Описание
The Linux Kernel versions 2.6.38 through 4.14 have a problematic use of pmd_mkdirty() in the touch_pmd() function inside the THP implementation. touch_pmd() can be reached by get_user_pages(). In such case, the pmd will become dirty. This scenario breaks the new can_follow_write_pmd()'s logic - pmd can become dirty without going through a COW cycle. This bug is not as severe as the original "Dirty cow" because an ext4 file (or any other regular file) cannot be mapped using THP. Nevertheless, it does allow us to overwrite read-only huge pages. For example, the zero huge page and sealed shmem files can be overwritten (since their mapping can be populated using THP). Note that after the first write page-fault to the zero page, it will be replaced with a new fresh (and zeroed) thp.
A flaw was found in the patches used to fix the 'dirtycow' vulnerability (CVE-2016-5195). An attacker, able to run local code, can exploit a race condition in transparent huge pages to modify usually read-only huge pages.
Отчет
From the initial flaw description released, this issue does not affect the Linux kernel packages as shipped with Red Hat Enterprise Linux 5, 6, 7 and Red Hat Enterprise MRG 2. They do not contain the upstream commit (8310d48b125d19fcd9521d83b8293e63eb1646aa) which allows the race condition. This issue affects the Linux kernel packages as shipped with Red Hat Enterprise Linux 7 for ARM and Red Hat Enterprise Linux for Power LE. Future Linux kernel updates for the respective releases may address this issue. For further information please refer to the vulnerability article in the Customer Portal: https://access.redhat.com/security/vulnerabilities/3253921
Меры по смягчению последствий
Disabling the use of zero page: It is possible to prevent the zero page from being mapped as a huge page, by modifying a configuration tunable in the /sys directory as shown below. This prevents the flaw from being exercised in this method.
echo 0 > /sys/kernel/mm/transparent_hugepage/use_zero_page
Disabling huge pages: It is possible to mitigate this flaw by disabling hugepages on a system. Some user applications may require hugepages to be performant, and may suffer a performance penalty when running without hugepages. Red Hat has existing solutions created instructing how to disable transparent How to disable transparent hugepages (THP) on Red Hat Enterprise Linux 7 https://access.redhat.com/solutions/1320153 If your application may require transparent huge pages, please consult your software vendor to confirm.
Затронутые пакеты
| Платформа | Пакет | Состояние | Рекомендация | Релиз |
|---|---|---|---|---|
| Red Hat Enterprise Linux 5 | kernel | Not affected | ||
| Red Hat Enterprise Linux 6 | kernel | Not affected | ||
| Red Hat Enterprise Linux 7 | kernel | Not affected | ||
| Red Hat Enterprise Linux 7 | kernel-rt | Not affected | ||
| Red Hat Enterprise MRG 2 | realtime-kernel | Not affected | ||
| Red Hat Enterprise Linux 7 | kernel-alt | Fixed | RHSA-2018:0180 | 25.01.2018 |
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Дополнительная информация
Статус:
EPSS
6.1 Medium
CVSS3
Связанные уязвимости
The Linux Kernel versions 2.6.38 through 4.14 have a problematic use of pmd_mkdirty() in the touch_pmd() function inside the THP implementation. touch_pmd() can be reached by get_user_pages(). In such case, the pmd will become dirty. This scenario breaks the new can_follow_write_pmd()'s logic - pmd can become dirty without going through a COW cycle. This bug is not as severe as the original "Dirty cow" because an ext4 file (or any other regular file) cannot be mapped using THP. Nevertheless, it does allow us to overwrite read-only huge pages. For example, the zero huge page and sealed shmem files can be overwritten (since their mapping can be populated using THP). Note that after the first write page-fault to the zero page, it will be replaced with a new fresh (and zeroed) thp.
The Linux Kernel versions 2.6.38 through 4.14 have a problematic use of pmd_mkdirty() in the touch_pmd() function inside the THP implementation. touch_pmd() can be reached by get_user_pages(). In such case, the pmd will become dirty. This scenario breaks the new can_follow_write_pmd()'s logic - pmd can become dirty without going through a COW cycle. This bug is not as severe as the original "Dirty cow" because an ext4 file (or any other regular file) cannot be mapped using THP. Nevertheless, it does allow us to overwrite read-only huge pages. For example, the zero huge page and sealed shmem files can be overwritten (since their mapping can be populated using THP). Note that after the first write page-fault to the zero page, it will be replaced with a new fresh (and zeroed) thp.
The Linux Kernel versions 2.6.38 through 4.14 have a problematic use o ...
The Linux Kernel versions 2.6.38 through 4.14 have a problematic use of pmd_mkdirty() in the touch_pmd() function inside the THP implementation. touch_pmd() can be reached by get_user_pages(). In such case, the pmd will become dirty. This scenario breaks the new can_follow_write_pmd()'s logic - pmd can become dirty without going through a COW cycle. This bug is not as severe as the original "Dirty cow" because an ext4 file (or any other regular file) cannot be mapped using THP. Nevertheless, it does allow us to overwrite read-only huge pages. For example, the zero huge page and sealed shmem files can be overwritten (since their mapping can be populated using THP). Note that after the first write page-fault to the zero page, it will be replaced with a new fresh (and zeroed) thp.
Уязвимость функции touch_pmd() ядра операционных систем Linux, позволяющая нарушителю получить доступ к конфиденциальным данным, нарушить их целостность, а также вызвать отказ в обслуживании
EPSS
6.1 Medium
CVSS3