Linux kernel XFS: exchange-range TO_EOF corrupts file sizes, letting shared blocks be rewritten

In the Linux kernel, the following vulnerability has been resolved:

xfs: fix exchange-range-to-eof file size exchange

Norbert Szetei posted a patch containing an incomplete description of a bug in XFS_IOC_EXCHANGE_RANGE's TO_EOF flag. The bug tricks exchange-range into modifying a file so that it has shared blocks starting beyond EOF, which is never allowed because files never have written data beyond EOF, and you can only share written data blocks. This is key to all the incorrect behavior that follows.

Eventually I got from Norbert a description of what's going wrong:

Fair. Here it is with physical block numbers, 4k blocks. peer is the file that gets rewritten and file1 is the one that ends up with the flag cleared.

Start, nothing shared:

peer size 4096 -> [100] file1 size 12288 -> [200] [201] [202] donor1 size 4096 -> [300] donor2 size 4096 -> [400]

1. FICLONERANGE peer's block into file1's middle slot:

peer size 4096 -> [100] file1 size 12288 -> [200] [100] [202] reflink flag set ^^^^^ both files own block 100

1. XFS_IOC_EXCHANGE_RANGE file1 against donor1, file1_offset 0, file2_offset 8192, length 0, TO_EOF. One block is exchanged, and the sizes are exchanged with it:

file1 size 4096 -> [200] [100] [300] reflink flag set ^^^^^^^^^^^ still owned, now past the size donor1 size 12288 -> [202]

file1 says it is one block long while it still owns three. Nothing was unmapped. file2_offset is block 2, so this exchange cannot clear a flag.

1. XFS_IOC_EXCHANGE_RANGE file1 against donor2, both offsets 0, length 4096. By i_disk_size this is two one-block files exchanging everything, so xmi_can_exchange_reflink_flags() agrees and the flag is cleared:

file1 size 4096 -> [400] [100] [300] reflink flag CLEARED donor2 size 4096 -> [200]

Block 100 is still shared with peer.

1. pwrite(file1, 4096, 4096), which is file1's second slot, block 100. xfs_is_cow_inode(file1) is false, so no CoW:

peer size 4096 -> [100] same block, new contents

Block 100 is the only one that matters here. PoC below, it reads a file called peer in the current directory. The filesystem needs reflink and exchange-range, which mkfs.xfs 7.x turns on by default.

So yes, TO_EOF is screwing up the file size even if it's actually exchanging the data block correctly. The following is the intended behavior of XFS_EXCHANGE_RANGE_TO_EOF, as described by its manpage:

"Ignore the length parameter. All bytes in file1_fd from file1_offset to EOF are moved to file2_fd, and file2's size is set to (file2_offset+(file1_length-file1_offset)). Meanwhile, all bytes in file2 from file2_offset to EOF are moved to file1 and file1's size is set to (file1_offset+(file2_length-file2_offset))."

In other words, if we only exchange a portion of the file data, then we should only exchange the portion of the file size that relates to the exchanged data. Unfortunately, the code swaps the ondisk file size and the incore file sizes, which is incorrect if the caller passes in a nonzero offset.

Having confused the file sizes, the PoC uses a second exchange-range call on what looks like a non-reflinked single-block file. Because the file size is set incorrectly, exchange-range thinks it's exchanging the full contents of two files and clears the reflink flag on the broken file. That enables an extending write of the broken file to rewrite the shared block that's just past EOF. This has become known colloquially as refluxfs.

Once the file sizes are set correctly, the second exchange-range no longer thinks that it's doing a full-contents swap, so it won't clear the reflink flag on either of its file arguments.

Norbert followed up to an earlier version of this patch with a second complaint about us not handling the i_disk_size state correctly:

The offsets are bounded against the incore i_size but subtracted from

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