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Message-Id: <1331587715-26069-8-git-send-email-wad@chromium.org> Date: Mon, 12 Mar 2012 16:28:30 -0500 From: Will Drewry <wad@...omium.org> To: linux-kernel@...r.kernel.org Cc: linux-arch@...r.kernel.org, linux-doc@...r.kernel.org, kernel-hardening@...ts.openwall.com, netdev@...r.kernel.org, x86@...nel.org, arnd@...db.de, davem@...emloft.net, hpa@...or.com, mingo@...hat.com, oleg@...hat.com, peterz@...radead.org, rdunlap@...otime.net, mcgrathr@...omium.org, tglx@...utronix.de, luto@....edu, eparis@...hat.com, serge.hallyn@...onical.com, djm@...drot.org, scarybeasts@...il.com, indan@....nu, pmoore@...hat.com, akpm@...ux-foundation.org, corbet@....net, eric.dumazet@...il.com, markus@...omium.org, coreyb@...ux.vnet.ibm.com, keescook@...omium.org, Will Drewry <wad@...omium.org> Subject: [PATCH v14 08/13] seccomp: add SECCOMP_RET_ERRNO This change adds the SECCOMP_RET_ERRNO as a valid return value from a seccomp filter. Additionally, it makes the first use of the lower 16-bits for storing a filter-supplied errno. 16-bits is more than enough for the errno-base.h calls. Returning errors instead of immediately terminating processes that violate seccomp policy allow for broader use of this functionality for kernel attack surface reduction. For example, a linux container could maintain a whitelist of pre-existing system calls but drop all new ones with errnos. This would keep a logically static attack surface while providing errnos that may allow for graceful failure without the downside of do_exit() on a bad call. v14: - no change/rebase v13: - rebase on to 88ebdda6159ffc15699f204c33feb3e431bf9bdc v12: - move to WARN_ON if filter is NULL (oleg@...hat.com, luto@....edu, keescook@...omium.org) - return immediately for filter==NULL (keescook@...omium.org) - change evaluation to only compare the ACTION so that layered errnos don't result in the lowest one being returned. (keeschook@...omium.org) v11: - check for NULL filter (keescook@...omium.org) v10: - change loaders to fn v9: - n/a v8: - update Kconfig to note new need for syscall_set_return_value. - reordered such that TRAP behavior follows on later. - made the for loop a little less indent-y v7: - introduced Reviewed-by: Kees Cook <keescook@...omium.org> Acked-by: Serge Hallyn <serge.hallyn@...onical.com> Signed-off-by: Will Drewry <wad@...omium.org> --- arch/Kconfig | 6 ++++-- include/linux/seccomp.h | 15 +++++++++++---- kernel/seccomp.c | 44 +++++++++++++++++++++++++++++++++++--------- 3 files changed, 50 insertions(+), 15 deletions(-) diff --git a/arch/Kconfig b/arch/Kconfig index 7c6bd48..dd4e067 100644 --- a/arch/Kconfig +++ b/arch/Kconfig @@ -203,8 +203,10 @@ config HAVE_ARCH_SECCOMP_FILTER bool help This symbol should be selected by an architecure if it provides - asm/syscall.h, specifically syscall_get_arguments() and - syscall_get_arch(). + asm/syscall.h, specifically syscall_get_arguments(), + syscall_get_arch(), and syscall_set_return_value(). Additionally, + its system call entry path must respect a return value of -1 from + __secure_computing_int() and/or secure_computing(). config SECCOMP_FILTER def_bool y diff --git a/include/linux/seccomp.h b/include/linux/seccomp.h index ce980a8..eb36185 100644 --- a/include/linux/seccomp.h +++ b/include/linux/seccomp.h @@ -12,13 +12,14 @@ /* * All BPF programs must return a 32-bit value. - * The bottom 16-bits are reserved for future