[PATCH 2/2] device-dax: implement ->split() to catch invalid munmap attempts

[Dan Williams <dan.j.williams@intel.com>]

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

Cc: <stable@vger.kernel.org>
Fixes: dee410792419 ("/dev/dax, core: file operations and dax-mmap")
Reported-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
---
 drivers/dax/device.c |   12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/drivers/dax/device.c b/drivers/dax/device.c
index 6833ada237ab..7b0bf825c4e7 100644
--- a/drivers/dax/device.c
+++ b/drivers/dax/device.c
@@ -428,9 +428,21 @@ static int dev_dax_fault(struct vm_fault *vmf)
 	return dev_dax_huge_fault(vmf, PE_SIZE_PTE);
 }
 
+static int dev_dax_split(struct vm_area_struct *vma, unsigned long addr)
+{
+	struct file *filp = vma->vm_file;
+	struct dev_dax *dev_dax = filp->private_data;
+	struct dax_region *dax_region = dev_dax->region;
+
+	if (!IS_ALIGNED(addr, dax_region->align))
+		return -EINVAL;
+	return 0;
+}
+
 static const struct vm_operations_struct dax_vm_ops = {
 	.fault = dev_dax_fault,
 	.huge_fault = dev_dax_huge_fault,
+	.split = dev_dax_split,
 };
 
 static int dax_mmap(struct file *filp, struct vm_area_struct *vma)

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