- Description
- In the Linux kernel, the following vulnerability has been resolved:
mm/huge_memory: use correct flags for device private PMD entry
Commit 65edfda6f3f2 ("mm/rmap: extend rmap and migration support
device-private entries") updated set_pmd_migration_entry() to use
pmdp_huge_get_and_clear() in the softleaf case, but made no further
adjustments to the function itself.
Therefore this function continues to incorrectly use pmd_write(),
pmd_soft_dirty() and pmd_uffd_wp() to determine whether the installed
migration entry should be marked writable, softdirty or uffd-wp
respectively.
Whilst all are incorrect, the most problematic of these is pmd_write(), as
this can lead to corrupted rmap state.
On x86-64 _PAGE_SWP_SOFT_DIRTY is aliased to _PAGE_RW. So calling
pmd_write() on a softleaf will return the softdirty state encoded in the
entry, assuming CONFIG_MEM_SOFT_DIRTY was enabled.
This was observed when running the hmm.hmm_device_private.anon_write_child
selftest:
1. The test faults in a range then migrates it such that a device-private
THP range is established.
2. The parent then migrates it to a device-private writable PMD entry whose
folio is entirely AnonExclusive with entire_mapcount=1, softdirty set
(accidentally correct write state).
3. The parent forks and the PMD entries are set to device-private read only
entries, entire_mapcount=2, softdirty still set.
4. [BUG] The child writes to the range then migrates to RAM - intending to
install non-writable migration entries - but replacing parent and child
PMD mappings with WRITABLE entries due to misinterpreting the softdirty
bit.
5. In remove_migration_pmd(), if !softleaf_is_migration_read(entry) we
set the RMAP_EXCLUSIVE flag when calling folio_add_anon_rmap_pmd() for
both parent and child, which are therefore AnonExclusive.
6. [SPLAT] Child sets migrated folio entire_mapcount=1, parent sets
entire_mapcount=2 and we end up with an AnonExclusive folio with
entire_mapcount=2! Assert fires in __folio_add_anon_rmap():
VM_WARN_ON_FOLIO(folio_test_large(folio) &&
folio_entire_mapcount(folio) > 1 &&
PageAnonExclusive(cur_page), folio)
This patch fixes the issue by correctly referencing the softleaf entry
fields for writable, softdirty and uffd-wp in set_pmd_migration_entry().
It also only updates A/D flags if the entry is present as these are
otherwise not meaningful for a softleaf entry.
This patch also flips the if (!present) { ... } else { ... } logic in
set_pmd_migration_entry() so it is easier to understand, and adds some
comments to make things clearer.
I was able to bisect this to commit 775465fd26a3 ("lib/test_hmm: add zone
device private THP test infrastructure") which first exposes this bug as
it was the commit that permitted test_hmm to generate the test.
However commit 65edfda6f3f2 ("mm/rmap: extend rmap and migration support
device-private entries") is the commit that actually enabled this
behaviour.
- Source
- 416baaa9-dc9f-4396-8d5f-8c081fb06d67
- NVD status
- Analyzed
- Products
- linux_kernel
[
{
"nodes": [
{
"cpeMatch": [
{
"criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
"matchCriteriaId": "6A64BF9F-3BCA-42FD-98CB-8F03474D2B1E",
"versionEndExcluding": "7.0.13",
"versionStartIncluding": "6.19",
"vulnerable": true
},
{
"criteria": "cpe:2.3:o:linux:linux_kernel:7.1:rc1:*:*:*:*:*:*",
"matchCriteriaId": "B1EF7059-E670-45F4-B422-54C40FA86390",
"vulnerable": true
},
{
"criteria": "cpe:2.3:o:linux:linux_kernel:7.1:rc2:*:*:*:*:*:*",
"matchCriteriaId": "0D38F0BF-A728-4133-A358-D44A2F7EE6D6",
"vulnerable": true
},
{
"criteria": "cpe:2.3:o:linux:linux_kernel:7.1:rc3:*:*:*:*:*:*",
"matchCriteriaId": "EC732D08-5F7B-46D9-B154-E60C7F4F0A97",
"vulnerable": true
},
{
"criteria": "cpe:2.3:o:linux:linux_kernel:7.1:rc4:*:*:*:*:*:*",
"matchCriteriaId": "E5910A9D-F60A-409A-B486-FE66BFEBA9B9",
"vulnerable": true
},
{
"criteria": "cpe:2.3:o:linux:linux_kernel:7.1:rc5:*:*:*:*:*:*",
"matchCriteriaId": "81DFF19E-9CF8-49C6-8C36-1E4038622933",
"vulnerable": true
},
{
"criteria": "cpe:2.3:o:linux:linux_kernel:7.1:rc6:*:*:*:*:*:*",
"matchCriteriaId": "B0E8FC71-3952-444C-83E9-718DBBBEC615",
"vulnerable": true
},
{
"criteria": "cpe:2.3:o:linux:linux_kernel:7.1:rc7:*:*:*:*:*:*",
"matchCriteriaId": "1039E95A-8CC3-4C88-8FF9-5C08EEB861C9",
"vulnerable": true
}
],
"negate": false,
"operator": "OR"
}
]
}
]