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Message-ID: <87h6wgcrv6.fsf@redhat.com>
Date: Tue, 24 Jan 2023 08:29:49 +0100
From: Giuseppe Scrivano <gscrivan@...hat.com>
To: Aleksa Sarai <cyphar@...har.com>
Cc: linux-kernel@...r.kernel.org, keescook@...omium.org,
bristot@...hat.com, ebiederm@...ssion.com, brauner@...nel.org,
viro@...iv.linux.org.uk, alexl@...hat.com, peterz@...radead.org,
bmasney@...hat.com
Subject: Re: [PATCH v3 1/2] exec: add PR_HIDE_SELF_EXE prctl
Aleksa Sarai <cyphar@...har.com> writes:
> On 2023-01-20, Giuseppe Scrivano <gscrivan@...hat.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.
>
> 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?
> [1]: https://github.com/cyphar/linux/tree/magiclink/open_how-reopen
>
>>
>> Signed-off-by: Giuseppe Scrivano <gscrivan@...hat.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
>>
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