chroot: failed to run command '/bin/bash': Permission denied

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$ sudo chroot /mnt/linux /bin/bash
chroot: failed to run command '/bin/bash': Permission denied

The error is what it says: /mnt/linux/bin/bash was found, but you are not allowed to execute it, even if you are root.
You will get the same error message if you run the command directly:

# /mnt/linux/bin/bash
bash: /mnt/linux/bin/bash: Permission denied

Most likely, the partition was mounted with the noexec flag, disallowing running any executable stored on that partition.
To check this, check the output of findmnt:

$ findmnt
|-/mnt/linux                  /dev/sdb1   ext4        rw,nosuid,nodev,noexec,noatime,data=ordered  

See the noexec attribute.

The solution is to umount the partition. Check /etc/fstab, and if the partition is there, replace auto by noauto, user by nouser, add exec.
Remount the partition and try again.