rdpkru - Intel 11th Gen Core CPU bug

repro.c

#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <assert.h>
#include <sys/mman.h>

#define BITS_PER_PKEY 2
static uint32_t rdpkru(void)
{
        uint32_t ecx = 0;
        uint32_t edx, pkru;

        /*
         * "rdpkru" instruction.  Places PKRU contents in to EAX,
         * clears EDX and requires that ecx=0.
         */
        asm volatile(".byte 0x0f,0x01,0xee\n\t"
                        : "=a" (pkru), "=d" (edx)
                        : "c" (ecx));
        return pkru;
}

static void print_pkru()
{
        uint32_t pkru = rdpkru();
        for (int i = 0; i < 32; i+=2) {
                printf("(pkey: %d val:%d) ",i/2, pkru >> i & 3);
        }
        printf("\n");
}

int main(void) {
        int pkey = pkey_alloc(0,0);
        assert(pkey >= 0);
        for (int iterations = 0; true ;iterations++) {
                uint32_t pkru = rdpkru();
                if (((pkru >> (pkey*BITS_PER_PKEY)) & 3) != 0) {
                        print_pkru();
                        fprintf(stderr, "pkey %d has an unexpected value on iteration %d\n", pkey, iterations);
                        exit(1);
                }
        }
        return 0;
}