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linux-user, fix: Hide helper threads through saved procfs fds #419

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linux-user, fix: Hide helper threads through saved procfs fds #419

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110 changes: 104 additions & 6 deletions linux-user/syscall.c
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Original file line number Diff line number Diff line change
Expand Up @@ -66,6 +66,7 @@
#include <linux/in6.h>
#include <linux/errqueue.h>
#include <linux/random.h>
#include <linux/magic.h>
#include <linux/sctp.h>
#ifdef CONFIG_TIMERFD
#include <sys/timerfd.h>
Expand Down Expand Up @@ -12641,12 +12642,65 @@ static int is_proc_myself(const char *filename, const char *entry)
return 0;
}
#ifdef CONFIG_LATX
static bool latx_stat_same_object(const struct stat *a, const struct stat *b)
{
return a->st_dev == b->st_dev && a->st_ino == b->st_ino;
}

static bool latx_stat_is_proc_self_task(const struct stat *st)
{
struct stat task_st;

return stat("/proc/self/task", &task_st) == 0 &&
st->st_dev == task_st.st_dev && st->st_ino == task_st.st_ino;
latx_stat_same_object(st, &task_st);
}

static bool latx_stat_is_proc_self_task_fd(int fd, const struct stat *st)
{
struct stat fd_st;
struct stat task_st;
struct statfs fs;

/*
* A saved /proc/self/task fd remains usable after a shared chroot makes
* the absolute procfs path unreachable. Verify both the filesystem and
* the directory's identity: ../../self/task resolves to the current
* process task directory, while another process's task fd does not.
*/
return fd >= 0 && fstatfs(fd, &fs) == 0 &&
fs.f_type == PROC_SUPER_MAGIC && fstat(fd, &fd_st) == 0 &&
latx_stat_same_object(st, &fd_st) &&
fstatat(fd, "../../self/task", &task_st, 0) == 0 &&
latx_stat_same_object(st, &task_st);
}

static bool latx_stat_is_proc_self_task_at(int dirfd, const char *pathname,
int flags,
const struct stat *st)
{
struct stat task_st;
struct statfs fs;

if (latx_stat_is_proc_self_task(st)) {
return true;
}

if (pathname && pathname[0] == '\0') {
return (flags & AT_EMPTY_PATH) &&
latx_stat_is_proc_self_task_fd(dirfd, st);
}

/*
* Chromium keeps an fd for the procfs root before sharing a sandbox
* chroot with CLONE_FS. At that point /proc/self/task is no longer
* reachable by an absolute path, but self/task remains reachable through
* the saved procfd. Verify that the dirfd is procfs before using that
* relative reference so an ordinary self/task directory is not rewritten.
*/
return pathname && pathname[0] != '/' && dirfd >= 0 &&
fstatfs(dirfd, &fs) == 0 && fs.f_type == PROC_SUPER_MAGIC &&
fstatat(dirfd, "self/task", &task_st, 0) == 0 &&
latx_stat_same_object(st, &task_st);
}

static bool latx_statx_is_proc_self_task(const struct target_statx *stx)
Expand All @@ -12659,12 +12713,55 @@ static bool latx_statx_is_proc_self_task(const struct target_statx *stx)
stx->stx_ino == task_st.st_ino;
}

static bool latx_statx_is_proc_self_task_at(int dirfd, const char *pathname,
int flags,
const struct target_statx *stx)
{
struct stat task_st;
struct statfs fs;

if (latx_statx_is_proc_self_task(stx)) {
return true;
}

if (pathname && pathname[0] == '\0') {
return (flags & AT_EMPTY_PATH) && fstat(dirfd, &task_st) == 0 &&
latx_stat_is_proc_self_task_fd(dirfd, &task_st) &&
stx->stx_dev_major == major(task_st.st_dev) &&
stx->stx_dev_minor == minor(task_st.st_dev) &&
stx->stx_ino == task_st.st_ino;
}

return pathname && pathname[0] != '/' && dirfd >= 0 &&
fstatfs(dirfd, &fs) == 0 && fs.f_type == PROC_SUPER_MAGIC &&
fstatat(dirfd, "self/task", &task_st, 0) == 0 &&
stx->stx_dev_major == major(task_st.st_dev) &&
stx->stx_dev_minor == minor(task_st.st_dev) &&
stx->stx_ino == task_st.st_ino;
}

static void latx_adjust_proc_self_task_stat(struct stat *st)
{
if (latx_stat_is_proc_self_task(st)) {
st->st_nlink = latx_guest_thread_count() + 2;
}
}
static void latx_adjust_proc_self_task_fstat(int fd, struct stat *st)
{
if (latx_stat_is_proc_self_task(st) ||
latx_stat_is_proc_self_task_fd(fd, st)) {
st->st_nlink = latx_guest_thread_count() + 2;
}
}

static void latx_adjust_proc_self_task_stat_at(int dirfd, const char *pathname,
int flags,
struct stat *st)
{
if (latx_stat_is_proc_self_task_at(dirfd, pathname, flags, st)) {
st->st_nlink = latx_guest_thread_count() + 2;
}
}
#endif
static int is_proc_other(const char *filename, const char *entry)
{
Expand Down Expand Up @@ -16979,7 +17076,7 @@ static abi_long do_syscall1(void *cpu_env, int num, abi_long arg1,
}
#ifdef CONFIG_LATX
if (!is_error(ret)) {
latx_adjust_proc_self_task_stat(&st);
latx_adjust_proc_self_task_fstat(arg1, &st);
}
#endif
#if defined(TARGET_NR_stat) || defined(TARGET_NR_lstat)
Expand Down Expand Up @@ -18531,7 +18628,7 @@ static abi_long do_syscall1(void *cpu_env, int num, abi_long arg1,
}
#ifdef CONFIG_LATX
if (!is_error(ret)) {
latx_adjust_proc_self_task_stat(&st);
latx_adjust_proc_self_task_fstat(arg1, &st);
}
#endif
if (!is_error(ret))
Expand All @@ -18552,7 +18649,7 @@ static abi_long do_syscall1(void *cpu_env, int num, abi_long arg1,

if (!is_error(ret)) {
#ifdef CONFIG_LATX
latx_adjust_proc_self_task_stat(&st);
latx_adjust_proc_self_task_stat_at(arg1, p, arg4, &st);
#else
if (rcu_call_thread_is_running() &&
(!strcmp((const char *)p, "self/task/") ||
Expand Down Expand Up @@ -18589,7 +18686,8 @@ static abi_long do_syscall1(void *cpu_env, int num, abi_long arg1,
ret = get_errno(sys_statx(dirfd, p, flags, mask, &host_stx));
if (!is_error(ret)) {
#ifdef CONFIG_LATX
if (latx_statx_is_proc_self_task(&host_stx)) {
if (latx_statx_is_proc_self_task_at(dirfd, p, flags,
&host_stx)) {
host_stx.stx_nlink = latx_guest_thread_count() + 2;
}
#endif
Expand All @@ -18609,7 +18707,7 @@ static abi_long do_syscall1(void *cpu_env, int num, abi_long arg1,

#ifdef CONFIG_LATX
if (!is_error(ret)) {
latx_adjust_proc_self_task_stat(&st);
latx_adjust_proc_self_task_stat_at(dirfd, p, flags, &st);
}
#endif
unlock_user(p, arg2, 0);
Expand Down
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