The xen_iret function in arch/x86/xen/xen-asm_32.S in the Linux kernel before 3.7.9 on 32-bit Xen paravirt_ops platforms does not properly handle an invalid value in the DS segment register, which allows guest OS users to gain guest OS privileges via a crafted application.
The dispatch_discard_io function in drivers/block/xen-blkback/blkback.c in the Xen blkback implementation in the Linux kernel before 3.10.5 allows guest OS users to cause a denial of service (data loss) via filesystem write operations on a read-only disk that supports the (1) BLKIF_OP_DISCARD (aka discard or TRIM) or (2) SCSI UNMAP feature.