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    SubjectRe: [PATCH v3 1/2] exec: add PR_HIDE_SELF_EXE prctl
    On 2023-01-24, Giuseppe Scrivano <gscrivan@redhat.com> wrote:
    > Aleksa Sarai <cyphar@cyphar.com> writes:
    >
    > > On 2023-01-20, Giuseppe Scrivano <gscrivan@redhat.com> wrote:
    > >> This patch adds a new prctl called PR_HIDE_SELF_EXE which allows
    > >> processes to hide their own /proc/*/exe file. When this prctl is
    > >> used, every access to /proc/*/exe for the calling process will
    > >> fail with ENOENT.
    > >>
    > >> This is useful for preventing issues like CVE-2019-5736, where an
    > >> attacker can gain host root access by overwriting the binary
    > >> in OCI runtimes through file-descriptor mishandling in containers.
    > >>
    > >> The current fix for CVE-2019-5736 is to create a read-only copy or
    > >> a bind-mount of the current executable, and then re-exec the current
    > >> process. With the new prctl, the read-only copy or bind-mount copy is
    > >> not needed anymore.
    > >>
    > >> While map_files/ also might contain symlinks to files in host,
    > >> proc_map_files_get_link() permissions checks are already sufficient.
    > >
    > > I suspect this doesn't protect against the execve("/proc/self/exe")
    > > tactic (because it clears the bit on execve), so I'm not sure this is
    > > much safer than PR_SET_DUMPABLE (yeah, it stops root in the source
    > > userns from accessing /proc/$pid/exe but the above attack makes that no
    > > longer that important).
    >
    > it protects against that attack too. It clears the bit _after_ the
    > execve() syscall is done.
    >
    > If you attempt execve("/proc/self/exe") you still get ENOENT:
    >
    > ```
    > #include <stdlib.h>
    > #include <stdio.h>
    > #include <sys/prctl.h>
    > #include <unistd.h>
    >
    > int main(void)
    > {
    > int ret;
    >
    > ret = prctl(65, 1, 0, 0, 0);
    > if (ret != 0)
    > exit(1);
    >
    > execl("/proc/self/exe", "foo", NULL);
    > exit(2);
    > }
    > ```
    >
    > # strace -e prctl,execve ./hide-self-exe
    > execve("./hide-self-exe", ["./hide-self-exe"], 0x7fff975a3690 /* 39 vars */) = 0
    > prctl(0x41 /* PR_??? */, 0x1, 0, 0, 0) = 0
    > execve("/proc/self/exe", ["foo"], 0x7ffcf51868b8 /* 39 vars */) = -1 ENOENT (No such file or directory)
    > +++ exited with 2 +++
    >
    > I've also tried execv'ing with a script that uses "#!/proc/self/exe" and
    > I get the same ENOENT.

    Ah, you're right. As you mentioned, you could still do the attack
    through /proc/self/map_files but that would require you to know where
    the binary will be located (and being non-dumpable blocks container
    processes from doing tricks to get the right path).

    I wonder if we should somehow require (or auto-apply) SUID_DUMP_NONE
    when setting this prctl, since it does currently depend on it to be
    properly secure...

    > > I think the only way to fix this properly is by blocking re-opens of
    > > magic links that have more permissions than they originally did. I just
    > > got back from vacation, but I'm working on fixing up [1] so it's ready
    > > to be an RFC so we can close this hole once and for all.
    >
    > so that relies on the fact opening /proc/self/exe with O_WRONLY fails
    > with ETXTBSY?

    Not quite, it relies on the fact that /proc/self/exe (and any other
    magiclink to /proc/self/exe) does not have a write mode (semantically,
    because of -ETXTBSY) and thus blocks any attempt to open it (or re-open
    it) with a write mode. It also fixes some other possible issues and lets
    you have upgrade masks (a-la capabilities) to file descriptors.

    Ultimately I think having a complete "no really, nobody can touch this"
    knob is also a good idea, and as this is is much simpler we can it in
    much quicker than the magiclink stuff (which I still think is necessary
    in general).

