RE: [PATCH 2/4 v3] exfat: separate the boot sector analysis

[Sungjong Seo]

> Separate the boot sector analysis to read_boot_sector().
> And add a check for the fs_name field.
> Furthermore, add a strict consistency check, because overlapping areas can
> cause serious corruption.
> 
> Signed-off-by: Tetsuhiro Kohada <kohada.t2@xxxxxxxxx>

Reviewed-by: Sungjong Seo <sj1557.seo@xxxxxxxxxxx>

> ---
> Changes in v2:
>  - rebase with patch 'optimize dir-cache' applied Changes in v3:
>  - add a check for the fs_name field
> 
>  fs/exfat/exfat_raw.h |  2 +
>  fs/exfat/super.c     | 97 ++++++++++++++++++++++++--------------------
>  2 files changed, 56 insertions(+), 43 deletions(-)
> 
> diff --git a/fs/exfat/exfat_raw.h b/fs/exfat/exfat_raw.h index
> 07f74190df44..350ce59cc324 100644
> --- a/fs/exfat/exfat_raw.h
> +++ b/fs/exfat/exfat_raw.h
> @@ -10,11 +10,13 @@
> 
>  #define BOOT_SIGNATURE		0xAA55
>  #define EXBOOT_SIGNATURE	0xAA550000
> +#define STR_EXFAT		"EXFAT   "	/* size should be 8 */
> 
>  #define EXFAT_MAX_FILE_LEN	255
> 
>  #define VOL_CLEAN		0x0000
>  #define VOL_DIRTY		0x0002
> +#define ERR_MEDIUM		0x0004
> 
>  #define EXFAT_EOF_CLUSTER	0xFFFFFFFFu
>  #define EXFAT_BAD_CLUSTER	0xFFFFFFF7u
> diff --git a/fs/exfat/super.c b/fs/exfat/super.c index
> e60d28e73ff0..6a1330be5a9a 100644
> --- a/fs/exfat/super.c
> +++ b/fs/exfat/super.c
> @@ -366,25 +366,20 @@ static int exfat_read_root(struct inode *inode)
>  	return 0;
>  }
> 
> -static struct boot_sector *exfat_read_boot_with_logical_sector(
> -		struct super_block *sb)
> +static int exfat_calibrate_blocksize(struct super_block *sb, int
> +logical_sect)
>  {
>  	struct exfat_sb_info *sbi = EXFAT_SB(sb);
> -	struct boot_sector *p_boot = (struct boot_sector *)sbi->boot_bh-
> >b_data;
> -	unsigned short logical_sect = 0;
> -
> -	logical_sect = 1 << p_boot->sect_size_bits;
> 
>  	if (!is_power_of_2(logical_sect) ||
>  	    logical_sect < 512 || logical_sect > 4096) {
>  		exfat_err(sb, "bogus logical sector size %u", logical_sect);
> -		return NULL;
> +		return -EIO;
>  	}
> 
>  	if (logical_sect < sb->s_blocksize) {
>  		exfat_err(sb, "logical sector size too small for device
> (logical sector size = %u)",
>  			  logical_sect);
> -		return NULL;
> +		return -EIO;
>  	}
> 
>  	if (logical_sect > sb->s_blocksize) {
> @@ -394,24 +389,20 @@ static struct boot_sector
> *exfat_read_boot_with_logical_sector(
>  		if (!sb_set_blocksize(sb, logical_sect)) {
>  			exfat_err(sb, "unable to set blocksize %u",
>  				  logical_sect);
> -			return NULL;
> +			return -EIO;
>  		}
>  		sbi->boot_bh = sb_bread(sb, 0);
>  		if (!sbi->boot_bh) {
>  			exfat_err(sb, "unable to read boot sector (logical
> sector size = %lu)",
>  				  sb->s_blocksize);
> -			return NULL;
> +			return -EIO;
>  		}
> -
> -		p_boot = (struct boot_sector *)sbi->boot_bh->b_data;
>  	}
> -	return p_boot;
> +	return 0;
>  }
> 
> -/* mount the file system volume */
> -static int __exfat_fill_super(struct super_block *sb)
> +static int exfat_read_boot_sector(struct super_block *sb)
>  {
> -	int ret;
>  	struct boot_sector *p_boot;
>  	struct exfat_sb_info *sbi = EXFAT_SB(sb);
> 
> @@ -424,51 +415,41 @@ static int __exfat_fill_super(struct super_block
*sb)
>  		exfat_err(sb, "unable to read boot sector");
>  		return -EIO;
>  	}
> -
> -	/* PRB is read */
>  	p_boot = (struct boot_sector *)sbi->boot_bh->b_data;
> 
>  	/* check the validity of BOOT */
>  	if (le16_to_cpu((p_boot->signature)) != BOOT_SIGNATURE) {
>  		exfat_err(sb, "invalid boot record signature");
> -		ret = -EINVAL;
> -		goto free_bh;
> +		return -EINVAL;
>  	}
> 
> -
