On Thu, Oct 05, 2017 at 05:57:10PM +0100, Will Deacon wrote: > Loading the pmd without holding the pmd_lock exposes us to races with > concurrent updaters of the page tables but, worse still, it also allows > the compiler to cache the pmd value in a register and reuse it later on, > even if we've performed a READ_ONCE in between and seen a more recent > value. > > In the case of page_vma_mapped_walk, this leads to the following crash > when the pmd loaded for the initial pmd_trans_huge check is all zeroes > and a subsequent valid table entry is loaded by check_pmd. We then > proceed into map_pte, but the compiler re-uses the zero entry inside > pte_offset_map, resulting in a junk pointer being installed in pvmw->pte: > > [ 254.032812] PC is at check_pte+0x20/0x170 > [ 254.032948] LR is at page_vma_mapped_walk+0x2e0/0x540 > [...] > [ 254.036114] Process doio (pid: 2463, stack limit = 0xffff00000f2e8000) > [ 254.036361] Call trace: > [ 254.038977] [<ffff000008233328>] check_pte+0x20/0x170 > [ 254.039137] [<ffff000008233758>] page_vma_mapped_walk+0x2e0/0x540 > [ 254.039332] [<ffff000008234adc>] page_mkclean_one+0xac/0x278 > [ 254.039489] [<ffff000008234d98>] rmap_walk_file+0xf0/0x238 > [ 254.039642] [<ffff000008236e74>] rmap_walk+0x64/0xa0 > [ 254.039784] [<ffff0000082370c8>] page_mkclean+0x90/0xa8 > [ 254.040029] [<ffff0000081f3c64>] clear_page_dirty_for_io+0x84/0x2a8 > [ 254.040311] [<ffff00000832f984>] mpage_submit_page+0x34/0x98 > [ 254.040518] [<ffff00000832fb4c>] mpage_process_page_bufs+0x164/0x170 > [ 254.040743] [<ffff00000832fc8c>] mpage_prepare_extent_to_map+0x134/0x2b8 > [ 254.040969] [<ffff00000833530c>] ext4_writepages+0x484/0xe30 > [ 254.041175] [<ffff0000081f6ab4>] do_writepages+0x44/0xe8 > [ 254.041372] [<ffff0000081e5bd4>] __filemap_fdatawrite_range+0xbc/0x110 > [ 254.041568] [<ffff0000081e5e68>] file_write_and_wait_range+0x48/0xd8 > [ 254.041739] [<ffff000008324310>] ext4_sync_file+0x80/0x4b8 > [ 254.041907] [<ffff0000082bd434>] vfs_fsync_range+0x64/0xc0 > [ 254.042106] [<ffff0000082332b4>] SyS_msync+0x194/0x1e8 > > This patch fixes the problem by ensuring that READ_ONCE is used before > the initial checks on the pmd, and this value is subsequently used when > checking whether or not the pmd is present. pmd_check is removed and the > pmd_present check is inlined directly. > > Cc: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx> > Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> > Cc: <stable@xxxxxxxxxxxxxxx> > Fixes: f27176cfc363 ("mm: convert page_mkclean_one() to use page_vma_mapped_walk()") > Tested-by: Yury Norov <ynorov@xxxxxxxxxxxxxxxxxx> > Tested-by: Richard Ruigrok <rruigrok@xxxxxxxxxxxxxx> > Signed-off-by: Will Deacon <will.deacon@xxxxxxx> Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> -- Kirill A. Shutemov