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[vfio] vfio: Use get_user_pages_longterm correctly

Message ID 20180629173150.GA24833@ziepe.ca (mailing list archive)
State New, archived
Headers show

Commit Message

Jason Gunthorpe June 29, 2018, 5:31 p.m. UTC 
The patch noted in the fixes below converted get_user_pages_fast() to
get_user_pages_longterm(), however the two calls differ in a few ways.

First _fast() is documented to not require the mmap_sem, while _longterm()
is documented to need it. Hold the mmap sem as required.

Second, _fast accepts an 'int write' while _longterm uses 'unsigned int
gup_flags', so the expression '!!(prot & IOMMU_WRITE)' is only working by
luck as FOLL_WRITE is currently == 0x1. Use the expected FOLL_WRITE
constant instead.

Fixes: 94db151dc892 ("vfio: disable filesystem-dax page pinning")
Cc: <stable@vger.kernel.org>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
---
 drivers/vfio/vfio_iommu_type1.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

I noticed this while trying to review some RDMA code that was touching
our get_user_pages_longterm() call site and wanted to see what others
are doing..

If someone can explain that get_user_pages_longterm() is safe to call
without the mmap_sem held I'd love to here it!

The comments in gup.c do seem to pretty clearly state the
__get_user_pages_locked() called internally by
get_user_pages_longterm() needs mmap_sem held..

This is confusing me because this is the only
get_user_pages_longterm() callsite that doesn't hold the mmap_sem, and
if it really isn't required I'd like to remove it from the RDMA code
as well :)

Thanks,
Jason
diff mbox

Patch

diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
index 2c75b33db4ac19..2612d63b1ba989 100644
--- a/drivers/vfio/vfio_iommu_type1.c
+++ b/drivers/vfio/vfio_iommu_type1.c
@@ -345,16 +345,17 @@  static int vaddr_get_pfn(struct mm_struct *mm, unsigned long vaddr,
 	struct vm_area_struct *vmas[1];
 	int ret;
 
+	down_read(&mm->mmap_sem);
 	if (mm == current->mm) {
-		ret = get_user_pages_longterm(vaddr, 1, !!(prot & IOMMU_WRITE),
-					      page, vmas);
+		ret = get_user_pages_longterm(
+			vaddr, 1, (prot & IOMMU_WRITE) ? FOLL_WRITE : 0, page,
+			vmas);
 	} else {
 		unsigned int flags = 0;
 
 		if (prot & IOMMU_WRITE)
 			flags |= FOLL_WRITE;
 
-		down_read(&mm->mmap_sem);
 		ret = get_user_pages_remote(NULL, mm, vaddr, 1, flags, page,
 					    vmas, NULL);
 		/*
@@ -368,8 +369,8 @@  static int vaddr_get_pfn(struct mm_struct *mm, unsigned long vaddr,
 			ret = -EOPNOTSUPP;
 			put_page(page[0]);
 		}
-		up_read(&mm->mmap_sem);
 	}
+	up_read(&mm->mmap_sem);
 
 	if (ret == 1) {
 		*pfn = page_to_pfn(page[0]);