63659fe74e
lo_setattr() invokes fchmod() in a rarely used code path, so it should
be whitelisted or virtiofsd will crash with EBADSYS.
Said code path can be triggered for example as follows:
On the host, in the shared directory, create a file with the sticky bit
set and a security.capability xattr:
(1) # touch foo
(2) # chmod u+s foo
(3) # setcap '' foo
Then in the guest let some process truncate that file after it has
dropped all of its capabilities (at least CAP_FSETID):
int main(int argc, char *argv[])
{
capng_setpid(getpid());
capng_clear(CAPNG_SELECT_BOTH);
capng_updatev(CAPNG_ADD, CAPNG_PERMITTED | CAPNG_EFFECTIVE, 0);
capng_apply(CAPNG_SELECT_BOTH);
ftruncate(open(argv[1], O_RDWR), 0);
}
This will cause the guest kernel to drop the sticky bit (i.e. perform a
mode change) as part of the truncate (where FATTR_FH is set), and that
will cause virtiofsd to invoke fchmod() instead of fchmodat().
(A similar configuration exists further below with futimens() vs.
utimensat(), but the former is not a syscall but just a wrapper for the
latter, so no further whitelisting is required.)
Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1842667
Reported-by: Qian Cai <caiqian@redhat.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20200608093111.14942-1-mreitz@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
173 lines
4.1 KiB
C
173 lines
4.1 KiB
C
/*
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* Seccomp sandboxing for virtiofsd
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*
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* Copyright (C) 2019 Red Hat, Inc.
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*
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* SPDX-License-Identifier: GPL-2.0-or-later
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*/
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#include "qemu/osdep.h"
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#include "seccomp.h"
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#include "fuse_i.h"
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#include "fuse_log.h"
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#include <errno.h>
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#include <glib.h>
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#include <seccomp.h>
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#include <stdlib.h>
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/* Bodge for libseccomp 2.4.2 which broke ppoll */
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#if !defined(__SNR_ppoll) && defined(__SNR_brk)
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#ifdef __NR_ppoll
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#define __SNR_ppoll __NR_ppoll
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#else
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#define __SNR_ppoll __PNR_ppoll
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#endif
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#endif
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static const int syscall_whitelist[] = {
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/* TODO ireg sem*() syscalls */
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SCMP_SYS(brk),
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SCMP_SYS(capget), /* For CAP_FSETID */
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SCMP_SYS(capset),
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SCMP_SYS(clock_gettime),
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SCMP_SYS(clone),
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#ifdef __NR_clone3
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SCMP_SYS(clone3),
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#endif
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SCMP_SYS(close),
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SCMP_SYS(copy_file_range),
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SCMP_SYS(dup),
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SCMP_SYS(eventfd2),
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SCMP_SYS(exit),
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SCMP_SYS(exit_group),
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SCMP_SYS(fallocate),
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SCMP_SYS(fchdir),
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SCMP_SYS(fchmod),
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SCMP_SYS(fchmodat),
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SCMP_SYS(fchownat),
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SCMP_SYS(fcntl),
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SCMP_SYS(fdatasync),
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SCMP_SYS(fgetxattr),
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SCMP_SYS(flistxattr),
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SCMP_SYS(flock),
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SCMP_SYS(fremovexattr),
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SCMP_SYS(fsetxattr),
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SCMP_SYS(fstat),
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SCMP_SYS(fstatfs),
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SCMP_SYS(fsync),
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SCMP_SYS(ftruncate),
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SCMP_SYS(futex),
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SCMP_SYS(getdents),
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SCMP_SYS(getdents64),
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SCMP_SYS(getegid),
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SCMP_SYS(geteuid),
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SCMP_SYS(getpid),
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SCMP_SYS(gettid),
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SCMP_SYS(gettimeofday),
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SCMP_SYS(getxattr),
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SCMP_SYS(linkat),
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SCMP_SYS(listxattr),
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SCMP_SYS(lseek),
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SCMP_SYS(madvise),
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SCMP_SYS(mkdirat),
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SCMP_SYS(mknodat),
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SCMP_SYS(mmap),
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SCMP_SYS(mprotect),
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SCMP_SYS(mremap),
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SCMP_SYS(munmap),
