From mboxrd@z Thu Jan 1 00:00:00 1970 From: Kees Cook Subject: Re: [RFC PATCH] seccomp: Add protection keys into seccomp_data Date: Mon, 29 Oct 2018 09:25:15 -0700 Message-ID: References: <20181029112343.27454-1-msammler@mpi-sws.org> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Return-path: In-Reply-To: <20181029112343.27454-1-msammler@mpi-sws.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: linuxppc-dev-bounces+glppe-linuxppc-embedded-2=m.gmane.org@lists.ozlabs.org Sender: "Linuxppc-dev" To: Michael Sammler Cc: Will Drewry , Linux API , Dave Hansen , Ram Pai , Andy Lutomirski , PowerPC List-Id: linux-api@vger.kernel.org On Mon, Oct 29, 2018 at 4:23 AM, Michael Sammler 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. > > 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 > Cc: Andy Lutomirski > Cc: Will Drewry > Cc: Ram Pai > Cc: linuxppc-dev@lists.ozlabs.org > Cc: linux-api@vger.kernel.org > Signed-off-by: Michael Sammler > --- > 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