[PATCH] block: Deny writable memory mapping if block is read-only

[Loic Poulain <loic.poulain@linaro.org>]

User should not be able to write block device if it is read-only at
block level (e.g force_ro attribute). This is ensured in the regular
fops write operation (blkdev_write_iter) but not when writing via
user mapping (mmap), allowing user to actually write a read-only
block device via a PROT_WRITE mapping.

Example: This can lead to integrity issue of eMMC boot partition
(e.g mmcblk0boot0) which is read-only by default.

To fix this issue, simply deny shared writable mapping if the block
is readonly.

Note: Block remains writable if switch to read-only is performed
after the initial mapping, but this is expected behavior according
to commit a32e236eb93e ("Partially revert "block: fail op_is_write()
requests to read-only partitions"")'.

Signed-off-by: Loic Poulain <loic.poulain@linaro.org>
---
 block/fops.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/block/fops.c b/block/fops.c
index d2e6be4e3d1c..58d0aebc7313 100644
--- a/block/fops.c
+++ b/block/fops.c
@@ -678,6 +678,16 @@ static long blkdev_fallocate(struct file *file, int mode, loff_t start,
 	return error;
 }
 
+static int blkdev_mmap(struct file *file, struct vm_area_struct *vma)
+{
+	struct inode *bd_inode = bdev_file_inode(file);
+
+	if (bdev_read_only(I_BDEV(bd_inode)))
+		return generic_file_readonly_mmap(file, vma);
+
+	return generic_file_mmap(file, vma);
+}
+
 const struct file_operations def_blk_fops = {
 	.open		= blkdev_open,
 	.release	= blkdev_close,
@@ -685,7 +695,7 @@ const struct file_operations def_blk_fops = {
 	.read_iter	= blkdev_read_iter,
 	.write_iter	= blkdev_write_iter,
 	.iopoll		= iocb_bio_iopoll,
-	.mmap		= generic_file_mmap,
+	.mmap		= blkdev_mmap,
 	.fsync		= blkdev_fsync,
 	.unlocked_ioctl	= blkdev_ioctl,
 #ifdef CONFIG_COMPAT
-- 
2.34.1