• Eric Biggers's avatar
    mm/madvise.c: fix freeing of locked page with MADV_FREE · 263630e8
    Eric Biggers authored
    If madvise(..., MADV_FREE) split a transparent hugepage, it called
    put_page() before unlock_page().
    
    This was wrong because put_page() can free the page, e.g. if a
    concurrent madvise(..., MADV_DONTNEED) has removed it from the memory
    mapping. put_page() then rightfully complained about freeing a locked
    page.
    
    Fix this by moving the unlock_page() before put_page().
    
    This bug was found by syzkaller, which encountered the following splat:
    
        BUG: Bad page state in process syzkaller412798  pfn:1bd800
        page:ffffea0006f60000 count:0 mapcount:0 mapping:          (null) index:0x20a00
        flags: 0x200000000040019(locked|uptodate|dirty|swapbacked)
        raw: 0200000000040019 0000000000000000 0000000000020a00 00000000ffffffff
        raw: ffffea0006f60020 ffffea0006f60020 0000000000000000 0000000000000000
        page dumped because: PAGE_FLAGS_CHECK_AT_FREE flag(s) set
        bad because of flags: 0x1(locked)
        Modules linked in:
        CPU: 1 PID: 3037 Comm: syzkaller412798 Not tainted 4.13.0-rc5+ #35
        Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
        Call Trace:
         __dump_stack lib/dump_stack.c:16 [inline]
         dump_stack+0x194/0x257 lib/dump_stack.c:52
         bad_page+0x230/0x2b0 mm/page_alloc.c:565
         free_pages_check_bad+0x1f0/0x2e0 mm/page_alloc.c:943
         free_pages_check mm/page_alloc.c:952 [inline]
         free_pages_prepare mm/page_alloc.c:1043 [inline]
         free_pcp_prepare mm/page_alloc.c:1068 [inline]
         free_hot_cold_page+0x8cf/0x12b0 mm/page_alloc.c:2584
         __put_single_page mm/swap.c:79 [inline]
         __put_page+0xfb/0x160 mm/swap.c:113
         put_page include/linux/mm.h:814 [inline]
         madvise_free_pte_range+0x137a/0x1ec0 mm/madvise.c:371
         walk_pmd_range mm/pagewalk.c:50 [inline]
         walk_pud_range mm/pagewalk.c:108 [inline]
         walk_p4d_range mm/pagewalk.c:134 [inline]
         walk_pgd_range mm/pagewalk.c:160 [inline]
         __walk_page_range+0xc3a/0x1450 mm/pagewalk.c:249
         walk_page_range+0x200/0x470 mm/pagewalk.c:326
         madvise_free_page_range.isra.9+0x17d/0x230 mm/madvise.c:444
         madvise_free_single_vma+0x353/0x580 mm/madvise.c:471
         madvise_dontneed_free mm/madvise.c:555 [inline]
         madvise_vma mm/madvise.c:664 [inline]
         SYSC_madvise mm/madvise.c:832 [inline]
         SyS_madvise+0x7d3/0x13c0 mm/madvise.c:760
         entry_SYSCALL_64_fastpath+0x1f/0xbe
    
    Here is a C reproducer:
    
        #define _GNU_SOURCE
        #include <pthread.h>
        #include <sys/mman.h>
        #include <unistd.h>
    
        #define MADV_FREE	8
        #define PAGE_SIZE	4096
    
        static void *mapping;
        static const size_t mapping_size = 0x1000000;
    
        static void *madvise_thrproc(void *arg)
        {
            madvise(mapping, mapping_size, (long)arg);
        }
    
        int main(void)
        {
            pthread_t t[2];
    
            for (;;) {
                mapping = mmap(NULL, mapping_size, PROT_WRITE,
                               MAP_POPULATE|MAP_ANONYMOUS|MAP_PRIVATE, -1, 0);
    
                munmap(mapping + mapping_size / 2, PAGE_SIZE);
    
                pthread_create(&t[0], 0, madvise_thrproc, (void*)MADV_DONTNEED);
                pthread_create(&t[1], 0, madvise_thrproc, (void*)MADV_FREE);
                pthread_join(t[0], NULL);
                pthread_join(t[1], NULL);
                munmap(mapping, mapping_size);
            }
        }
    
    Note: to see the splat, CONFIG_TRANSPARENT_HUGEPAGE=y and
    CONFIG_DEBUG_VM=y are needed.
    
    Google Bug Id: 64696096
    
    Link: http://lkml.kernel.org/r/20170823205235.132061-1-ebiggers3@gmail.com
    Fixes: 854e9ed0 ("mm: support madvise(MADV_FREE)")
    Signed-off-by: default avatarEric Biggers <ebiggers@google.com>
    Acked-by: default avatarDavid Rientjes <rientjes@google.com>
    Acked-by: default avatarMinchan Kim <minchan@kernel.org>
    Acked-by: default avatarMichal Hocko <mhocko@suse.com>
    Cc: Dmitry Vyukov <dvyukov@google.com>
    Cc: Hugh Dickins <hughd@google.com>
    Cc: Andrea Arcangeli <aarcange@redhat.com>
    Cc: <stable@vger.kernel.org>	[v4.5+]
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    263630e8
madvise.c 21.6 KB