Date:   Wed, 17 Aug 2022 05:54:47 +0000
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Russell Currey <ruscur@...sell.cc>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
CC:     "mpe@...erman.id.au" <mpe@...erman.id.au>,
        "ajd@...ux.ibm.com" <ajd@...ux.ibm.com>,
        "aneesh.kumar@...ux.ibm.com" <aneesh.kumar@...ux.ibm.com>,
        "npiggin@...il.com" <npiggin@...il.com>,
        "anshuman.khandual@....com" <anshuman.khandual@....com>,
        "linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>,
        "nicholas@...ux.ibm.com" <nicholas@...ux.ibm.com>
Subject: Re: [PATCH v4 2/2] selftests/powerpc: Add a test for execute-only
 memory



Le 17/08/2022 à 07:06, Russell Currey a écrit :
> From: Nicholas Miehlbradt <nicholas@...ux.ibm.com>
> 
> This selftest is designed to cover execute-only protections
> on the Radix MMU but will also work with Hash.
> 
> The tests are based on those found in pkey_exec_test with modifications
> to use the generic mprotect() instead of the pkey variants.
> 
> Signed-off-by: Nicholas Miehlbradt <nicholas@...ux.ibm.com>
> Signed-off-by: Russell Currey <ruscur@...sell.cc>
> ---
> v4: new
> 
>   tools/testing/selftests/powerpc/mm/Makefile   |   3 +-
>   .../testing/selftests/powerpc/mm/exec_prot.c  | 231 ++++++++++++++++++

There is a lot of code in common with pkey_exec_prot.c
Isn't there a way to refactor ?

>   2 files changed, 233 insertions(+), 1 deletion(-)
>   create mode 100644 tools/testing/selftests/powerpc/mm/exec_prot.c
> 
> diff --git a/tools/testing/selftests/powerpc/mm/Makefile b/tools/testing/selftests/powerpc/mm/Makefile
> index 27dc09d0bfee..19dd0b2ea397 100644
> --- a/tools/testing/selftests/powerpc/mm/Makefile
> +++ b/tools/testing/selftests/powerpc/mm/Makefile
> @@ -3,7 +3,7 @@ noarg:
>   	$(MAKE) -C ../
>   
>   TEST_GEN_PROGS := hugetlb_vs_thp_test subpage_prot prot_sao segv_errors wild_bctr \
> -		  large_vm_fork_separation bad_accesses pkey_exec_prot \
> +		  large_vm_fork_separation bad_accesses exec_prot pkey_exec_prot \
>   		  pkey_siginfo stack_expansion_signal stack_expansion_ldst \
>   		  large_vm_gpr_corruption
>   TEST_PROGS := stress_code_patching.sh
> @@ -22,6 +22,7 @@ $(OUTPUT)/wild_bctr: CFLAGS += -m64
>   $(OUTPUT)/large_vm_fork_separation: CFLAGS += -m64
>   $(OUTPUT)/large_vm_gpr_corruption: CFLAGS += -m64
>   $(OUTPUT)/bad_accesses: CFLAGS += -m64
> +$(OUTPUT)/exec_prot: CFLAGS += -m64
>   $(OUTPUT)/pkey_exec_prot: CFLAGS += -m64
>   $(OUTPUT)/pkey_siginfo: CFLAGS += -m64
>   
> diff --git a/tools/testing/selftests/powerpc/mm/exec_prot.c b/tools/testing/selftests/powerpc/mm/exec_prot.c
> new file mode 100644
> index 000000000000..db75b2225de1
> --- /dev/null
> +++ b/tools/testing/selftests/powerpc/mm/exec_prot.c
> @@ -0,0 +1,231 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +/*
> + * Copyright 2022, Nicholas Miehlbradt, IBM Corporation
> + * based on pkey_exec_prot.c
> + *
> + * Test if applying execute protection on pages works as expected.
