Messages in this thread | | | Date | Tue, 24 Jan 2023 12:53:48 +1100 | From | Aleksa Sarai <> | Subject | Re: [PATCH v3 1/2] exec: add PR_HIDE_SELF_EXE prctl |
| |
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).
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.
[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] | |