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SCMP_SYS(newfstatat),
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SCMP_SYS(open),
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SCMP_SYS(openat),
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SCMP_SYS(ppoll),
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SCMP_SYS(prctl), /* TODO restrict to just PR_SET_NAME? */
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SCMP_SYS(preadv),
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SCMP_SYS(pread64),
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SCMP_SYS(pwritev),
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SCMP_SYS(pwrite64),
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SCMP_SYS(read),
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SCMP_SYS(readlinkat),
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SCMP_SYS(recvmsg),
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SCMP_SYS(renameat),
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SCMP_SYS(renameat2),
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SCMP_SYS(removexattr),
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SCMP_SYS(rt_sigaction),
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SCMP_SYS(rt_sigprocmask),
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SCMP_SYS(rt_sigreturn),
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SCMP_SYS(sendmsg),
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SCMP_SYS(setresgid),
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SCMP_SYS(setresuid),
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#ifdef __NR_setresgid32
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SCMP_SYS(setresgid32),
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#endif
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#ifdef __NR_setresuid32
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SCMP_SYS(setresuid32),
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#endif
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SCMP_SYS(set_robust_list),
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SCMP_SYS(setxattr),
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SCMP_SYS(symlinkat),
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SCMP_SYS(time), /* Rarely needed, except on static builds */
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SCMP_SYS(tgkill),
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SCMP_SYS(unlinkat),
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SCMP_SYS(unshare),
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SCMP_SYS(utimensat),
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SCMP_SYS(write),
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SCMP_SYS(writev),
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};
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/* Syscalls used when --syslog is enabled */
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static const int syscall_whitelist_syslog[] = {
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SCMP_SYS(sendto),
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};
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static void add_whitelist(scmp_filter_ctx ctx, const int syscalls[], size_t len)
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{
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size_t i;
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for (i = 0; i < len; i++) {
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if (seccomp_rule_add(ctx, SCMP_ACT_ALLOW, syscalls[i], 0) != 0) {
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fuse_log(FUSE_LOG_ERR, "seccomp_rule_add syscall %d failed\n",
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syscalls[i]);
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exit(1);
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}
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}
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}
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void setup_seccomp(bool enable_syslog)
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{
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scmp_filter_ctx ctx;
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#ifdef SCMP_ACT_KILL_PROCESS
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ctx = seccomp_init(SCMP_ACT_KILL_PROCESS);
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/* Handle a newer libseccomp but an older kernel */
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if (!ctx && errno == EOPNOTSUPP) {
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ctx = seccomp_init(SCMP_ACT_TRAP);
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}
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#else
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ctx = seccomp_init(SCMP_ACT_TRAP);
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#endif
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if (!ctx) {
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fuse_log(FUSE_LOG_ERR, "seccomp_init() failed\n");
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exit(1);
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}
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add_whitelist(ctx, syscall_whitelist, G_N_ELEMENTS(syscall_whitelist));
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if (enable_syslog) {
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add_whitelist(ctx, syscall_whitelist_syslog,
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G_N_ELEMENTS(syscall_whitelist_syslog));
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}
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/* libvhost-user calls this for post-copy migration, we don't need it */
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if (seccomp_rule_add(ctx, SCMP_ACT_ERRNO(ENOSYS),
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SCMP_SYS(userfaultfd), 0) != 0) {
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fuse_log(FUSE_LOG_ERR, "seccomp_rule_add userfaultfd failed\n");
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exit(1);
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}
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if (seccomp_load(ctx) < 0) {
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fuse_log(FUSE_LOG_ERR, "seccomp_load() failed\n");
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exit(1);
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}
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seccomp_release(ctx);
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}
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