CVE ID : CVE-2024-50250

Published : Nov. 9, 2024, 11:15 a.m. | 1 hour, 1 minute ago

Description : 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 bfoster’s 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.

We’re not done yet — there’s no call to invalidate_inode_pages2_range,
so programs that have the file range mmap’d 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 we’re
operating on.

Severity: 0.0 | NA

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