isofs: fix Rock Ridge CE extent validation on multisession media

Commit a36d990f5913 ("isofs: validate Rock Ridge CE continuation extent
against volume size") compares the CE extent directly with s_nzones.  The
extent is an absolute block number, while s_nzones is the number of blocks
relative to the selected ISO session.  The comparison is therefore wrong
when a multisession disc starts at a non-zero block.

isofs_get_last_session() selects the last session on multisession
media, and its volume descriptors describe a volume beginning at that
session's LBA.
For example, a session beginning at LBA 45447 with 64 blocks can contain a
valid CE at absolute LBA 45467.  The existing check rejects that CE, so the
ER continuation record is not read, Rock Ridge is disabled, and the mount
falls back to Joliet names.

Save the selected session start in filesystem-block units and validate the
CE extent against the half-open interval [session_start, session_end).
Scale the session length to the same block units and retain a separate
block-device limit.  The lower bound is intentional: accepting arbitrary
blocks before the selected session could make a CE read data from a
previous session or another filesystem on the device.

Only apply the bounds check when cont_extent is non-zero.  A zero extent is
the in-memory sentinel indicating that no CE continuation was found, rather
than a request to read block zero.

For a single-session image, session_start is zero and the effective volume
boundary remains unchanged.

Build-tested with:

    make CONFIG_RUST= CONFIG_RUST_DRIVERS= fs/isofs/

Fixes: a36d990f5913 ("isofs: validate Rock Ridge CE continuation extent against volume size")
Cc: stable@vger.kernel.org
Signed-off-by: Baolin Liu <liubaolin@kylinos.cn>
Link: https://patch.msgid.link/20260907082602.3777551-1-liubaolin12138@163.com
Signed-off-by: Jan Kara <jack@suse.cz>

3 files changed, 13 insertions, 2 deletions

diff --git a/fs/isofs/inode.c b/fs/isofs/inode.c
index 337836a0a170..b766b5c9c593 100644
--- a/fs/isofs/inode.c
+++ b/fs/isofs/inode.c
@@ -821,6 +821,8 @@ root_found:
 	if (!sb_set_blocksize(s, orig_zonesize))
 		goto out_freesbi;
 
+	sbi->s_session_start = (sector_t)vol_desc_start <<
+				(ISOFS_BLOCK_BITS - s->s_blocksize_bits);
 	sbi->s_nls_iocharset = NULL;
 
 #ifdef CONFIG_JOLIET
diff --git a/fs/isofs/isofs.h b/fs/isofs/isofs.h
index dacb9cdae4fd..79ca0256843a 100644
--- a/fs/isofs/isofs.h
+++ b/fs/isofs/isofs.h
@@ -35,6 +35,8 @@ struct isofs_sb_info {
 	unsigned long s_firstdatazone;
 	unsigned long s_log_zone_size;
 	unsigned long s_max_size;
+	/* Session start in filesystem block units. */
+	sector_t s_session_start;
 	
 	int           s_rock_offset; /* offset of SUSP fields within SU area */
 	s32           s_sbsector;
diff --git a/fs/isofs/rock.c b/fs/isofs/rock.c
index 2628f31bd3a5..84e0d764c210 100644
--- a/fs/isofs/rock.c
+++ b/fs/isofs/rock.c
@@ -9,6 +9,7 @@
 
 #include <linux/slab.h>
 #include <linux/pagemap.h>
+#include <linux/blkdev.h>
 
 #include "isofs.h"
 #include "rock.h"
@@ -84,6 +85,10 @@ static void init_rock_state(struct rock_state *rs, struct inode *inode)
  */
 static int rock_continue(struct rock_state *rs)
 {
+	struct super_block *sb = rs->inode->i_sb;
+	struct isofs_sb_info *sbi = ISOFS_SB(sb);
+	sector_t session_end = sbi->s_session_start +
+		((sector_t)sbi->s_nzones << (ISOFS_BLOCK_BITS - sb->s_blocksize_bits));
 	int ret = 1;
 	int blocksize = 1 << rs->inode->i_blkbits;
 	const int min_de_size = offsetof(struct rock_ridge, u);
@@ -101,11 +106,13 @@ static int rock_continue(struct rock_state *rs)
 		goto out;
 	}
 
-	if ((unsigned)rs->cont_extent >= ISOFS_SB(rs->inode->i_sb)->s_nzones) {
+	if (rs->cont_extent &&
+	    (rs->cont_extent < sbi->s_session_start ||
+	     rs->cont_extent >= session_end)) {
 		printk(KERN_NOTICE "rock: corrupted directory entry. "
 			"extent=%u out of volume (nzones=%lu)\n",
 			(unsigned)rs->cont_extent,
-			ISOFS_SB(rs->inode->i_sb)->s_nzones);
+			sbi->s_nzones);
 		ret = -EIO;
 		goto out;
 	}