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;