> -	/* check logical sector size */
> -	p_boot = exfat_read_boot_with_logical_sector(sb);
> -	if (!p_boot) {
> -		ret = -EIO;
> -		goto free_bh;
> +	if (memcmp(p_boot->fs_name, STR_EXFAT, BOOTSEC_FS_NAME_LEN)) {
> +		exfat_err(sb, "invalid fs_name"); /* fs_name may unprintable
> */
> +		return -EINVAL;
>  	}
> 
>  	/*
> -	 * res_zero field must be filled with zero to prevent mounting
> +	 * must_be_zero field must be filled with zero to prevent mounting
>  	 * from FAT volume.
>  	 */
> -	if (memchr_inv(p_boot->must_be_zero, 0,
> -			sizeof(p_boot->must_be_zero))) {
> -		ret = -EINVAL;
> -		goto free_bh;
> -	}
> +	if (memchr_inv(p_boot->must_be_zero, 0, sizeof(p_boot-
> >must_be_zero)))
> +		return -EINVAL;
> 
> -	p_boot = (struct boot_sector *)p_boot;
> -	if (!p_boot->num_fats) {
> +	if (p_boot->num_fats != 1 && p_boot->num_fats != 2) {
>  		exfat_err(sb, "bogus number of FAT structure");
> -		ret = -EINVAL;
> -		goto free_bh;
> +		return -EINVAL;
>  	}
> 
>  	sbi->sect_per_clus = 1 << p_boot->sect_per_clus_bits;
>  	sbi->sect_per_clus_bits = p_boot->sect_per_clus_bits;
> -	sbi->cluster_size_bits = sbi->sect_per_clus_bits + sb-
> >s_blocksize_bits;
> +	sbi->cluster_size_bits = p_boot->sect_per_clus_bits +
> +		p_boot->sect_size_bits;
>  	sbi->cluster_size = 1 << sbi->cluster_size_bits;
>  	sbi->num_FAT_sectors = le32_to_cpu(p_boot->fat_length);
>  	sbi->FAT1_start_sector = le32_to_cpu(p_boot->fat_offset);
> -	sbi->FAT2_start_sector = p_boot->num_fats == 1 ?
> -		sbi->FAT1_start_sector :
> -			sbi->FAT1_start_sector + sbi->num_FAT_sectors;
> +	sbi->FAT2_start_sector = le32_to_cpu(p_boot->fat_offset);
> +	if (p_boot->num_fats == 2)
> +		sbi->FAT2_start_sector += sbi->num_FAT_sectors;
>  	sbi->data_start_sector = le32_to_cpu(p_boot->clu_offset);
>  	sbi->num_sectors = le64_to_cpu(p_boot->vol_length);
>  	/* because the cluster index starts with 2 */ @@ -483,15 +464,45 @@
> static int __exfat_fill_super(struct super_block *sb)
>  	sbi->clu_srch_ptr = EXFAT_FIRST_CLUSTER;
>  	sbi->used_clusters = EXFAT_CLUSTERS_UNTRACKED;
> 
> -	if (le16_to_cpu(p_boot->vol_flags) & VOL_DIRTY) {
> -		sbi->vol_flag |= VOL_DIRTY;
> -		exfat_warn(sb, "Volume was not properly unmounted. Some data
> may be corrupt. Please run fsck.");
> +	/* check consistencies */
> +	if (sbi->num_FAT_sectors << p_boot->sect_size_bits <
> +	    sbi->num_clusters * 4) {
> +		exfat_err(sb, "bogus fat length");
> +		return -EINVAL;
> +	}
> +	if (sbi->data_start_sector <
> +	    sbi->FAT1_start_sector + sbi->num_FAT_sectors *
p_boot->num_fats)
> {
> +		exfat_err(sb, "bogus data start sector");
> +		return -EINVAL;
>  	}
> +	if (sbi->vol_flag & VOL_DIRTY)
> +		exfat_warn(sb, "Volume was not properly unmounted. Some data
> may be corrupt. Please run fsck.");
> +	if (sbi->vol_flag & ERR_MEDIUM)
> +		exfat_warn(sb, "Medium has reported failures. Some data may
> be
> +lost.");
> 
>  	/* exFAT file size is limited by a disk volume size */
>  	sb->s_maxbytes = (u64)(sbi->num_clusters - EXFAT_RESERVED_CLUSTERS)
> <<
>  		sbi->cluster_size_bits;
> 
> +	/* check logical sector size */
> +	if (exfat_calibrate_blocksize(sb, 1 << p_boot->sect_size_bits))
> +		return -EIO;
> +
> +	return 0;
> +}
> +
> +/* mount the file system volume */
> +static int __exfat_fill_super(struct super_block *sb) {
> +	int ret;
> +	struct exfat_sb_info *sbi = EXFAT_SB(sb);
> +
> +	ret = exfat_read_boot_sector(sb);
> +	if (ret) {
> +		exfat_err(sb, "failed to read boot sector");
> +		goto free_bh;
> +	}
> +
>  	ret = exfat_create_upcase_table(sb);
>  	if (ret) {
>  		exfat_err(sb, "failed to load upcase table");
> --
> 2.25.1