[RFC PATCH] seccomp: Add protection keys into seccomp_data
Michael Sammler
msammler at mpi-sws.org
Tue Oct 30 04:05:57 AEDT 2018
On 10/29/2018 05:48 PM, Ram Pai wrote:
> On Mon, Oct 29, 2018 at 09:25:15AM -0700, Kees Cook wrote:
>> On Mon, Oct 29, 2018 at 4:23 AM, Michael Sammler <msammler at mpi-sws.org> wrote:
>>> Add the current value of an architecture specific protection keys
>>> register (currently PKRU on x86) to data available for seccomp-bpf
>>> programs to work on. This allows filters based on the currently
>>> enabled protection keys.
>>>
>>> Support for protection keys on the POWER architecture is not part of
>>> this patch since I do not have access to a PowerPC, but adding support
>>> for it can be achieved by setting sd->pkeys to the AMR register in
>>> populate_seccomp_data.
> Maybe you can use a generic read_pkey() function, and each arch can
> provide their own implementation. In the case of powerpc it is
> mfspr(SPRN_AMR);
>
> something like the code below might help?
>
> diff --git a/arch/powerpc/include/asm/pkeys.h b/arch/powerpc/include/asm/pkeys.h
> index 92a9962..d79efe3 100644
> --- a/arch/powerpc/include/asm/pkeys.h
> +++ b/arch/powerpc/include/asm/pkeys.h
> @@ -89,6 +89,11 @@ static inline u16 pte_to_pkey_bits(u64 pteflags)
> #define __mm_pkey_is_reserved(pkey) (reserved_allocation_mask & \
> pkey_alloc_mask(pkey))
>
> +static inline u64 read_pkey()
> +{
> + return mfspr(SPRN_AMR);
> +}
> +
> static inline bool mm_pkey_is_allocated(struct mm_struct *mm, int pkey)
> {
> if (pkey < 0 || pkey >= arch_max_pkey())
> diff --git a/arch/x86/include/asm/pkeys.h b/arch/x86/include/asm/pkeys.h
> index 19b137f..feea114 100644
> --- a/arch/x86/include/asm/pkeys.h
> +++ b/arch/x86/include/asm/pkeys.h
> @@ -9,6 +9,11 @@
> extern int arch_set_user_pkey_access(struct task_struct *tsk, int pkey,
> unsigned long init_val);
>
> +static inline u64 read_pkey()
> +{
> + return read_pkru();
> +}
> +
> static inline bool arch_pkeys_enabled(void)
> {
> return boot_cpu_has(X86_FEATURE_OSPKE);
> diff --git a/include/linux/pkeys.h b/include/linux/pkeys.h
> index 2955ba97..f1538c7 100644
> --- a/include/linux/pkeys.h
> +++ b/include/linux/pkeys.h
> @@ -13,6 +13,11 @@
> #define PKEY_DEDICATED_EXECUTE_ONLY 0
> #define ARCH_VM_PKEY_FLAGS 0
>
> +static inline u64 read_pkey()
> +{
> + return 0;
> +}
> +
> static inline int vma_pkey(struct vm_area_struct *vma)
> {
> return 0;
>
> RP
Thank you very much, this helps indeed. I will add this to the next
version of this patch.
>>> One use case for this patch is disabling unnecessary system calls for a
>>> library (e.g. network i/o for a crypto library) while the library runs
>>> without disabling the system calls for the whole program (by changing
>>> the protection keys before and after the library executes). Using this
>>> one could ensure that the library behaves a expected (e.g. the crypto
>>> library not sending private keys to a malicious server).
>>>
>>> This patch also enables lightweight sandboxing of untrusted code using
>>> memory protection keys: Protection keys provide memory isolation but
>>> for a sandbox system call isolation is needed as well. This patch
>>> allows writing a seccomp filter to prevent system calls by the
>>> untrusted code while still allowing system calls for the trusted code.
>> Isn't PKU instruction based? Couldn't a malicious library just change
>> the state of the MPK registers? This seems like an easy way to bypass
>> any filters that used PKU. I'm not convinced this is a meaningful
>> barrier that should be enforced by seccomp.
>>
>> This can also be done with a signal handler with SECCOMP_RET_TRAP and
>> check instruction pointer vs PKU state.
>>
>> -Kees
>>
>>> An alternative design would be to extend (c)BPF with a new instruction
>>> to read the state of a protection key. This alternate design would
>>> provide a simpler interface to the user space since the BPF program
>>> would not need to deal with the architecture specific pkeys field in
>>> seccomp_data, but the question is, how much of an advantage this would
>>> be as the nr field in seccomp_data is already architecture specific.
>>> Adding a new instruction for BPF programs is more complicated than
>>> this patch and might be a breaking change.
>>>
>>> Results of selftests/seccomp_benchmark.c on a x86 machine with pkeys
>>> support:
>>>
>>> With patch:
>>> Benchmarking 33554432 samples...
