommu.c  | 4 ++--
 mm/util.c   | 2 +-
 5 files changed, 7 insertions(+), 7 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 7e1f4849463a..d47acdff7af3 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2559,7 +2559,7 @@ int vm_iomap_memory(struct vm_area_struct *vma, phys_addr_t start, unsigned long
 	unsigned long vm_len, pfn, pages;
 
 	/* Check that the physical memory area passed in looks valid */
-	if (start + len < start)
+	if (add_would_overflow(start, len))
 		return -EINVAL;
 	/*
 	 * You *really* shouldn't map things that aren't page-aligned,
@@ -2569,7 +2569,7 @@ int vm_iomap_memory(struct vm_area_struct *vma, phys_addr_t start, unsigned long
 	len += start & ~PAGE_MASK;
 	pfn = start >> PAGE_SHIFT;
 	pages = (len + ~PAGE_MASK) >> PAGE_SHIFT;
-	if (pfn + pages < pfn)
+	if (add_would_overflow(pfn, pages))
 		return -EINVAL;
 
 	/* We start the mapping 'vm_pgoff' pages into the area */
diff --git a/mm/mmap.c b/mm/mmap.c
index b78e83d351d2..16501fcaf511 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -3023,7 +3023,7 @@ SYSCALL_DEFINE5(remap_file_pages, unsigned long, start, unsigned long, size,
 		return ret;
 
 	/* Does pgoff wrap? */
-	if (pgoff + (size >> PAGE_SHIFT) < pgoff)
+	if (add_would_overflow(pgoff, (size >> PAGE_SHIFT)))
 		return ret;
 
 	if (mmap_write_lock_killable(mm))
diff --git a/mm/mremap.c b/mm/mremap.c
index 38d98465f3d8..efa27019a05d 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -848,7 +848,7 @@ static struct vm_area_struct *vma_to_resize(unsigned long addr,
 	/* Need to be careful about a growing mapping */
 	pgoff = (addr - vma->vm_start) >> PAGE_SHIFT;
 	pgoff += vma->vm_pgoff;
-	if (pgoff + (new_len >> PAGE_SHIFT) < pgoff)
+	if (add_would_overflow(pgoff, (new_len >> PAGE_SHIFT)))
 		return ERR_PTR(-EINVAL);
 
 	if (vma->vm_flags & (VM_DONTEXPAND | VM_PFNMAP))
diff --git a/mm/nommu.c b/mm/nommu.c
index b6dc558d3144..299bcfe19eed 100644
--- a/mm/nommu.c
+++ b/mm/nommu.c
@@ -202,7 +202,7 @@ EXPORT_SYMBOL(vmalloc_to_pfn);
 long vread_iter(struct iov_iter *iter, const char *addr, size_t count)
 {
 	/* Don't allow overflow */
-	if ((unsigned long) addr + count < count)
+	if (add_would_overflow(count, (unsigned long)addr))
 		count = -(unsigned long) addr;
 
 	return copy_to_iter(addr, count, iter);
@@ -1705,7 +1705,7 @@ int access_process_vm(struct task_struct *tsk, unsigned long addr, void *buf, in
 {
 	struct mm_struct *mm;
 
-	if (addr + len < addr)
+	if (add_would_overflow(addr, len))
 		return 0;
 
 	mm = get_task_mm(tsk);
diff --git a/mm/util.c b/mm/util.c
index 5a6a9802583b..e6beeb23b48b 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -567,7 +567,7 @@ unsigned long vm_mmap(struct file *file, unsigned long addr,
 	unsigned long len, unsigned long prot,
 	unsigned long flag, unsigned long offset)
 {
-	if (unlikely(offset + PAGE_ALIGN(len) < offset))
+	if (unlikely(add_would_overflow(offset, PAGE_ALIGN(len))))
 		return -EINVAL;
 	if (unlikely(offset_in_page(offset)))
 		return -EINVAL;
-- 
2.34.1