Re: [PATCH v4] fat: stop reading directory entries past the end-of-directory marker

[OGAWA Hirofumi]

Matteo Croce <technoboy85@xxxxxxxxx> writes:

> The FAT specification[1] (FAT Directory Structure -> "DIR_Name[0]") states:
>
>     If DIR_Name[0] == 0x00, then the directory entry is free (same as for
>     0xE5), and there are no allocated directory entries after this one
>     (all of the DIR_Name[0] bytes in all of the entries after this one
>     are also set to 0).
>
>     The special 0 value, rather than the 0xE5 value, indicates to FAT
>     file system driver code that the rest of the entries in this
>     directory do not need to be examined because they are all free.
>
> Linux did not honour this. fat_get_entry() kept advancing past the 0x00
> terminator; if the trailing on-disk slots were not zero-filled (buggy
> formatters, read-only media written by other operating systems, on-disk
> corruption) the driver surfaced arbitrary bytes as real directory
> entries. On a typical affected image, `ls /mnt` returns ~150 bogus
> entries with random binary names, multi-gigabyte sizes, dates ranging
> from 1980 to 2106, and a flood of -EIO from stat().
>
> Earlier attempts (v1..v3, see [2][3][4]) added `de->name[0] == 0` guards
> at each call site. As Hirofumi pointed out on v3, those guards reject
> the entry but fat_get_entry() has already advanced *pos past it; the
> next readdir() resumes after the marker and walks straight back into
> the garbage. His suggestion was to centralise the check.
>
> This patch:
>
>   * Adds fat_get_entry_eod(), a small wrapper around fat_get_entry()
>     that returns -1 when name[0] == 0 and seeks *pos to dir->i_size.
>     Per spec every slot after the 0x00 marker is also zero, so jumping
>     to the end of the directory is correct: subsequent reads return -1
>     from fat_bmap() without re-fetching trailing zero slots, and
>     callers persisting *pos across invocations (notably readdir's
>     ctx->pos) keep reporting end-of-directory on re-entry.
>
>   * Converts the read/search paths to use the new wrapper:
>       fat_parse_long(), fat_search_long(), __fat_readdir(),
>       and fat_get_short_entry() -- the last covers
>       fat_get_dotdot_entry(), fat_dir_empty(), fat_subdirs(),
>       fat_scan(), and fat_scan_logstart() transitively.
>
>   * Leaves fat_add_entries() and __fat_remove_entries() on raw
>     fat_get_entry(): the write paths legitimately need to operate on
>     free/zero slots. fat_add_entries() additionally detects an
>     allocated entry past a 0x00 marker (the spec violation that
>     produces the garbage) and treats it as filesystem corruption:
>     fat_fs_error_ratelimit() is called -- which honours the configured
>     errors= mount option (panic / remount-ro / continue) -- and the
>     operation returns -EIO so we don't write fresh entries into an
>     already-corrupt directory.
>
> [1] https://download.microsoft.com/download/1/6/1/161ba512-40e2-4cc9-843a-923143f3456c/fatgen103.doc
> [2] https://lore.kernel.org/lkml/20181207013410.7050-1-mcroce@xxxxxxxxxx/
> [3] https://lore.kernel.org/lkml/20181216231510.26854-1-mcroce@xxxxxxxxxx/
> [4] https://lore.kernel.org/lkml/20190201001408.7453-1-mcroce@xxxxxxxxxx/
>
> Reported-by: Timothy Redaelli <tredaelli@xxxxxxxxxx>
> Suggested-by: OGAWA Hirofumi <hirofumi@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Matteo Croce <teknoraver@xxxxxxxx>

Thanks.

