CVE-2021-26932

An issue was discovered in the Linux kernel 3.2 through 5.10.16, as used by
Xen. Grant mapping operations often occur in batch hypercalls, where a
number of operations are done in a single hypercall, the success or failure
of each one is reported to the backend driver, and the backend driver then
loops over the results, performing follow-up actions based on the success
or failure of each operation. Unfortunately, when running in PV mode, the
Linux backend drivers mishandle this: Some errors are ignored, effectively
implying their success from the success of related batch elements. In other
cases, errors resulting from one batch element lead to further batch
elements not being inspected, and hence successful ones to not be possible
to properly unmap upon error recovery. Only systems with Linux backends
running in PV mode are vulnerable. Linux backends run in HVM / PVH modes
are not vulnerable. This affects arch/*/xen/p2m.c and drivers/xen/gntdev.c.