AZL-97788

In the Linux kernel, the following vulnerability has been resolved:

openrisc: signal: do not restore privileged SR bits on sigreturn

restoresigcontext() copies the whole supervision register (SR) from the signal frame and only clears SPRSR_SM before the value is reloaded into the hardware SR (through ESR and l.rfe) on the return to user space. All other SR bits are left under user control.

An unprivileged task can thus return from a signal handler through a crafted sigframe that clears SPRSRDME. With the data MMU disabled the CPU performs no translation or protection on data accesses, so the task gains read and write access to arbitrary physical memory, a local privilege escalation. SPRSRIME, SPRSRSUMRA, SPRSRLEE, SPRSREPH and the cache-enable bits are exposed the same way. The ptrace GPR regset already refuses any change to SR for exactly this reason.

Restore only the arithmetic flag bits (F, CY, OV) from the signal frame and take every privileged control bit from the SR the kernel saved on signal entry.

Verified with qemu-system-or1k -M or1k-sim: before this change an unprivileged PoC clears SPRSRDME in rt_sigreturn and writes a marker to physical address 0x03000000 (beyond the kernel's mem=32M); afterwards the same PoC receives SIGSEGV and physical memory is unchanged.