>>> 28.019505558 - 18.676858522 = 9342647036
>>> getpid native: 278 ns
>>> 42.279109885 - 28.019657031 = 14259452854
>>> getpid RET_ALLOW: 424 ns
>>> Estimated seccomp overhead per syscall: 146 ns
>>>
>>> Without patch:
>>> Benchmarking 33554432 samples...
>>> 28.059619466 - 18.706769155 = 9352850311
>>> getpid native: 278 ns
>>> 42.299228279 - 28.059761804 = 14239466475
>>> getpid RET_ALLOW: 424 ns
>>> Estimated seccomp overhead per syscall: 146 ns
>>>
>>> Cc: Kees Cook <keescook at chromium.org>
>>> Cc: Andy Lutomirski <luto at amacapital.net>
>>> Cc: Will Drewry <wad at chromium.org>
>>> Cc: Ram Pai <linuxram at us.ibm.com>
>>> Cc: linuxppc-dev at lists.ozlabs.org
>>> Cc: linux-api at vger.kernel.org
>>> Signed-off-by: Michael Sammler <msammler at mpi-sws.org>
>>> ---
>>> Changes to the previous version:
>>> - added motivation, notes about POWER, alternative design and benchmark results to the commit log
>>> - renamed pkru field in seccomp_data to pkeys
>>> - changed size of pkru field to __u64 and removed reserved field
>>> - added test for x86
>>>
>>> arch/mips/kernel/ptrace.c | 1 +
>>> arch/x86/entry/common.c | 1 +
>>> include/uapi/linux/seccomp.h | 3 +
>>> kernel/seccomp.c | 1 +
>>> tools/testing/selftests/seccomp/seccomp_bpf.c | 107 +++++++++++++++++++++++++-
>>> 5 files changed, 112 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/arch/mips/kernel/ptrace.c b/arch/mips/kernel/ptrace.c
>>> index e5ba56c0..a58dd04d 100644
>>> --- a/arch/mips/kernel/ptrace.c
>>> +++ b/arch/mips/kernel/ptrace.c
>>> @@ -1277,6 +1277,7 @@ asmlinkage long syscall_trace_enter(struct pt_regs *regs, long syscall)
>>> for (i = 0; i < 6; i++)
>>> sd.args[i] = args[i];
>>> sd.instruction_pointer = KSTK_EIP(current);
>>> + sd.pkeys = 0;
>>>
>>> ret = __secure_computing(&sd);
>>> if (ret == -1)
>>> diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
>>> index 3b2490b8..20c51bf2 100644
>>> --- a/arch/x86/entry/common.c
>>> +++ b/arch/x86/entry/common.c
>>> @@ -98,6 +98,7 @@ static long syscall_trace_enter(struct pt_regs *regs)
>>> sd.arch = arch;
>>> sd.nr = regs->orig_ax;
>>> sd.instruction_pointer = regs->ip;
>>> + sd.pkeys = read_pkru();
>>> #ifdef CONFIG_X86_64
>>> if (arch == AUDIT_ARCH_X86_64) {
>>> sd.args[0] = regs->di;
>>> diff --git a/include/uapi/linux/seccomp.h b/include/uapi/linux/seccomp.h
>>> index 9efc0e73..3aa2d934 100644
>>> --- a/include/uapi/linux/seccomp.h
>>> +++ b/include/uapi/linux/seccomp.h
>>> @@ -52,12 +52,15 @@
>>> * @instruction_pointer: at the time of the system call.
>>> * @args: up to 6 system call arguments always stored as 64-bit values
>>> * regardless of the architecture.