> + */
> +
> +#define _GNU_SOURCE
> +#include <stdio.h>
> +#include <stdlib.h>
> +#include <string.h>
> +#include <signal.h>
> +
> +#include <unistd.h>
> +#include <sys/mman.h>
> +
> +#include "pkeys.h"
> +
> +
> +#define PPC_INST_NOP	0x60000000
> +#define PPC_INST_TRAP	0x7fe00008
> +#define PPC_INST_BLR	0x4e800020
> +
> +static volatile sig_atomic_t fault_code;
> +static volatile sig_atomic_t remaining_faults;
> +static volatile unsigned int *fault_addr;
> +static unsigned long pgsize, numinsns;
> +static unsigned int *insns;
> +static bool pkeys_supported;
> +
> +static bool is_fault_expected(int fault_code)
> +{
> +	if (fault_code == SEGV_ACCERR)
> +		return true;
> +
> +	/* Assume any pkey error is fine since pkey_exec_prot test covers them */
> +	if (fault_code == SEGV_PKUERR && pkeys_supported)
> +		return true;
> +
> +	return false;
> +}
> +
> +static void trap_handler(int signum, siginfo_t *sinfo, void *ctx)
> +{
> +	/* Check if this fault originated from the expected address */
> +	if (sinfo->si_addr != (void *)fault_addr)
> +		sigsafe_err("got a fault for an unexpected address\n");
> +
> +	_exit(1);
> +}
> +
> +static void segv_handler(int signum, siginfo_t *sinfo, void *ctx)
> +{
> +	fault_code = sinfo->si_code;
> +
> +	/* Check if this fault originated from the expected address */
> +	if (sinfo->si_addr != (void *)fault_addr) {
> +		sigsafe_err("got a fault for an unexpected address\n");
> +		_exit(1);
> +	}
> +
> +	/* Check if too many faults have occurred for a single test case */
> +	if (!remaining_faults) {
> +		sigsafe_err("got too many faults for the same address\n");
> +		_exit(1);
> +	}
> +
> +
> +	/* Restore permissions in order to continue */
> +	if (is_fault_expected(fault_code)) {
> +		if (mprotect(insns, pgsize, PROT_READ | PROT_WRITE | PROT_EXEC)) {
> +			sigsafe_err("failed to set access permissions\n");
> +			_exit(1);
> +		}
> +	} else {
> +		sigsafe_err("got a fault with an unexpected code\n");
> +		_exit(1);
> +	}
> +
> +	remaining_faults--;
> +}
> +
> +static int check_exec_fault(int rights)
> +{
> +	/*
> +	 * Jump to the executable region.
> +	 *
> +	 * The first iteration also checks if the overwrite of the
> +	 * first instruction word from a trap to a no-op succeeded.
> +	 */
> +	fault_code = -1;
> +	remaining_faults = 0;
> +	if (!(rights & PROT_EXEC))
> +		remaining_faults = 1;
> +
> +	FAIL_IF(mprotect(insns, pgsize, rights) != 0);
> +	asm volatile("mtctr	%0; bctrl" : : "r"(insns));
> +
> +	FAIL_IF(remaining_faults != 0);
> +	if (!(rights & PROT_EXEC))
> +		FAIL_IF(!is_fault_expected(fault_code));
> +
> +	return 0;
> +}
> +
> +static int test(void)
> +{
> +	struct sigaction segv_act, trap_act;
> +	int i;
> +
> +	/* Skip the test if the CPU doesn't support Radix */
> +	SKIP_IF(!have_hwcap2(PPC_FEATURE2_ARCH_3_00));
> +
> +	/* Check if pkeys are supported */
> +	pkeys_supported = pkeys_unsupported() == 0;
> +
> +	/* Setup SIGSEGV handler */
> +	segv_act.sa_handler = 0;
> +	segv_act.sa_sigaction = segv_handler;
> +	FAIL_IF(sigprocmask(SIG_SETMASK, 0, &segv_act.sa_mask) != 0);
> +	segv_act.sa_flags = SA_SIGINFO;
> +	segv_act.sa_restorer = 0;
> +	FAIL_IF(sigaction(SIGSEGV, &segv_act, NULL) != 0);
> +
> +	/* Setup SIGTRAP handler */
> +	trap_act.sa_handler = 0;
> +	trap_act.sa_sigaction = trap_handler;
> +	FAIL_IF(sigprocmask(SIG_SETMASK, 0, &trap_act.sa_mask) != 0);
> +	trap_act.sa_flags = SA_SIGINFO;
> +	trap_act.sa_restorer = 0;
> +	FAIL_IF(sigaction(SIGTRAP, &trap_act, NULL) != 0);
> +
> +	/* Setup executable region */
> +	pgsize = getpagesize();
> +	numinsns = pgsize / sizeof(unsigned int);
> +	insns = (unsigned int *)mmap(NULL, pgsize, PROT_READ | PROT_WRITE,
> +				      MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
> +	FAIL_IF(insns == MAP_FAILED);
> +
> +	/* Write the instruction words */
> +	for (i = 1; i < numinsns - 1; i++)
> +		insns[i] = PPC_INST_NOP;
> +
> +	/*
> +	 * Set the first instruction as an unconditional trap. If
> +	 * the last write to this address succeeds, this should
> +	 * get overwritten by a no-op.
