Re: [PATCH] mm/madvise: fix freeing of locked page with MADV_FREE

[Minchan Kim]

On Wed, Aug 23, 2017 at 01:52:35PM -0700, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@xxxxxxxxxx>
> 
> 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.
> 
> Fixes: 854e9ed09ded ("mm: support madvise(MADV_FREE)")
> Google-Bug-Id: 64696096
> Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Minchan Kim <minchan@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx # v4.5+
> Acked-by: David Rientjes <rientjes@xxxxxxxxxx>
> Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>
Acked-by: Minchan Kim <minchan@xxxxxxxxxx>

Thanks!