[PATCH v6 07/17] mm: Close race in generic_access_phys

[Daniel Vetter]

Way back it was a reasonable assumptions that iomem mappings never
change the pfn range they point at. But this has changed:

- gpu drivers dynamically manage their memory nowadays, invalidating
  ptes with unmap_mapping_range when buffers get moved

- contiguous dma allocations have moved from dedicated carvetouts to
  cma regions. This means if we miss the unmap the pfn might contain
  pagecache or anon memory (well anything allocated with GFP_MOVEABLE)

- even /dev/mem now invalidates mappings when the kernel requests that
  iomem region when CONFIG_IO_STRICT_DEVMEM is set, see 3234ac664a87
  ("/dev/mem: Revoke mappings when a driver claims the region")

Accessing pfns obtained from ptes without holding all the locks is
therefore no longer a good idea. Fix this.

Since ioremap might need to manipulate pagetables too we need to drop
the pt lock and have a retry loop if we raced.

While at it, also add kerneldoc and improve the comment for the
vma_ops->access function. It's for accessing, not for moving the
memory from iomem to system memory, as the old comment seemed to
suggest.

References: 28b2ee20c7cb ("access_process_vm device memory infrastructure")
Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
Cc: Jason Gunthorpe <jgg at ziepe.ca>
Cc: Dan Williams <dan.j.williams at intel.com>
Cc: Kees Cook <keescook at chromium.org>
Cc: Benjamin Herrensmidt <benh at kernel.crashing.org>
Cc: Dave Airlie <airlied at linux.ie>
Cc: Andrew Morton <akpm at linux-foundation.org>
Cc: John Hubbard <jhubbard at nvidia.com>
Cc: Jérôme Glisse <jglisse at redhat.com>
Cc: Jan Kara <jack at suse.cz>
Cc: Dan Williams <dan.j.williams at intel.com>
Cc: linux-mm at kvack.org
Cc: linux-arm-kernel at lists.infradead.org
Cc: linux-samsung-soc at vger.kernel.org
Cc: linux-media at vger.kernel.org
Cc: Chris Wilson <chris at chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter at ffwll.ch>
--
v2: Fix inversion in the retry check (John).

v4: While at it, use offset_in_page (Chris Wilson)
---
 include/linux/mm.h |  3 ++-
 mm/memory.c        | 46 +++++++++++++++++++++++++++++++++++++++++++---
 2 files changed, 45 insertions(+), 4 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index b1a4a140863d..06cc6f30fc78 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -574,7 +574,8 @@ struct vm_operations_struct {
 	vm_fault_t (*pfn_mkwrite)(struct vm_fault *vmf);
 
 	/* called by access_process_vm when get_user_pages() fails, typically
-	 * for use by special VMAs that can switch between memory and hardware
+	 * for use by special VMAs. See also generic_access_phys() for a generic
+	 * implementation useful for any iomem mapping.
 	 */
 	int (*access)(struct vm_area_struct *vma, unsigned long addr,
 		      void *buf, int len, int write);
diff --git a/mm/memory.c b/mm/memory.c
index c48f8df6e502..ac32039ce941 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4847,28 +4847,68 @@ int follow_phys(struct vm_area_struct *vma,
 	return ret;
 }
 
+/**
+ * generic_access_phys - generic implementation for iomem mmap access
+ * @vma: the vma to access
+ * @addr: userspace addres, not relative offset within @vma
+ * @buf: buffer to read/write
+ * @len: length of transfer
+ * @write: set to FOLL_WRITE when writing, otherwise reading
+ *
+ * This is a generic implementation for &vm_operations_struct.access for an
+ * iomem mapping. This callback is used by access_process_vm() when the @vma is
+ * not page based.
+ */
 int generic_access_phys(struct vm_area_struct *vma, unsigned long addr,
 			void *buf, int len, int write)
 {
 	resource_size_t phys_addr;
 	unsigned long prot = 0;
 	void __iomem *maddr;
-	int offset = addr & (PAGE_SIZE-1);
+	pte_t *ptep, pte;
+	spinlock_t *ptl;
+	int offset = offset_in_page(addr);
+	int ret = -EINVAL;
+
+	if (!(vma->vm_flags & (VM_IO | VM_PFNMAP)))
+		return -EINVAL;
+
+retry:
+	if (follow_pte(vma->vm_mm, addr, &ptep, &ptl))
+		return -EINVAL;
+	pte = *ptep;
+	pte_unmap_unlock(ptep, ptl);
 
-	if (follow_phys(vma, addr, write, &prot, &phys_addr))
+	prot = pgprot_val(pte_pgprot(pte));
+	phys_addr = (resource_size_t)pte_pfn(pte) << PAGE_SHIFT;
+
+	if ((write & FOLL_WRITE) && !pte_write(pte))
 		return -EINVAL;
 
 	maddr = ioremap_prot(phys_addr, PAGE_ALIGN(len + offset), prot);
 	if (!maddr)
 		return -ENOMEM;
 
+	if (follow_pte(vma->vm_mm, addr, &ptep, &ptl))
+		goto out_unmap;
+
+	if (!pte_same(pte, *ptep)) {
+		pte_unmap_unlock(ptep, ptl);
+		iounmap(maddr);
+
+		goto retry;
+	}
+
 	if (write)
 		memcpy_toio(maddr + offset, buf, len);
 	else
 		memcpy_fromio(buf, maddr + offset, len);
+	ret = len;
+	pte_unmap_unlock(ptep, ptl);
+out_unmap:
 	iounmap(maddr);
 
-	return len;
+	return ret;
 }
 EXPORT_SYMBOL_GPL(generic_access_phys);
 #endif
-- 
2.29.2