CVE-2024-39298 in Linux
Riassunto
di VulDB • 15/06/2026
Based on the crash log and the analysis provided, here is a detailed breakdown of the root cause, the race condition, and potential solutions.
### 1. Root Cause Analysis
The crash is a **Use-After-Free / Double-Free / Invalid State** bug in the Linux kernel's memory failure handling path, specifically involving **HugePages** and the **Buddy Allocator**.
#### Key Components Involved: - **`memory_failure`**: The kernel's mechanism to handle hardware memory errors (e.g., ECC errors). - **`try_memory_failure_hugetlb` / `me_huge_page`**: Handles memory failure for HugePages. - **`__page_handle_poison`**: Core logic to handle a poisoned page. - **`dissolve_free_hugetlb_folio`**: Attempts to dissolve a free HugePage back into individual base pages so they can be returned to the buddy allocator. - **`drain_all_pages`**: Ensures all per-CPU page lists are drained before operating on pages. - **`take_page_off_buddy`**: Removes a page from the buddy allocator's free lists.
#### The Race Condition: The core issue is a **race between `dissolve_free_hugetlb_folio` and the buddy allocator's compaction/isolation mechanisms**.
1. **Scenario**: - A HugePage is identified as poisoned. - The kernel tries to dissolve it into base pages (`dissolve_free_hugetlb_folio`). - Before `dissolve_free_hugetlb_folio` completes, **another CPU** (or the same CPU in a different context) is performing **memory compaction** or **buddy allocator isolation**. - During compaction, pages are **isolated** from the buddy lists (removed temporarily) to be moved or merged. - `dissolve_free_hugetlb_folio` calls `take_page_off_buddy` on a page that is **already isolated** (not in the buddy list). - `take_page_off_buddy` fails or behaves incorrectly because the page is not in the expected state. - The page is then freed or returned to the buddy allocator in an **invalid state** (e.g., `mapcount` is non-zero, or flags are inconsistent). - Later, when the buddy allocator tries to free or use this page, it detects the bad state (`BUG: Bad page state`), leading to a panic.
2. **Why `mapcount` is non-zero?** - The `mapcount` field tracks how many times a page is mapped into user space. - If a page is isolated for compaction, its `mapcount` should still reflect its mapping status. - However, if the page was incorrectly freed or its state was corrupted during the dissolve process, the `mapcount` might be left in an inconsistent state (e.g., non-zero when it should be zero, or vice versa). - The `BUG: Bad page state` with `nonzero mapcount` indicates that the page is being freed or returned to the buddy allocator while still being mapped or in an invalid state.
### 2. Detailed Call Trace Analysis
``` free_unref_page+0x36e/0x5c0 unpoison_memory+0x50b/0x630 simple_attr_write_xsigned.constprop.0.isra.0+0xb3/0x110 debugfs_attr_write+0x42/0x60 full_proxy_write+0x5b/0x80 vfs_write+0xcd/0x550 ksys_write+0x64/0xe0 do_syscall_64+0xc2/0x1d0 entry_SYSCALL_64_after_hwframe+0x77/0x7f ```
- **`unpoison_memory`**: This is called when a user writes to `/sys/kernel/debug/hwpoison/unpoison` (or similar debugfs entry) to "unpoison" a page after it has been handled. - **`free_unref_page`**: This is the buddy allocator's function to free a page. - **`BUG: Bad page state`**: Triggered in `bad_page()` because the page's `mapcount` is non-zero, which is invalid for a page being freed.
This suggests that `unpoison_memory` is trying to free a page that is still in an invalid state, likely due to the earlier race condition during `dissolve_free_hugetlb_folio`.
### 3. Potential Fixes
#### Fix 1: Serialize `dissolve_free_hugetlb_folio` with Compaction Ensure that
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