linux-ext4 - tune2fs crash in error situation

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Message-ID: <55a8dd42bcb78c1b965ba5ca159e7a9b@be409.mail.saunalahti.fi>
Date:   Thu, 28 Sep 2017 15:01:15 +0300
From:   eru@...ti.fi
To:     linux-ext4@...r.kernel.org
Subject: tune2fs crash in error situation

tune2fs seems to coredump if the file system is not clean, so that it 
goes
to the branch calling ext2fs_run_ext3_journal(), and this then fails for
some reason while calling ext2fs_open2().

misc/tunefs.c:
..
         /* Recover the journal if possible. */
         if ((open_flag & EXT2_FLAG_RW) && !(mount_flags & 
EXT2_MF_MOUNTED) &&
             ext2fs_has_feature_journal_needs_recovery(fs->super)) {
                 errcode_t err;

                 printf(_("Recovering journal.\n"));
                 err = ext2fs_run_ext3_journal(&fs);
                 if (err) {
                         com_err("tune2fs", err, "while recovering 
journal.\n");
                         printf(_("Please run e2fsck -fy %s.\n"), 
argv[1]);
                         goto closefs;
                 }
                 ext2fs_clear_feature_journal_needs_recovery(fs->super);
                 ext2fs_mark_super_dirty(fs);
         }

Failures in ext2fs_open2() zero the fs pointer, and the goto closefs
leads to code that will dereference fs:

closefs:
         if (rc) {
                 ext2fs_mmp_stop(fs);
#ifndef BUILD_AS_LIB
                 exit(1);
#endif
         }

         convert_64bit(fs, feature_64bit);
         return (ext2fs_close_free(&fs) ? 1 : 0);

calling ext2fs_close_free(&fs) leads to

lib/ext2fs/closefs.c:
..
errcode_t ext2fs_close2(ext2_filsys fs, int flags)
{
         errcode_t       retval;
         int             meta_blks;
         io_stats stats = 0;

         EXT2_CHECK_MAGIC(fs, EXT2_ET_MAGIC_EXT2FS_FILSYS);

and the dereference happens in that macro.

A crash because of this was observed when the device passed to tune2fs
was a /dev/disk/by-id/* link, and apparently udevd happened to 
manipulate
it while tune2fs was running. Or that at least is my theory for the
randomly observed error message:

  tune2fs: No such file or directory while recovering journal

that preceded the crash. This error situation could be reproduced
nondeterministically by a loop that calls tunefs with an unclean file
system (created by mounting an image to a VM, and the suddenly killing
the VM), and having the loop make the path invalid while tune2fs is 
running.

while true; do
   cp dirtyfs.img x.img
   ln -s /root/x.img /tmp/disk
   ./tune2fs -L somelabel /tmp/disk 2>>tunefs.error &
   sleep 0.1
   rm /tmp/disk
done

At least versions e2fsprogs-1.43.5 and e2fsprogs-1.43.6 are affected.
An obvious patch for the problem:

diff -ru e2fsprogs-1.43.6.orig/misc/tune2fs.c 
e2fsprogs-1.43.6/misc/tune2fs.c
--- e2fsprogs-1.43.6.orig/misc/tune2fs.c	2017-08-29 16:05:52.000000000 
+0300
+++ e2fsprogs-1.43.6/misc/tune2fs.c	2017-09-21 12:00:37.068873101 +0300
@@ -3256,6 +3256,9 @@
  #endif
  	}

-	convert_64bit(fs, feature_64bit);
-	return (ext2fs_close_free(&fs) ? 1 : 0);
+	if (fs) {
+		convert_64bit(fs, feature_64bit);
+		return (ext2fs_close_free(&fs) ? 1 : 0);
+	}
+	return 1;
  }

-- 
Erkki Ruohtula