If multiple threads in userspace keep changing the protection keys
mapping a range, there can be a scenario where kernel takes a key fault
but the pkey value found in the siginfo struct is a permissive one.

This can confuse the userspace as shown in the below test case.

/* use this to control the number of test iterations */

static void pkeyreg_set(int pkey, unsigned long rights)
{
	unsigned long reg, shift;

	shift = (NR_PKEYS - pkey - 1) * PKEY_BITS_PER_PKEY;
	asm volatile("mfspr	%0, 0xd" : "=r"(reg));
	reg &= ~(((unsigned long) PKEY_BITS_MASK) << shift);
	reg |= (rights & PKEY_BITS_MASK) << shift;
	asm volatile("mtspr	0xd, %0" : : "r"(reg));
}

static unsigned long pkeyreg_get(void)
{
	unsigned long reg;

	asm volatile("mfspr	%0, 0xd" : "=r"(reg));
	return reg;
}

static int sys_pkey_mprotect(void *addr, size_t len, int prot, int pkey)
{
	return syscall(SYS_pkey_mprotect, addr, len, prot, pkey);
}

static int sys_pkey_alloc(unsigned long flags, unsigned long access_rights)
{
	return syscall(SYS_pkey_alloc, flags, access_rights);
}

static int sys_pkey_free(int pkey)
{
	return syscall(SYS_pkey_free, pkey);
}

static int faulting_pkey;
static int permissive_pkey;
static pthread_barrier_t pkey_set_barrier;
static pthread_barrier_t mprotect_barrier;

static void pkey_handle_fault(int signum, siginfo_t *sinfo, void *ctx)
{
	unsigned long pkeyreg;

	/* FIXME: printf is not signal-safe but for the current purpose,
	          it gets the job done. */
	printf("pkey: exp = %d, got = %d\n", faulting_pkey, sinfo->si_pkey);
	fflush(stdout);

	assert(sinfo->si_code == SEGV_PKUERR);
	assert(sinfo->si_pkey == faulting_pkey);

	/* clear pkey permissions to let the faulting instruction continue */
	pkeyreg_set(faulting_pkey, 0x0);
}

static void *do_mprotect_fault(void *p)
{
	unsigned long rights, pkeyreg, pgsize;
	unsigned int i;
	void *region;
	int pkey;

	srand(time(NULL));
	pgsize = sysconf(_SC_PAGESIZE);
	rights = PKEY_DISABLE_WRITE;
	region = p;

	/* allocate key, no permissions */
	assert((pkey = sys_pkey_alloc(0, PKEY_DISABLE_ACCESS)) > 0);
	pkeyreg_set(4, 0x0);

	/* cache the pkey here as the faulting pkey for future reference
	   in the signal handler */
	faulting_pkey = pkey;
	printf("%s: faulting pkey = %d\n", __func__, faulting_pkey);

	/* try to allocate, mprotect and free pkeys repeatedly */
	for (i = 0; i < NUM_ITERATIONS; i++) {
		/* sync up with the other thread here */
		pthread_barrier_wait(&pkey_set_barrier);

		/* make sure that the pkey used by the non-faulting thread
		   is made permissive for this thread's context too so that
		   no faults are triggered because it still might have been
		   set to a restrictive value */
//		pkeyreg_set(permissive_pkey, 0x0);

		/* sync up with the other thread here */
		pthread_barrier_wait(&mprotect_barrier);

		/* perform mprotect */
		assert(!sys_pkey_mprotect(region, pgsize, PROT_READ | PROT_WRITE, pkey));

		/* choose a random byte from the protected region and
		   attempt to write to it, this will generate a fault */
		*((char *) region + (rand() % pgsize)) = rand();

		/* restore pkey permissions as the signal handler may have
		   cleared the bit out for the sake of continuing */
		pkeyreg_set(pkey, PKEY_DISABLE_WRITE);
	}

	/* free pkey */
	sys_pkey_free(pkey);

	return NULL;
}

static void *do_mprotect_nofault(void *p)
{
	unsigned long pgsize;
	unsigned int i, j;
	void *region;
	int pkey;

	pgsize = sysconf(_SC_PAGESIZE);
	region = p;

	/* try to allocate, mprotect and free pkeys repeatedly */
	for (i = 0; i < NUM_ITERATIONS; i++) {
		/* allocate pkey, all permissions */
		assert((pkey = sys_pkey_alloc(0, 0)) > 0);
		permissive_pkey = pkey;

		/* sync up with the other thread here */
		pthread_barrier_wait(&pkey_set_barrier);
		pthread_barrier_wait(&mprotect_barrier);

		/* perform mprotect on the common page, no faults will
		   be triggered as this is most permissive */
		assert(!sys_pkey_mprotect(region, pgsize, PROT_READ | PROT_WRITE, pkey));

		/* free pkey */
		assert(!sys_pkey_free(pkey));
	}

	return NULL;
}

int main(int argc, char **argv)
{
	pthread_t fault_thread, nofault_thread;
	unsigned long pgsize;
	struct sigaction act;
	pthread_attr_t attr;
	cpu_set_t fault_cpuset, nofault_cpuset;
	unsigned int i;
	void *region;

	/* allocate memory region to protect */
	pgsize = sysconf(_SC_PAGESIZE);
	assert(region = memalign(pgsize, pgsize));

	CPU_ZERO(&fault_cpuset);
	CPU_SET(0, &fault_cpuset);
	CPU_ZERO(&nofault_cpuset);
	CPU_SET(8, &nofault_cpuset);
	assert(!pthread_attr_init(&attr));

	/* setup sigsegv signal handler */
	act.sa_handler = 0;
	act.sa_sigaction = pkey_handle_fault;
	assert(!sigprocmask(SIG_SETMASK, 0, &act.sa_mask));
	act.sa_flags = SA_SIGINFO;
	act.sa_restorer = 0;
	assert(!sigaction(SIGSEGV, &act, NULL));

	/* setup barrier for the two threads */
	pthread_barrier_init(&pkey_set_barrier, NULL, 2);
	pthread_barrier_init(&mprotect_barrier, NULL, 2);

	/* setup and start threads */
	assert(!pthread_create(&fault_thread, &attr, &do_mprotect_fault, region));
	assert(!pthread_setaffinity_np(fault_thread, sizeof(cpu_set_t), &fault_cpuset));
	assert(!pthread_create(&nofault_thread, &attr, &do_mprotect_nofault, region));
	assert(!pthread_setaffinity_np(nofault_thread, sizeof(cpu_set_t), &nofault_cpuset));

	/* cleanup */
	assert(!pthread_attr_destroy(&attr));
	assert(!pthread_join(fault_thread, NULL));
	assert(!pthread_join(nofault_thread, NULL));
	assert(!pthread_barrier_destroy(&pkey_set_barrier));
	assert(!pthread_barrier_destroy(&mprotect_barrier));
	free(region);

	puts("PASS");

	return EXIT_SUCCESS;
}

The above test can result the below failure without this patch.

pkey: exp = 3, got = 3
pkey: exp = 3, got = 4
a.out: pkey-siginfo-race.c:100: pkey_handle_fault: Assertion `sinfo->si_pkey == faulting_pkey' failed.
Aborted

Check for vma access before considering this a key fault. If vma pkey allow
access retry the acess again.

Test case is written by Sandipan Das <sandipan@linux.ibm.com> hence added SOB
from him.
Signed-off-by: Sandipan Das <sandipan@linux.ibm.com>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200505071729.54912-3-aneesh.kumar@linux.ibm.com