CVE-2024-50250

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

fsdax: dax_unshare_iter needs to copy entire blocks

The code that copies data from srcmap to iomap in dax_unshare_iter is very very broken, which bfosters recent fsx changes have exposed.

If the pos and len passed to dax_file_unshare are not aligned to an fsblock boundary, the iter pos and length in the _iter function will reflect this unalignment.

dax_iomap_direct_access always returns a pointer to the start of the kmapped fsdax page, even if its pos argument is in the middle of that page. This is catastrophic for data integrity when iter->pos is not aligned to a page, because daddr/saddr do not point to the same byte in the file as iter->pos. Hence we corrupt user data by copying it to the wrong place.

If iter->pos + iomap_length() in the _iter function not aligned to a page, then we fail to copy a full block, and only partially populate the destination block. This is catastrophic for data confidentiality because we expose stale pmem contents.

Fix both of these issues by aligning copy_pos/copy_len to a page boundary (remember, this is fsdax so 1 fsblock == 1 base page) so that we always copy full blocks.

Were not done yet – theres no call to invalidate_inode_pages2_range, so programs that have the file range mmapd will continue accessing the old memory mapping after the file metadata updates have completed.

Be careful with the return value – if the unshare succeeds, we still need to return the number of bytes that the iomap iter thinks were operating on.

Affected Software

References

• https://git.kernel.org/stable/c/50793801fc7f6d08def48754fb0f0706b0cfc394
• https://git.kernel.org/stable/c/8e9c0f500b42216ef930f5c0d1703989a451913d
• https://git.kernel.org/stable/c/9bc18bb476e50e32e5d08f2734d63d63e0fa528c
• https://git.kernel.org/stable/c/bdbc96c23197d773a7d1bf03e4f11de593b0ff28