> +	 */
> +	insns[0] = PPC_INST_TRAP;
> +
> +	/*
> +	 * Later, to jump to the executable region, we use a branch
> +	 * and link instruction (bctrl) which sets the return address
> +	 * automatically in LR. Use that to return back.
> +	 */
> +	insns[numinsns - 1] = PPC_INST_BLR;
> +
> +	/*
> +	 * Pick the first instruction's address from the executable
> +	 * region.
> +	 */
> +	fault_addr = insns;
> +
> +	/*
> +	 * Read an instruction word from the address when the page
> +	 * is execute only. This should generate an access fault.
> +	 */
> +	fault_code = -1;
> +	remaining_faults = 1;
> +	printf("Testing read on --x, should fault...");
> +	FAIL_IF(mprotect(insns, pgsize, PROT_EXEC) != 0);
> +	i = *fault_addr;
> +	FAIL_IF(remaining_faults != 0 || !is_fault_expected(fault_code));
> +	printf("ok!\n");
> +
> +	/*
> +	 * Write an instruction word to the address when the page
> +	 * execute only. This should also generate an access fault.
> +	 */
> +	fault_code = -1;
> +	remaining_faults = 1;
> +	printf("Testing write on --x, should fault...");
> +	FAIL_IF(mprotect(insns, pgsize, PROT_EXEC) != 0);
> +	*fault_addr = PPC_INST_NOP;
> +	FAIL_IF(remaining_faults != 0 || !is_fault_expected(fault_code));
> +	printf("ok!\n");
> +
> +	printf("Testing exec on ---, should fault...");
> +	FAIL_IF(check_exec_fault(PROT_NONE));
> +	printf("ok!\n");
> +
> +	printf("Testing exec on r--, should fault...");
> +	FAIL_IF(check_exec_fault(PROT_READ));
> +	printf("ok!\n");
> +
> +	printf("Testing exec on -w-, should fault...");
> +	FAIL_IF(check_exec_fault(PROT_WRITE));
> +	printf("ok!\n");
> +
> +	printf("Testing exec on rw-, should fault...");
> +	FAIL_IF(check_exec_fault(PROT_READ | PROT_WRITE));
> +	printf("ok!\n");
> +
> +	printf("Testing exec on --x, should succeed...");
> +	FAIL_IF(check_exec_fault(PROT_EXEC));
> +	printf("ok!\n");
> +
> +	printf("Testing exec on r-x, should succeed...");
> +	FAIL_IF(check_exec_fault(PROT_READ | PROT_EXEC));
> +	printf("ok!\n");
> +
> +	printf("Testing exec on -wx, should succeed...");
> +	FAIL_IF(check_exec_fault(PROT_WRITE | PROT_EXEC));
> +	printf("ok!\n");
> +
> +	printf("Testing exec on rwx, should succeed...");
> +	FAIL_IF(check_exec_fault(PROT_READ | PROT_WRITE | PROT_EXEC));
> +	printf("ok!\n");
> +
> +	/* Cleanup */
> +	FAIL_IF(munmap((void *)insns, pgsize));
> +
> +	return 0;
> +}
> +
> +int main(void)
> +{
> +	return test_harness(test, "exec_prot");
> +}

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