queue-4.19/fs-fix-guard_bio_eod-to-check-for-real-eod-errors.patch

   1 From c5a82a95fb919c4c32009bb32bff3abcb9534d0e Mon Sep 17 00:00:00 2001
   2 From: Carlos Maiolino <cmaiolino@redhat.com>
   3 Date: Tue, 26 Feb 2019 11:51:50 +0100
   4 Subject: fs: fix guard_bio_eod to check for real EOD errors
   5
   6 [ Upstream commit dce30ca9e3b676fb288c33c1f4725a0621361185 ]
   7
   8 guard_bio_eod() can truncate a segment in bio to allow it to do IO on
   9 odd last sectors of a device.
  10
  11 It already checks if the IO starts past EOD, but it does not consider
  12 the possibility of an IO request starting within device boundaries can
  13 contain more than one segment past EOD.
  14
  15 In such cases, truncated_bytes can be bigger than PAGE_SIZE, and will
  16 underflow bvec->bv_len.
  17
  18 Fix this by checking if truncated_bytes is lower than PAGE_SIZE.
  19
  20 This situation has been found on filesystems such as isofs and vfat,
  21 which doesn't check the device size before mount, if the device is
  22 smaller than the filesystem itself, a readahead on such filesystem,
  23 which spans EOD, can trigger this situation, leading a call to
  24 zero_user() with a wrong size possibly corrupting memory.
  25
  26 I didn't see any crash, or didn't let the system run long enough to
  27 check if memory corruption will be hit somewhere, but adding
  28 instrumentation to guard_bio_end() to check truncated_bytes size, was
  29 enough to see the error.
  30
  31 The following script can trigger the error.
  32
  33 MNT=/mnt
  34 IMG=./DISK.img
  35 DEV=/dev/loop0
  36
  37 mkfs.vfat $IMG
  38 mount $IMG $MNT
  39 cp -R /etc $MNT &> /dev/null
  40 umount $MNT
  41
  42 losetup -D
  43
  44 losetup --find --show --sizelimit 16247280 $IMG
  45 mount $DEV $MNT
  46
  47 find $MNT -type f -exec cat {} + >/dev/null
  48
  49 Kudos to Eric Sandeen for coming up with the reproducer above
  50
  51 Reviewed-by: Ming Lei <ming.lei@redhat.com>
  52 Signed-off-by: Carlos Maiolino <cmaiolino@redhat.com>
  53 Signed-off-by: Jens Axboe <axboe@kernel.dk>
  54 Signed-off-by: Sasha Levin <sashal@kernel.org>
  55 ---
  56  fs/buffer.c | 7 +++++++
  57  1 file changed, 7 insertions(+)
  58
  59 diff --git a/fs/buffer.c b/fs/buffer.c
  60 index c083c4b3c1e7..a550e0d8e965 100644
  61 --- a/fs/buffer.c
  62 +++ b/fs/buffer.c
  63 @@ -3027,6 +3027,13 @@ void guard_bio_eod(int op, struct bio *bio)
  64         /* Uhhuh. We've got a bio that straddles the device size! */
  65         truncated_bytes = bio->bi_iter.bi_size - (maxsector << 9);
  66
  67 +       /*
  68 +        * The bio contains more than one segment which spans EOD, just return
  69 +        * and let IO layer turn it into an EIO
  70 +        */
  71 +       if (truncated_bytes > bvec->bv_len)
  72 +               return;
  73 +
  74         /* Truncate the bio.. */
  75         bio->bi_iter.bi_size -= truncated_bytes;
  76         bvec->bv_len -= truncated_bytes;
  77 --
  78 2.19.1
  79