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[88.114.211.119]) by smtp.gmail.com with ESMTPSA id x69sm1453725lff.54.2020.11.01.03.41.32 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sun, 01 Nov 2020 03:41:33 -0800 (PST) Subject: Re: [PATCH] mm: optionally disable brk() To: David Hildenbrand , David Laight , Michal Hocko , Kees Cook Cc: "akpm@linux-foundation.org" , "linux-mm@kvack.org" , "linux-kernel@vger.kernel.org" References: <20201002171921.3053-1-toiwoton@gmail.com> <653873ef-2a57-37e0-1ac3-fba763652b35@redhat.com> <2a0f5ade-d770-c36e-50bc-ff0c8e9dacbf@gmail.com> <20201005061248.GN4555@dhcp22.suse.cz> <888e62e0-3979-207b-c516-ddfc6b9f3345@redhat.com> <4d325e3e-3139-eded-6781-435fb04fb915@gmail.com> <9dc586f4-38f0-7956-0ab6-bd7921491606@redhat.com> <5fb32353b1964299809fce0c7579a092@AcuMS.aculab.com> From: Topi Miettinen Message-ID: Date: Sun, 1 Nov 2020 13:41:30 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.4.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 5.10.2020 15.18, David Hildenbrand wrote: > On 05.10.20 13:21, David Laight wrote: >> From: David Hildenbrand >>> Sent: 05 October 2020 10:55 >> ... >>>> If hardening and compatibility are seen as tradeoffs, perhaps there >>>> could be a top level config choice (CONFIG_HARDENING_TRADEOFF) for this. >>>> It would have options >>>> - "compatibility" (default) to gear questions for maximum compatibility, >>>> deselecting any hardening options which reduce compatibility >>>> - "hardening" to gear questions for maximum hardening, deselecting any >>>> compatibility options which reduce hardening >>>> - "none/manual": ask all questions like before >>> >>> I think the general direction is to avoid an exploding set of config >>> options. So if there isn't a *real* demand, I guess gluing this to a >>> single option ("CONFIG_SECURITY_HARDENING") might be good enough. >> >> Wouldn't that be better achieved by run-time clobbering >> of the syscall vectors? > > You mean via something like a boot parameter? Possibly yes. > This may be obvious, but a global seccomp filter which doesn't affect NNP can be installed in initrd with a simple program with no changes to kernel: #include #include #include #include #include int main(int argc, char **argv) { if (argc < 3) { fprintf(stderr, "Usage: %s syscall [syscall]... program\n", argv[0]); return EXIT_FAILURE; } scmp_filter_ctx ctx = seccomp_init(SCMP_ACT_ALLOW); if (ctx == NULL) { fprintf(stderr, "failed to init filter\n"); return EXIT_FAILURE; } int r; r = seccomp_attr_set(ctx, SCMP_FLTATR_CTL_NNP, 0); if (r != 0) { fprintf(stderr, "failed to disable NNP\n"); return EXIT_FAILURE; } fprintf(stderr, "filtering"); for (int i = 1; i < argc - 1; i++) { const char *syscall = argv[i]; int syscall_nr = seccomp_syscall_resolve_name(syscall); if (syscall_nr == __NR_SCMP_ERROR) { //fprintf(stderr, "unknown syscall %s, ignoring\n", syscall); continue; } r = seccomp_rule_add_exact(ctx, SCMP_ACT_ERRNO(ENOSYS), syscall_nr, 0); if (r != 0) { //fprintf(stderr, "failed to filter syscall %s, ignoring\n", syscall); continue; } fprintf(stderr, " %s", syscall); } fprintf(stderr, "\n"); r = seccomp_load(ctx); if (r != 0) { fprintf(stderr, "failed to apply filter\n"); return EXIT_FAILURE; } seccomp_release(ctx); char *program = argv[argc - 1]; char *new_argv[] = { program, NULL }; execv(program, new_argv); fprintf(stderr, "failed to exec %s\n", program); return EXIT_FAILURE; } This can be inserted in initrd to disable some obsolete and old system calls like this: #!/bin/sh exec /usr/local/sbin/seccomp-exec _sysctl afs_syscall bdflush break create_module ftime get_kernel_syms getpmsg gtty idle lock mpx prof profil putpmsg query_module security sgetmask ssetmask stty sysfs tuxcall ulimit uselib ustat vserver epoll_ctl_old epoll_wait_old old_adjtimex old_getpagesize oldfstat oldlstat oldolduname oldstat oldumount olduname osf_old_creat osf_old_fstat osf_old_getpgrp osf_old_killpg osf_old_lstat osf_old_open osf_old_sigaction osf_old_sigblock osf_old_sigreturn osf_old_sigsetmask osf_old_sigvec osf_old_stat osf_old_vadvise osf_old_vtrace osf_old_wait osf_oldquota vm86old brk /init -Topi