use. + * The bottom 16-bits are for optional return data. * The upper 16-bits are ordered from least permissive values to most. * * The ordering ensures that a min_t() over composed return values always * selects the least permissive choice. */ #define SECCOMP_RET_KILL 0x00000000U /* kill the task immediately */ +#define SECCOMP_RET_ERRNO 0x00030000U /* returns an errno */ #define SECCOMP_RET_ALLOW 0x7fff0000U /* allow */ /* Masks for the return value sections. */ @@ -64,11 +65,17 @@ struct seccomp { struct seccomp_filter *filter; }; -extern void __secure_computing(int); -static inline void secure_computing(int this_syscall) +/* + * Direct callers to __secure_computing should be updated as + * CONFIG_HAVE_ARCH_SECCOMP_FILTER propagates. + */ +extern void __secure_computing(int) __deprecated; +extern int __secure_computing_int(int); +static inline int secure_computing(int this_syscall) { if (unlikely(test_thread_flag(TIF_SECCOMP))) - __secure_computing(this_syscall); + return __secure_computing_int(this_syscall); + return 0; } extern long prctl_get_seccomp(void); diff --git a/kernel/seccomp.c b/kernel/seccomp.c index 82f5a36..8cb9b69 100644 --- a/kernel/seccomp.c +++ b/kernel/seccomp.c @@ -181,15 +181,20 @@ static int seccomp_chk_filter(struct sock_filter *filter, unsigned int flen) static u32 seccomp_run_filters(int syscall) { struct seccomp_filter *f; - u32 ret = SECCOMP_RET_KILL; + u32 ret = SECCOMP_RET_ALLOW; + + /* Ensure unexpected behavior doesn't result in failing open. */ + if (WARN_ON(current->seccomp.filter == NULL)) + return SECCOMP_RET_KILL; + /* * All filters are evaluated in order of youngest to oldest. The lowest - * BPF return value always takes priority. + * BPF return value (ignoring the DATA) always takes priority. */ for (f = current->seccomp.filter; f; f = f->prev) { - ret = sk_run_filter(NULL, f->insns); - if (ret != SECCOMP_RET_ALLOW) - break; + u32 cur_ret = sk_run_filter(NULL, f->insns); + if ((cur_ret & SECCOMP_RET_ACTION) < (ret & SECCOMP_RET_ACTION)) + ret = cur_ret; } return ret; } @@ -331,6 +336,13 @@ static int mode1_syscalls_32[] = { void __secure_computing(int this_syscall) { + /* Filter calls should never use this function. */ + BUG_ON(current->seccomp.mode == SECCOMP_MODE_FILTER); + __secure_computing_int(this_syscall); +} + +int __secure_computing_int(int this_syscall) +{ int mode = current->seccomp.mode; int exit_code = SIGKILL; int *syscall; @@ -344,16 +356,29 @@ void __secure_computing(int this_syscall) #endif do { if (*syscall == this_syscall) - return; + return 0; } while (*++syscall); break; #ifdef CONFIG_SECCOMP_FILTER - case SECCOMP_MODE_FILTER: - if (seccomp_run_filters(this_syscall) == SECCOMP_RET_ALLOW) - return; + case SECCOMP_MODE_FILTER: { + u32 action = seccomp_run_filters(this_syscall); + switch (action & SECCOMP_RET_ACTION) { + case SECCOMP_RET_ERRNO: + /* Set the low-order 16-bits as a errno. */ + syscall_set_return_value(current, task_pt_regs(current), + -(action & SECCOMP_RET_DATA), + 0); + return -1; + case SECCOMP_RET_ALLOW: + return 0; + case SECCOMP_RET_KILL: + default: + break; + } seccomp_filter_log_failure(this_syscall); exit_code = SIGSYS; break; + } #endif default: BUG(); @@ -364,6 +389,7 @@ void __secure_computing(int this_syscall) #endif audit_seccomp(this_syscall); do_exit(exit_code); + return -1; /* never reached */ } long prctl_get_seccomp(void) -- 1.7.5.4
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