dax: explain how read(2)/write(2) addresses are validated
Add a comment explaining how the user addresses provided to read(2) and write(2) are validated in the DAX I/O path. We call dax_copy_from_iter() or copy_to_iter() on these without calling access_ok() first in the DAX code, and there was a concern that the user might be able to read/write to arbitrary kernel addresses with this path. Link: http://lkml.kernel.org/r/20170816173615.10098-1-ross.zwisler@linux.intel.com Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com> Reviewed-by: Jan Kara <jack@suse.cz> Cc: Christoph Hellwig <hch@lst.de> Cc: Dan Williams <dan.j.williams@intel.com> Cc: Matthew Wilcox <mawilcox@microsoft.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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fs/dax.c
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fs/dax.c
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@ -1004,6 +1004,11 @@ dax_iomap_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
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if (map_len > end - pos)
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map_len = end - pos;
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/*
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* The userspace address for the memory copy has already been
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* validated via access_ok() in either vfs_read() or
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* vfs_write(), depending on which operation we are doing.
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*/
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if (iov_iter_rw(iter) == WRITE)
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map_len = dax_copy_from_iter(dax_dev, pgoff, kaddr,
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map_len, iter);
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