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>

3 files changed, 20 insertions, 10 deletions

diff --git a/fs/fat/cache.c b/fs/fat/cache.c
index e9bed49df6b7..78d501c1fb65 100644
--- a/fs/fat/cache.c
+++ b/fs/fat/cache.c
@@ -225,7 +225,8 @@ static inline void cache_init(struct fat_cache_id *cid, int fclus, int dclus)
 int fat_get_cluster(struct inode *inode, int cluster, int *fclus, int *dclus)
 {
         struct super_block *sb = inode->i_sb;
-        const int limit = sb->s_maxbytes >> MSDOS_SB(sb)->cluster_bits;
+        struct msdos_sb_info *sbi = MSDOS_SB(sb);
+        const int limit = sb->s_maxbytes >> sbi->cluster_bits;
         struct fat_entry fatent;
         struct fat_cache_id cid;
         int nr;
@@ -234,6 +235,12 @@ int fat_get_cluster(struct inode *inode, int cluster, int *fclus, int *dclus)
 
         *fclus = 0;
         *dclus = MSDOS_I(inode)->i_start;
+        if (!fat_valid_entry(sbi, *dclus)) {
+                fat_fs_error_ratelimit(sb,
+                        "%s: invalid start cluster (i_pos %lld, start %08x)",
+                        __func__, MSDOS_I(inode)->i_pos, *dclus);
+                return -EIO;
+        }
         if (cluster == 0)
                 return 0;
 
@@ -250,9 +257,8 @@ int fat_get_cluster(struct inode *inode, int cluster, int *fclus, int *dclus)
                 /* prevent the infinite loop of cluster chain */
                 if (*fclus > limit) {
                         fat_fs_error_ratelimit(sb,
-                                        "%s: detected the cluster chain loop"
-                                        " (i_pos %lld)", __func__,
-                                        MSDOS_I(inode)->i_pos);
+                                "%s: detected the cluster chain loop (i_pos %lld)",
+                                __func__, MSDOS_I(inode)->i_pos);
                         nr = -EIO;
                         goto out;
                 }
@@ -262,9 +268,8 @@ int fat_get_cluster(struct inode *inode, int cluster, int *fclus, int *dclus)
                         goto out;
                 else if (nr == FAT_ENT_FREE) {
                         fat_fs_error_ratelimit(sb,
-                                       "%s: invalid cluster chain (i_pos %lld)",
-                                       __func__,
-                                       MSDOS_I(inode)->i_pos);
+                                "%s: invalid cluster chain (i_pos %lld)",
+                                __func__, MSDOS_I(inode)->i_pos);
                         nr = -EIO;
                         goto out;
                 } else if (nr == FAT_ENT_EOF) {
diff --git a/fs/fat/fat.h b/fs/fat/fat.h
index 154ae54a6b3a..df84d5710b59 100644
--- a/fs/fat/fat.h
+++ b/fs/fat/fat.h
@@ -348,6 +348,11 @@ static inline void fatent_brelse(struct fat_entry *fatent)
         fatent->fat_inode = NULL;
 }
 
+static inline bool fat_valid_entry(struct msdos_sb_info *sbi, int entry)
+{
+        return FAT_START_ENT <= entry && entry < sbi->max_cluster;
+}
+
 extern void fat_ent_access_init(struct super_block *sb);
 extern int fat_ent_read(struct inode *inode, struct fat_entry *fatent,
                         int entry);
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;