Wind River Support Network

Description

In the Linux kernel, the following vulnerability has been resolved:  x86/mm: Drop unnecessary PMD page copy when freeing  On a box with a discrete GPU, lockdep reports a possible deadlock as soon as kswapd shrinks the TTM page pool. The immediate cause is an x86 commit that added an mmap_read_lock() to kernel page protection munging code.  The huge vmap code holds the same lock over a GFP_KERNEL allocation, which is a no-no now that reclaim can take it. That allocation is in a page table *free* path and ends up being for dubious purposes[1]. Basically, it tries to avoid hardware setting Accessed=1 in page table entries that are unreachable by the hardware, a non-issue.  Remove the PMD copy. Detach the original PMD page at the PUD, flush the mid-level caches, and free the PTE tables straight from the detached PMD page. With no allocation left, the locking issue is gone.  Lockdep splat/analysis:    WARNING: possible circular locking dependency detected   7.3.0-rc3-f6e7b42bf05b+ #183 Tainted: G     U   ------------------------------------------------------   kswapd0/269 is trying to acquire lock:   ((init_mm).mmap_lock){++++}-{4:4}, at: change_page_attr_set_clr+0x29a/0x4a0   but task is already holding lock:   (pool_shrink_rwsem){.+.+}-{4:4}, at: ttm_pool_shrink+0xb2/0x330 [ttm]   Chain exists of:     (init_mm).mmap_lock --> fs_reclaim --> pool_shrink_rwsem  The cycle is built from three edges:    1) pool_shrink_rwsem -> (init_mm).mmap_lock       The TTM shrinker restores the caching attribute of every page it      frees, while holding pool_shrink_rwsem:         ttm_pool_shrink()        -> ttm_pool_dispose_list()        -> ttm_pool_free_page()        -> set_pages_wb()        -> change_page_attr_set_clr()   [ init_mm mmap read lock ]    2) fs_reclaim -> pool_shrink_rwsem       The same shrinker, called from reclaim.    3) (init_mm).mmap_lock -> fs_reclaim       ioremap() installing a huge PUD mapping over an existing PMD table:         ioremap_page_range()        -> vmap_range_noflush()        -> vmap_try_huge_pud()          [ init_mm mmap read lock ]        -> pud_free_pmd_page()        -> __get_free_page(GFP_KERNEL)  [ enters reclaim ]  [ dhansen: Lots of changelog munging/trimming and merged comments from my 	   version of the fix. ]