[4.9-stable PATCH 04/11] device-dax: implement ->split() to catch invalid munmap attempts

[Dan Williams]

commit 9702cffdbf2129516db679e4467db81e1cd287da upstream.

Similar to how device-dax enforces that the 'address', 'offset', and
'len' parameters to mmap() be aligned to the device's fundamental
alignment, the same constraints apply to munmap().  Implement ->split()
to fail munmap calls that violate the alignment constraint.

Otherwise, we later fail VM_BUG_ON checks in the unmap_page_range() path
with crash signatures of the form:

    vma ffff8800b60c8a88 start 00007f88c0000000 end 00007f88c0e00000
    next           (null) prev           (null) mm ffff8800b61150c0
    prot 8000000000000027 anon_vma           (null) vm_ops ffffffffa0091240
    pgoff 0 file ffff8800b638ef80 private_data           (null)
    flags: 0x380000fb(read|write|shared|mayread|maywrite|mayexec|mayshare|softdirty|mixedmap|hugepage)
    ------------[ cut here ]------------
    kernel BUG at mm/huge_memory.c:2014!
    [..]
    RIP: 0010:__split_huge_pud+0x12a/0x180
    [..]
    Call Trace:
     unmap_page_range+0x245/0xa40
     ? __vma_adjust+0x301/0x990
     unmap_vmas+0x4c/0xa0
     unmap_region+0xae/0x120
     ? __vma_rb_erase+0x11a/0x230
     do_munmap+0x276/0x410
     vm_munmap+0x6a/0xa0
     SyS_munmap+0x1d/0x30

Link: http://lkml.kernel.org/r/151130418681.4029.7118245855057952010.stgit@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Fixes: dee410792419 ("/dev/dax, core: file operations and dax-mmap")
Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
Reported-by: Jeff Moyer <jmoyer@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
---
 drivers/dax/dax.c |   12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/drivers/dax/dax.c b/drivers/dax/dax.c
index 40be3747724d..473b44c008dd 100644
--- a/drivers/dax/dax.c
+++ b/drivers/dax/dax.c
@@ -453,9 +453,21 @@ static int dax_dev_pmd_fault(struct vm_area_struct *vma, unsigned long addr,
 	return rc;
 }
 
+static int dax_dev_split(struct vm_area_struct *vma, unsigned long addr)
+{
+	struct file *filp = vma->vm_file;
+	struct dax_dev *dax_dev = filp->private_data;
+	struct dax_region *dax_region = dax_dev->region;
+
+	if (!IS_ALIGNED(addr, dax_region->align))
+		return -EINVAL;
+	return 0;
+}
+
 static const struct vm_operations_struct dax_dev_vm_ops = {
 	.fault = dax_dev_fault,
 	.pmd_fault = dax_dev_pmd_fault,
+	.split = dax_dev_split,
 };
 
 static int dax_mmap(struct file *filp, struct vm_area_struct *vma)