ROMFS: romfs_lookup() shouldn't be doing a partial name comparison

romfs_lookup() should be using a routine akin to strcmp() on the backing store,
rather than one akin to strncmp().  If it uses the latter, it's liable to match
/bin/shutdown when looking up /bin/sh.

Signed-off-by: David Howells <dhowells@redhat.com>
Tested-by: Michal Simek <monstr@monstr.eu>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

3 files changed, 53 insertions, 22 deletions

diff --git a/fs/romfs/internal.h b/fs/romfs/internal.h
index 06044a9dc62d..95217b830118 100644
--- a/fs/romfs/internal.h
+++ b/fs/romfs/internal.h
@@ -43,5 +43,5 @@ extern int romfs_dev_read(struct super_block *sb, unsigned long pos,
                           void *buf, size_t buflen);
 extern ssize_t romfs_dev_strnlen(struct super_block *sb,
                                  unsigned long pos, size_t maxlen);
-extern int romfs_dev_strncmp(struct super_block *sb, unsigned long pos,
-                             const char *str, size_t size);
+extern int romfs_dev_strcmp(struct super_block *sb, unsigned long pos,
+                            const char *str, size_t size);
diff --git a/fs/romfs/storage.c b/fs/romfs/storage.c
index 7e3e1e12a081..66ce9ddfe504 100644
--- a/fs/romfs/storage.c
+++ b/fs/romfs/storage.c
@@ -67,26 +67,35 @@ static ssize_t romfs_mtd_strnlen(struct super_block *sb,
  * compare a string to one in a romfs image on MTD
  * - return 1 if matched, 0 if differ, -ve if error
  */
-static int romfs_mtd_strncmp(struct super_block *sb, unsigned long pos,
-                             const char *str, size_t size)
+static int romfs_mtd_strcmp(struct super_block *sb, unsigned long pos,
+                            const char *str, size_t size)
 {
-        u_char buf[16];
+        u_char buf[17];
         size_t len, segment;
         int ret;
 
-        /* scan the string up to 16 bytes at a time */
+        /* scan the string up to 16 bytes at a time, and attempt to grab the
+         * trailing NUL whilst we're at it */
+        buf[0] = 0xff;
+
         while (size > 0) {
-                segment = min_t(size_t, size, 16);
+                segment = min_t(size_t, size + 1, 17);
                 ret = ROMFS_MTD_READ(sb, pos, segment, &len, buf);
                 if (ret < 0)
                         return ret;
+                len--;
                 if (memcmp(buf, str, len) != 0)
                         return 0;
+                buf[0] = buf[len];
                 size -= len;
                 pos += len;
                 str += len;
         }
 
+        /* check the trailing NUL was */
+        if (buf[0])
+                return 0;
+
         return 1;
 }
 #endif /* CONFIG_ROMFS_ON_MTD */
@@ -154,28 +163,48 @@ static ssize_t romfs_blk_strnlen(struct super_block *sb,
  * compare a string to one in a romfs image on a block device
  * - return 1 if matched, 0 if differ, -ve if error
  */
-static int romfs_blk_strncmp(struct super_block *sb, unsigned long pos,
-                             const char *str, size_t size)
+static int romfs_blk_strcmp(struct super_block *sb, unsigned long pos,
+                            const char *str, size_t size)
 {
         struct buffer_head *bh;
         unsigned long offset;
         size_t segment;
-        bool x;
+        bool matched, terminated = false;
 
-        /* scan the string up to 16 bytes at a time */
+        /* compare string up to a block at a time */
         while (size > 0) {
                 offset = pos & (ROMBSIZE - 1);
                 segment = min_t(size_t, size, ROMBSIZE - offset);
                 bh = sb_bread(sb, pos >> ROMBSBITS);
                 if (!bh)
                         return -EIO;
-                x = (memcmp(bh->b_data + offset, str, segment) != 0);
-                brelse(bh);
-                if (x)
-                        return 0;
+                matched = (memcmp(bh->b_data + offset, str, segment) == 0);
+
                 size -= segment;
                 pos += segment;
                 str += segment;
+                if (matched && size == 0 && offset + segment < ROMBSIZE) {
+                        if (!bh->b_data[offset + segment])
+                                terminated = true;
+                        else
+                                matched = false;
+                }
+                brelse(bh);
+                if (!matched)
+                        return 0;
+        }
+
+        if (!terminated) {
+                /* the terminating NUL must be on the first byte of the next
+                 * block */
+                BUG_ON((pos & (ROMBSIZE - 1)) != 0);
+                bh = sb_bread(sb, pos >> ROMBSBITS);
+                if (!bh)
+                        return -EIO;
+                matched = !bh->b_data[0];
+                brelse(bh);
+                if (!matched)
+                        return 0;
         }
 
         return 1;
@@ -234,10 +263,12 @@ ssize_t romfs_dev_strnlen(struct super_block *sb,
 
 /*
  * compare a string to one in romfs
+ * - the string to be compared to, str, may not be NUL-terminated; instead the
+ *   string is of the specified size
  * - return 1 if matched, 0 if differ, -ve if error
  */
-int romfs_dev_strncmp(struct super_block *sb, unsigned long pos,
-                      const char *str, size_t size)
+int romfs_dev_strcmp(struct super_block *sb, unsigned long pos,
+                     const char *str, size_t size)
 {
         size_t limit;
 
@@ -246,16 +277,16 @@ int romfs_dev_strncmp(struct super_block *sb, unsigned long pos,
                 return -EIO;
         if (size > ROMFS_MAXFN)
                 return -ENAMETOOLONG;
-        if (size > limit - pos)
+        if (size + 1 > limit - pos)
                 return -EIO;
 
 #ifdef CONFIG_ROMFS_ON_MTD
         if (sb->s_mtd)
-                return romfs_mtd_strncmp(sb, pos, str, size);
+                return romfs_mtd_strcmp(sb, pos, str, size);
 #endif
 #ifdef CONFIG_ROMFS_ON_BLOCK
         if (sb->s_bdev)
-                return romfs_blk_strncmp(sb, pos, str, size);
+                return romfs_blk_strcmp(sb, pos, str, size);
 #endif
         return -EIO;
 }
diff --git a/fs/romfs/super.c b/fs/romfs/super.c
index 10ca7d984a8b..c53b5ef8a02f 100644
--- a/fs/romfs/super.c
+++ b/fs/romfs/super.c
@@ -240,8 +240,8 @@ static struct dentry *romfs_lookup(struct inode *dir, struct dentry *dentry,
                         goto error;
 
                 /* try to match the first 16 bytes of name */
-                ret = romfs_dev_strncmp(dir->i_sb, offset + ROMFH_SIZE, name,
-                                        len);
+                ret = romfs_dev_strcmp(dir->i_sb, offset + ROMFH_SIZE, name,
+                                       len);
                 if (ret < 0)
                         goto error;
                 if (ret == 1)