    > > [1]: https://github.com/cyphar/linux/tree/magiclink/open_how-reopen
    > >
    > >>
    > >> Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>
    > >> ---
    > >> v2: https://lkml.org/lkml/2023/1/19/849
    > >>
    > >> Differences from v2:
    > >>
    > >> - fixed the test to check PR_SET_HIDE_SELF_EXE after fork
    > >>
    > >> v1: https://lkml.org/lkml/2023/1/4/334
    > >>
    > >> Differences from v1:
    > >>
    > >> - amended more information in the commit message wrt map_files not
    > >> requiring the same protection.
    > >> - changed the test to verify PR_HIDE_SELF_EXE cannot be unset after
    > >> a fork.
    > >>
    > >> fs/exec.c | 1 +
    > >> fs/proc/base.c | 8 +++++---
    > >> include/linux/sched.h | 5 +++++
    > >> include/uapi/linux/prctl.h | 3 +++
    > >> kernel/sys.c | 9 +++++++++
    > >> tools/include/uapi/linux/prctl.h | 3 +++
    > >> 6 files changed, 26 insertions(+), 3 deletions(-)
    > >>
    > >> diff --git a/fs/exec.c b/fs/exec.c
    > >> index ab913243a367..5a5dd964c3a3 100644
    > >> --- a/fs/exec.c
    > >> +++ b/fs/exec.c
    > >> @@ -1855,6 +1855,7 @@ static int bprm_execve(struct linux_binprm *bprm,
    > >> /* execve succeeded */
    > >> current->fs->in_exec = 0;
    > >> current->in_execve = 0;
    > >> + task_clear_hide_self_exe(current);
    > >> rseq_execve(current);
    > >> acct_update_integrals(current);
    > >> task_numa_free(current, false);
    > >> diff --git a/fs/proc/base.c b/fs/proc/base.c
    > >> index 9e479d7d202b..959968e2da0d 100644
    > >> --- a/fs/proc/base.c
    > >> +++ b/fs/proc/base.c
    > >> @@ -1723,19 +1723,21 @@ static int proc_exe_link(struct dentry *dentry, struct path *exe_path)
    > >> {
    > >> struct task_struct *task;
    > >> struct file *exe_file;
    > >> + long hide_self_exe;
    > >>
    > >> task = get_proc_task(d_inode(dentry));
    > >> if (!task)
    > >> return -ENOENT;
    > >> exe_file = get_task_exe_file(task);
    > >> + hide_self_exe = task_hide_self_exe(task);
    > >> put_task_struct(task);
    > >> - if (exe_file) {
    > >> + if (exe_file && !hide_self_exe) {
    > >> *exe_path = exe_file->f_path;
    > >> path_get(&exe_file->f_path);
    > >> fput(exe_file);
    > >> return 0;
    > >> - } else
    > >> - return -ENOENT;
    > >> + }
    > >> + return -ENOENT;
    > >> }
    > >>
    > >> static const char *proc_pid_get_link(struct dentry *dentry,
    > >> diff --git a/include/linux/sched.h b/include/linux/sched.h
    > >> index 853d08f7562b..8db32d5fc285 100644
    > >> --- a/include/linux/sched.h
    > >> +++ b/include/linux/sched.h
    > >> @@ -1790,6 +1790,7 @@ static __always_inline bool is_percpu_thread(void)
    > >> #define PFA_SPEC_IB_DISABLE 5 /* Indirect branch speculation restricted */
    > >> #define PFA_SPEC_IB_FORCE_DISABLE 6 /* Indirect branch speculation permanently restricted */
    > >> #define PFA_SPEC_SSB_NOEXEC 7 /* Speculative Store Bypass clear on execve() */
    > >> +#define PFA_HIDE_SELF_EXE 8 /* Hide /proc/self/exe for the process */
    > >>
    > >> #define TASK_PFA_TEST(name, func) \
    > >> static inline bool task_##func(struct task_struct *p) \
    > >> @@ -1832,6 +1833,10 @@ TASK_PFA_CLEAR(SPEC_IB_DISABLE, spec_ib_disable)
    > >> TASK_PFA_TEST(SPEC_IB_FORCE_DISABLE, spec_ib_force_disable)
    > >> TASK_PFA_SET(SPEC_IB_FORCE_DISABLE, spec_ib_force_disable)
    > >>
    > >> +TASK_PFA_TEST(HIDE_SELF_EXE, hide_self_exe)
    > >> +TASK_PFA_SET(HIDE_SELF_EXE, hide_self_exe)
    > >> +TASK_PFA_CLEAR(HIDE_SELF_EXE, hide_self_exe)
    > >> +
    > >> static inline void
    > >> current_restore_flags(unsigned long orig_flags, unsigned long flags)
    > >> {
    > >> diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
    > >> index a5e06dcbba13..f12f3df12468 100644
    > >> --- a/include/uapi/linux/prctl.h
    > >> +++ b/include/uapi/linux/prctl.h
    > >> @@ -284,4 +284,7 @@ struct prctl_mm_map {
    > >> #define PR_SET_VMA 0x53564d41
    > >> # define PR_SET_VMA_ANON_NAME 0
    > >>
    > >> +#define PR_SET_HIDE_SELF_EXE 65
    > >> +#define PR_GET_HIDE_SELF_EXE 66
    > >> +
    > >> #endif /* _LINUX_PRCTL_H */
    > >> diff --git a/kernel/sys.c b/kernel/sys.c
    > >> index 5fd54bf0e886..e992f1b72973 100644
    > >> --- a/kernel/sys.c
    > >> +++ b/kernel/sys.c
    > >> @@ -2626,6 +2626,15 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
    > >> case PR_SET_VMA:
    > >> error = prctl_set_vma(arg2, arg3, arg4, arg5);
    > >> break;
    > >> + case PR_SET_HIDE_SELF_EXE:
    > >> + if (arg2 != 1 || arg3 || arg4 || arg5)
    > >> + return -EINVAL;
    > >> + task_set_hide_self_exe(current);
    > >> + break;
    > >> + case PR_GET_HIDE_SELF_EXE:
    > >> + if (arg2 || arg3 || arg4 || arg5)
    > >> + return -EINVAL;
    > >> + return task_hide_self_exe(current) ? 1 : 0;
    > >> default:
    > >> error = -EINVAL;
    > >> break;
    > >> diff --git a/tools/include/uapi/linux/prctl.h b/tools/include/uapi/linux/prctl.h
    > >> index a5e06dcbba13..f12f3df12468 100644
    > >> --- a/tools/include/uapi/linux/prctl.h
    > >> +++ b/tools/include/uapi/linux/prctl.h
    > >> @@ -284,4 +284,7 @@ struct prctl_mm_map {
    > >> #define PR_SET_VMA 0x53564d41
    > >> # define PR_SET_VMA_ANON_NAME 0
    > >>
    > >> +#define PR_SET_HIDE_SELF_EXE 65
    > >> +#define PR_GET_HIDE_SELF_EXE 66
    > >> +
    > >> #endif /* _LINUX_PRCTL_H */
    > >> --
    > >> 2.38.1
    > >>
    >

    --
    Aleksa Sarai
    Senior Software Engineer (Containers)
    SUSE Linux GmbH
    <https://www.cyphar.com/>
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