Messages in this thread | | | Date | Thu, 26 Jan 2023 02:28:47 +1100 | From | Aleksa Sarai <> | Subject | Re: [PATCH v3 1/2] exec: add PR_HIDE_SELF_EXE prctl |
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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/> [unhandled content-type:application/pgp-signature] | |