>>> + * @pkeys: value of an architecture specific protection keys register
>>> + * (currently PKRU on x86)
>>> */
>>> struct seccomp_data {
>>> int nr;
>>> __u32 arch;
>>> __u64 instruction_pointer;
>>> __u64 args[6];
>>> + __u64 pkeys;
>>> };
>>>
>>> #endif /* _UAPI_LINUX_SECCOMP_H */
>>> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
>>> index fd023ac2..dfb8b0d6 100644
>>> --- a/kernel/seccomp.c
>>> +++ b/kernel/seccomp.c
>>> @@ -91,6 +91,7 @@ static void populate_seccomp_data(struct seccomp_data *sd)
>>> sd->args[4] = args[4];
>>> sd->args[5] = args[5];
>>> sd->instruction_pointer = KSTK_EIP(task);
>>> + sd->pkeys = 0;
>>> }
>>>
>>> /**
>>> diff --git a/tools/testing/selftests/seccomp/seccomp_bpf.c b/tools/testing/selftests/seccomp/seccomp_bpf.c
>>> index e1473234..f7f8fa6f 100644
>>> --- a/tools/testing/selftests/seccomp/seccomp_bpf.c
>>> +++ b/tools/testing/selftests/seccomp/seccomp_bpf.c
>>> @@ -82,6 +82,7 @@ struct seccomp_data {
>>> __u32 arch;
>>> __u64 instruction_pointer;
>>> __u64 args[6];
>>> + __u64 pkeys;
>>> };
>>> #endif
>>>
>>> @@ -732,7 +733,9 @@ TEST(KILL_process)
>>> TEST(arg_out_of_range)
>>> {
>>> struct sock_filter filter[] = {
>>> - BPF_STMT(BPF_LD|BPF_W|BPF_ABS, syscall_arg(6)),
>>> + BPF_STMT(BPF_LD|BPF_W|BPF_ABS,
>>> + offsetof(struct seccomp_data, pkeys)
>>> + + sizeof(__u64)),
>>> BPF_STMT(BPF_RET|BPF_K, SECCOMP_RET_ALLOW),
>>> };
>>> struct sock_fprog prog = {
>>> @@ -2933,6 +2936,108 @@ skip:
>>> ASSERT_EQ(0, kill(pid, SIGKILL));
>>> }
>>>
>>> +#if defined(__i386__) || defined(__x86_64__)
>>> +static inline void __cpuid(unsigned int *eax, unsigned int *ebx,
>>> + unsigned int *ecx, unsigned int *edx)
>>> +{
>>> + /* ecx is often an input as well as an output. */
>>> + asm volatile(
>>> + "cpuid;"
>>> + : "=a" (*eax),
>>> + "=b" (*ebx),
>>> + "=c" (*ecx),
>>> + "=d" (*edx)
>>> + : "0" (*eax), "2" (*ecx));
>>> +}
>>> +
>>> +/* Intel-defined CPU features, CPUID level 0x00000007:0 (ecx) */
>>> +#define X86_FEATURE_PKU (1<<3) /* Protection Keys for Userspace */
>>> +#define X86_FEATURE_OSPKE (1<<4) /* OS Protection Keys Enable */
>>> +
>>> +static inline int cpu_has_pku(void)
>>> +{
>>> + unsigned int eax;
>>> + unsigned int ebx;
>>> + unsigned int ecx;
>>> + unsigned int edx;
>>> +
>>> + eax = 0x7;
>>> + ecx = 0x0;
>>> + __cpuid(&eax, &ebx, &ecx, &edx);
>>> +
>>> + if (!(ecx & X86_FEATURE_PKU))
>>> + return 0;
>>> + if (!(ecx & X86_FEATURE_OSPKE))
>>> + return 0;
>>> + return 1;
>>> +}
>>> +
>>> +static inline __u32 read_pkru(void)
>>> +{
>>> + if (!cpu_has_pku())
>>> + return 0;
>>> +
>>> + __u32 ecx = 0;
>>> + __u32 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 inline void write_pkru(__u32 pkru)
>>> +{
>>> + if (!cpu_has_pku())
>>> + return;
>>> +
>>> + __u32 ecx = 0, edx = 0;
>>> +
>>> + /*
>>> + * "wrpkru" instruction. Loads contents in EAX to PKRU,
>>> + * requires that ecx = edx = 0.
>>> + */
>>> + asm volatile(".byte 0x0f,0x01,0xef\n\t"
>>> + : : "a" (pkru), "c"(ecx), "d"(edx));
>>> +}
>>> +
>>> +#define TEST_PKRU 0x55555550
>>> +
>>> +TEST_SIGNAL(pkeys_set, SIGSYS)
>>> +{
>>> + write_pkru(TEST_PKRU);
>>> + /* read back the written value because pkru might not be supported */
>>> + __u32 pkru = read_pkru();
>>> +
>>> + struct sock_filter filter[] = {
>>> + BPF_STMT(BPF_LD|BPF_W|BPF_ABS,
>>> + offsetof(struct seccomp_data, pkeys)),
>>> + BPF_JUMP(BPF_JMP|BPF_JEQ|BPF_K, pkru, 1, 0),
>>> + BPF_STMT(BPF_RET|BPF_K, SECCOMP_RET_ALLOW),
>>> + BPF_STMT(BPF_RET|BPF_K, SECCOMP_RET_KILL),
>>> + };
>>> + struct sock_fprog prog = {
>>> + .len = (unsigned short)ARRAY_SIZE(filter),
>>> + .filter = filter,
>>> + };
>>> + long ret;
>>> +
>>> + ret = prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0);
>>> + ASSERT_EQ(0, ret);
>>> +
>>> + ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &prog);
>>> + ASSERT_EQ(0, ret);
>>> +
>>> + /* should never return. */
>>> + EXPECT_EQ(0, syscall(__NR_getpid));
>>> +}
>>> +#endif
>>> +
>>> +
>>> /*
>>> * TODO:
>>> * - add microbenchmarks
>>> --
>>> 2.11.0
>>
>>
>> --
>> Kees Cook
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