[PATCH] ALSA: usx2y: bound the hwdep mmap fault offset

[Baul Lee <baul.lee@xbow.com>]

snd_us428ctls_vm_fault() turns the faulting page offset into a kernel
address with no bound of any kind:

	offset = vmf->pgoff << PAGE_SHIFT;
	vaddr = (char *)(...)->us428ctls_sharedmem + offset;
	page = virt_to_page(vaddr);
	get_page(page);
	vmf->page = page;

	return 0;

snd_us428ctls_mmap() checks only the length of the mapping, never the
offset, and us428ctls_sharedmem is a single page from
alloc_pages_exact().  For a character device file_mmap_size_max()
returns ULONG_MAX, so the mm layer imposes no ceiling either.  Every page
offset above zero resolves to a struct page outside the object, and the
handler installs it into the caller's address space read-write; the vma
is not marked read-only.

The caller picks the page frame with a single mmap() argument and gets
read-write access to a page of kernel memory it does not own; an offset
that lands in an unpopulated vmemmap region oopses instead.

A process that can open the hwdep node of an attached US-X2Y reaches
this after loading the FPGA image through the same node; no capability
check is involved.

On 7.2.0-rc5 (arm64), mmap() with a large offset:

  Unable to handle kernel paging request at virtual address fffffdffc45d5ac8
  pc : snd_us428ctls_vm_fault+0x68/0x140 [snd_usb_usx2y]
  Call trace:
   snd_us428ctls_vm_fault+0x68/0x140 [snd_usb_usx2y]
   __do_fault
   __handle_mm_fault
   handle_mm_fault
   el0_da

Reject any offset outside the shared region.  The pcm hwdep handler in
usx2yhwdeppcm.c computes its address the same way and needs the same
bound.

Discovered by XBOW, triaged by Baul Lee <baul.lee@xbow.com>

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-by: Federico Kirschbaum <federico.kirschbaum@xbow.com>
Reported-by: Baul Lee <baul.lee@xbow.com>
Cc: stable@vger.kernel.org
Signed-off-by: Baul Lee <baul.lee@xbow.com>
---
 sound/usb/usx2y/usX2Yhwdep.c   | 2 ++
 sound/usb/usx2y/usx2yhwdeppcm.c | 2 ++
 2 files changed, 4 insertions(+)

diff --git a/sound/usb/usx2y/usX2Yhwdep.c b/sound/usb/usx2y/usX2Yhwdep.c
index 4d7925184826..7f153a2fcdd8 100644
--- a/sound/usb/usx2y/usX2Yhwdep.c
+++ b/sound/usb/usx2y/usX2Yhwdep.c
@@ -22,12 +22,14 @@ static vm_fault_t snd_us428ctls_vm_fault(struct vm_fault *vmf)
 {
 	unsigned long offset;
 	struct page *page;
 	void *vaddr;
 
 	offset = vmf->pgoff << PAGE_SHIFT;
+	if (offset >= US428_SHAREDMEM_PAGES)
+		return VM_FAULT_SIGBUS;
 	vaddr = (char *)((struct usx2ydev *)vmf->vma->vm_private_data)->us428ctls_sharedmem + offset;
 	page = virt_to_page(vaddr);
 	get_page(page);
 	vmf->page = page;
 
 	return 0;
diff --git a/sound/usb/usx2y/usx2yhwdeppcm.c b/sound/usb/usx2y/usx2yhwdeppcm.c
index 7c90214485d9..f1d5b4b797ad 100644
--- a/sound/usb/usx2y/usx2yhwdeppcm.c
+++ b/sound/usb/usx2y/usx2yhwdeppcm.c
@@ -669,12 +669,14 @@ static void snd_usx2y_hwdep_pcm_vm_close(struct vm_area_struct *area)
 static vm_fault_t snd_usx2y_hwdep_pcm_vm_fault(struct vm_fault *vmf)
 {
 	unsigned long offset;
 	void *vaddr;
 
 	offset = vmf->pgoff << PAGE_SHIFT;
+	if (offset >= USX2Y_HWDEP_PCM_PAGES)
+		return VM_FAULT_SIGBUS;
 	vaddr = (char *)((struct usx2ydev *)vmf->vma->vm_private_data)->hwdep_pcm_shm + offset;
 	vmf->page = virt_to_page(vaddr);
 	get_page(vmf->page);
 	return 0;
 }
 
-- 
2.50.1 (Apple Git-155)