fat: validate ->i_start before using

On corrupted FATfs may have invalid ->i_start.  To handle it, this checks
->i_start before using, and return proper error code.

Link: http://lkml.kernel.org/r/87o9f8y1t5.fsf_-_@mail.parknet.co.jp
Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Reported-by: Anatoly Trosinenko <anatoly.trosinenko@gmail.com>
Tested-by: Anatoly Trosinenko <anatoly.trosinenko@gmail.com>
Cc: Alan Cox <gnomes@lxorguk.ukuu.org.uk>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 3 insertions, 3 deletions

diff --git a/fs/fat/fatent.c b/fs/fat/fatent.c
index 25d43a5e8a4d..defc2168de91 100644
--- a/fs/fat/fatent.c
+++ b/fs/fat/fatent.c
@@ -24,7 +24,7 @@ static void fat12_ent_blocknr(struct super_block *sb, int entry,
 {
         struct msdos_sb_info *sbi = MSDOS_SB(sb);
         int bytes = entry + (entry >> 1);
-        WARN_ON(entry < FAT_START_ENT || sbi->max_cluster <= entry);
+        WARN_ON(!fat_valid_entry(sbi, entry));
         *offset = bytes & (sb->s_blocksize - 1);
         *blocknr = sbi->fat_start + (bytes >> sb->s_blocksize_bits);
 }
@@ -34,7 +34,7 @@ static void fat_ent_blocknr(struct super_block *sb, int entry,
 {
         struct msdos_sb_info *sbi = MSDOS_SB(sb);
         int bytes = (entry << sbi->fatent_shift);
-        WARN_ON(entry < FAT_START_ENT || sbi->max_cluster <= entry);
+        WARN_ON(!fat_valid_entry(sbi, entry));
         *offset = bytes & (sb->s_blocksize - 1);
         *blocknr = sbi->fat_start + (bytes >> sb->s_blocksize_bits);
 }
@@ -354,7 +354,7 @@ int fat_ent_read(struct inode *inode, struct fat_entry *fatent, int entry)
         int err, offset;
         sector_t blocknr;
 
-        if (entry < FAT_START_ENT || sbi->max_cluster <= entry) {
+        if (!fat_valid_entry(sbi, entry)) {
                 fatent_brelse(fatent);
                 fat_fs_error(sb, "invalid access to FAT (entry 0x%08x)", entry);
                 return -EIO;