{"id":"CVE-2026-98939","title":"x86/mm: Drop unnecessary PMD page copy when freeing","summary":"In the Linux kernel, the following vulnerability has been resolved:\n\nx86/mm: Drop unnecessary PMD page copy when freeing\n\nOn a box with a discrete GPU, lockdep reports a possible deadlock as\nsoon as kswapd shrinks the TTM page pool. The …","severity":"none","vendor":"Linux","product":"Linux","affected":["Linux >= e21a9ea81426a85fdb0cdcfcc99d5d9c16b402ab < d3c2d2692bdf4af94bb27bc7a3aa0a348a567606","Linux >= cc73b2043d106b467accd38e9b1fc09e0607c056 < f38c57a28ebbd057471573e39555d5aa11ab03cf","Linux >= d5d8b8662e6e5a565b47a0388640e88402f23274 < e3ee38c1bc0b11a8d3e63dce7050759b61864286","Linux >= 6.18.53 < 6.18.56","Linux >= 7.2.7 < 7.2.10"],"published":"2026-10-23","updated":"2026-10-23","sourceUpdated":"2026-10-23T12:12:06.180Z","source":"CVEORG","sourceUrl":"https://www.cve.org/CVERecord?id=CVE-2026-98939","references":[{"url":"https://git.kernel.org/stable/c/d3c2d2692bdf4af94bb27bc7a3aa0a348a567606"},{"url":"https://git.kernel.org/stable/c/f38c57a28ebbd057471573e39555d5aa11ab03cf"},{"url":"https://git.kernel.org/stable/c/e3ee38c1bc0b11a8d3e63dce7050759b61864286"}],"tags":["cve.org"],"ingestedAt":"2026-10-23T13:24:55.562Z","slug":"CVE-2026-98939","body":"## Overview\n\nIn the Linux kernel, the following vulnerability has been resolved:\n\nx86/mm: Drop unnecessary PMD page copy when freeing\n\nOn a box with a discrete GPU, lockdep reports a possible deadlock as\nsoon as kswapd shrinks the TTM page pool. The immediate cause is an\nx86 commit that added an mmap_read_lock() to kernel page protection\nmunging code.\n\nThe huge vmap code holds the same lock over a GFP_KERNEL allocation,\nwhich is a no-no now that reclaim can take it. That allocation is in a\npage table *free* path and ends up being for dubious purposes[1].\nBasically, it tries to avoid hardware setting Accessed=1 in page table\nentries that are unreachable by the hardware, a non-issue.\n\nRemove the PMD copy. Detach the original PMD page at the PUD, flush\nthe mid-level caches, and free the PTE tables straight from the\ndetached PMD page. With no allocation left, the locking issue is gone.\n\nLockdep splat/analysis:\n\n  WARNING: possible circular locking dependency detected\n  7.3.0-rc3-f6e7b42bf05b+ #183 Tainted: G     U\n  ------------------------------------------------------\n  kswapd0/269 is trying to acquire lock:\n  ((init_mm).mmap_lock){++++}-{4:4}, at: change_page_attr_set_clr+0x29a/0x4a0\n  but task is already holding lock:\n  (pool_shrink_rwsem){.+.+}-{4:4}, at: ttm_pool_shrink+0xb2/0x330 [ttm]\n  Chain exists of:\n    (init_mm).mmap_lock --> fs_reclaim --> pool_shrink_rwsem\n\nThe cycle is built from three edges:\n\n  1) pool_shrink_rwsem -> (init_mm).mmap_lock\n\n     The TTM shrinker restores the caching attribute of every page it\n     frees, while holding pool_shrink_rwsem:\n\n       ttm_pool_shrink()\n       -> ttm_pool_dispose_list()\n       -> ttm_pool_free_page()\n       -> set_pages_wb()\n       -> change_page_attr_set_clr()   [ init_mm mmap read lock ]\n\n  2) fs_reclaim -> pool_shrink_rwsem\n\n     The same shrinker, called from reclaim.\n\n  3) (init_mm).mmap_lock -> fs_reclaim\n\n     ioremap() installing a huge PUD mapping over an existing PMD table:\n\n       ioremap_page_range()\n       -> vmap_range_noflush()\n       -> vmap_try_huge_pud()          [ init_mm mmap read lock ]\n       -> pud_free_pmd_page()\n       -> __get_free_page(GFP_KERNEL)  [ enters reclaim ]\n\n[ dhansen: Lots of changelog munging/trimming and merged comments from my\n\t   version of the fix. ]\n\n## Affected\n\n- `Linux >= e21a9ea81426a85fdb0cdcfcc99d5d9c16b402ab < d3c2d2692bdf4af94bb27bc7a3aa0a348a567606`\n- `Linux >= cc73b2043d106b467accd38e9b1fc09e0607c056 < f38c57a28ebbd057471573e39555d5aa11ab03cf`\n- `Linux >= d5d8b8662e6e5a565b47a0388640e88402f23274 < e3ee38c1bc0b11a8d3e63dce7050759b61864286`\n- `Linux >= 6.18.53 < 6.18.56`\n- `Linux >= 7.2.7 < 7.2.10`\n\n## Remediation\n\nRefer to the linked advisories for vendor-supplied fixes and affected version ranges.","depth":"sunlit","depthScore":3,"depthScoreParts":{"impact":2.8,"likelihood":0,"exploitation":0,"ransomware":0},"changes":[]}