Acked-by: OGAWA Hirofumi <hirofumi@xxxxxxxxxxxxxxxxxx>

> ---
>  fs/fat/dir.c | 44 ++++++++++++++++++++++++++++++++++++++++----
>  1 file changed, 40 insertions(+), 4 deletions(-)
>
> diff --git a/fs/fat/dir.c b/fs/fat/dir.c
> index 4f6f42f33613..c6cca5d00ffd 100644
> --- a/fs/fat/dir.c
> +++ b/fs/fat/dir.c
> @@ -130,6 +130,31 @@ static inline int fat_get_entry(struct inode *dir, loff_t *pos,
>  	return fat__get_entry(dir, pos, bh, de);
>  }
>  
> +/*
> + * Like fat_get_entry(), but honour the FAT end-of-directory marker:
> + * a dirent whose first name byte is NUL terminates iteration per the
> + * spec, which also guarantees that every following slot is zeroed.
> + * Skip straight to the end of the directory so the next call returns
> + * -1 from fat_bmap() without re-reading the trailing zero slots, and
> + * so callers that persist *pos across invocations (e.g. readdir's
> + * ctx->pos) keep reporting EOD.  Release *bh and set it to NULL to
> + * match fat_get_entry()'s contract that *bh is NULL on the -1 return.
> + */
> +static int fat_get_entry_eod(struct inode *dir, loff_t *pos,
> +			     struct buffer_head **bh,
> +			     struct msdos_dir_entry **de)
> +{
> +	int err = fat_get_entry(dir, pos, bh, de);
> +
> +	if (err == 0 && (*de)->name[0] == 0) {
> +		brelse(*bh);
> +		*bh = NULL;
> +		*pos = dir->i_size;
> +		return -1;
> +	}
> +	return err;
> +}
> +
>  /*
>   * Convert Unicode 16 to UTF-8, translated Unicode, or ASCII.
>   * If uni_xlate is enabled and we can't get a 1:1 conversion, use a
> @@ -327,7 +352,7 @@ static int fat_parse_long(struct inode *dir, loff_t *pos,
>  
>  		if (ds->id & 0x40)
>  			(*unicode)[offset + 13] = 0;
> -		if (fat_get_entry(dir, pos, bh, de) < 0)
> +		if (fat_get_entry_eod(dir, pos, bh, de) < 0)
>  			return PARSE_EOF;
>  		if (slot == 0)
>  			break;
> @@ -489,7 +514,7 @@ int fat_search_long(struct inode *inode, const unsigned char *name,
>  
>  	err = -ENOENT;
>  	while (1) {
> -		if (fat_get_entry(inode, &cpos, &bh, &de) == -1)
> +		if (fat_get_entry_eod(inode, &cpos, &bh, &de) == -1)
>  			goto end_of_dir;
>  parse_record:
>  		nr_slots = 0;
> @@ -601,7 +626,7 @@ static int __fat_readdir(struct inode *inode, struct file *file,
>  
>  	bh = NULL;
>  get_new:
> -	if (fat_get_entry(inode, &cpos, &bh, &de) == -1)
> +	if (fat_get_entry_eod(inode, &cpos, &bh, &de) == -1)
>  		goto end_of_dir;
>  parse_record:
>  	nr_slots = 0;
> @@ -885,7 +910,7 @@ static int fat_get_short_entry(struct inode *dir, loff_t *pos,
>  			       struct buffer_head **bh,
>  			       struct msdos_dir_entry **de)
>  {
> -	while (fat_get_entry(dir, pos, bh, de) >= 0) {
> +	while (fat_get_entry_eod(dir, pos, bh, de) >= 0) {
>  		/* free entry or long name entry or volume label */
>  		if (!IS_FREE((*de)->name) && !((*de)->attr & ATTR_VOLUME))
>  			return 0;
> @@ -1302,6 +1327,7 @@ int fat_add_entries(struct inode *dir, void *slots, int nr_slots,
>  	struct msdos_dir_entry *de;
>  	int err, free_slots, i, nr_bhs;
>  	loff_t pos;
> +	bool saw_eod;
>  
>  	sinfo->nr_slots = nr_slots;
>  
> @@ -1310,12 +1336,15 @@ int fat_add_entries(struct inode *dir, void *slots, int nr_slots,
>  	bh = prev = NULL;
>  	pos = 0;
>  	err = -ENOSPC;
> +	saw_eod = false;
>  	while (fat_get_entry(dir, &pos, &bh, &de) > -1) {
>  		/* check the maximum size of directory */
>  		if (pos >= FAT_MAX_DIR_SIZE)
>  			goto error;
>  
>  		if (IS_FREE(de->name)) {
> +			if (de->name[0] == 0)
> +				saw_eod = true;
>  			if (prev != bh) {
>  				get_bh(bh);
>  				bhs[nr_bhs] = prev = bh;
> @@ -1325,6 +1354,13 @@ int fat_add_entries(struct inode *dir, void *slots, int nr_slots,
>  			if (free_slots == nr_slots)
>  				goto found;
>  		} else {
> +			if (saw_eod) {
> +				fat_fs_error_ratelimit(sb,
> +					"allocated dir entry found after end-of-directory marker (i_pos %lld)",
> +					MSDOS_I(dir)->i_pos);
> +				err = -EIO;
> +				goto error;
> +			}
>  			for (i = 0; i < nr_bhs; i++)
>  				brelse(bhs[i]);
>  			prev = NULL;

-- 
OGAWA Hirofumi <hirofumi@xxxxxxxxxxxxxxxxxx>