[PATCH v2 1/2] exec: add PR_HIDE_SELF_EXE prctl

[PATCH v2 1/2] exec: add PR_HIDE_SELF_EXE prctl

[Giuseppe Scrivano]

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.

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>
---
 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(-)

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.

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

[PATCH v2 2/2] selftests: add tests for prctl(SET_HIDE_SELF_EXE)

[Giuseppe Scrivano]

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>
---
 tools/testing/selftests/prctl/Makefile        |   2 +-
 tools/testing/selftests/prctl/hide-self-exe.c | 101 ++++++++++++++++++
 2 files changed, 102 insertions(+), 1 deletion(-)
 create mode 100644 tools/testing/selftests/prctl/hide-self-exe.c

diff --git a/tools/testing/selftests/prctl/Makefile b/tools/testing/selftests/prctl/Makefile
index c7923b205222..024e107b26ec 100644
--- a/tools/testing/selftests/prctl/Makefile
+++ b/tools/testing/selftests/prctl/Makefile
@@ -5,7 +5,7 @@ ARCH ?= $(shell echo $(uname_M) | sed -e s/i.86/x86/ -e s/x86_64/x86/)
 
 ifeq ($(ARCH),x86)
 TEST_PROGS := disable-tsc-ctxt-sw-stress-test disable-tsc-on-off-stress-test \
-		disable-tsc-test
+		disable-tsc-test hide-self-exe
 all: $(TEST_PROGS)
 
 include ../lib.mk
diff --git a/tools/testing/selftests/prctl/hide-self-exe.c b/tools/testing/selftests/prctl/hide-self-exe.c
new file mode 100644
index 000000000000..f86cef8e061c
--- /dev/null
+++ b/tools/testing/selftests/prctl/hide-self-exe.c
@@ -0,0 +1,101 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Tests for prctl(PR_GET_HIDE_SELF_EXE, ...) / prctl(PR_SET_HIDE_SELF_EXE, ...)
+ *
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <signal.h>
+#include <inttypes.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <sys/wait.h>
+
+#include <sys/prctl.h>
+#include <linux/prctl.h>
+
+#ifndef PR_SET_HIDE_SELF_EXE
+# define PR_SET_HIDE_SELF_EXE		65
+# define PR_GET_HIDE_SELF_EXE		66
+#endif
+
+int main(void)
+{
+	int status;
+	pid_t pid;
+	int ret;
+
+	ret = open("/proc/self/exe", O_RDONLY);
+	if (ret < 0) {
+		perror("open /proc/self/exe");
+		exit(EXIT_FAILURE);
+	}
+	close(ret);
+
+	ret = prctl(PR_GET_HIDE_SELF_EXE, 0, 0, 0, 0);
+	if (ret != 0) {
+		perror("prctl(PR_GET_HIDE_SELF_EXE)");
+		exit(EXIT_FAILURE);
+	}
+
+	ret = prctl(PR_SET_HIDE_SELF_EXE, 1, 0, 0, 0);
+	if (ret != 0) {
+		perror("prctl(PR_SET_HIDE_SELF_EXE)");
+		exit(EXIT_FAILURE);
+	}
+
+	/* check it doesn't fail a second time.  */
+	ret = prctl(PR_SET_HIDE_SELF_EXE, 1, 0, 0, 0);
+	if (ret != 0) {
+		perror("prctl(PR_SET_HIDE_SELF_EXE)");
+		exit(EXIT_FAILURE);
+	}
+
+	ret = prctl(PR_GET_HIDE_SELF_EXE, 0, 0, 0, 0);
+	if (ret != 1) {
+		perror("prctl(PR_GET_HIDE_SELF_EXE)");
+		exit(EXIT_FAILURE);
+	}
+
+	ret = open("/proc/self/exe", O_RDONLY);
+	if (ret >= 0 || errno != ENOENT) {
+		perror("open /proc/self/exe succeeded");
+		exit(EXIT_FAILURE);
+	}
+
+	pid = fork();
+	if (pid < 0) {
+		perror("fork");
+		exit(EXIT_FAILURE);
+	}
+	if (pid == 0) {
+		/* Verify that it is still unreachable after fork().  */
+		ret = open("/proc/self/exe", O_RDONLY);
+		if (ret >= 0 || errno != ENOENT)
+			exit(EXIT_FAILURE);
+		close(ret);
+
+		/* And that it cannot be unset.  */
+		ret = prctl(PR_GET_HIDE_SELF_EXE, 0, 0, 0, 0);
+		if (ret != 1) {
+			perror("prctl(PR_GET_HIDE_SELF_EXE)");
+			exit(EXIT_FAILURE);
+		}
+
+		/* HIDE_SELF_EXE is cleared after execve.  */
+		ret = system("cat /proc/self/exe > /dev/null");
+		exit(ret == 0 ? EXIT_SUCCESS : EXIT_FAILURE);
+	}
+	if (waitpid(pid, &status, 0) != pid) {
+		perror("waitpid");
+		exit(EXIT_FAILURE);
+	}
+	if (status != 0) {
+		perror("child failed");
+		exit(EXIT_FAILURE);
+	}
+	exit(EXIT_SUCCESS);
+}
+
-- 
2.38.1

Re: [PATCH v2 2/2] selftests: add tests for prctl(SET_HIDE_SELF_EXE)

[Brian Masney]

On Thu, Jan 19, 2023 at 06:07:18PM +0100, Giuseppe Scrivano wrote:
> Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>
> ---
>  tools/testing/selftests/prctl/Makefile        |   2 +-
>  tools/testing/selftests/prctl/hide-self-exe.c | 101 ++++++++++++++++++
>  2 files changed, 102 insertions(+), 1 deletion(-)
>  create mode 100644 tools/testing/selftests/prctl/hide-self-exe.c
> 
> diff --git a/tools/testing/selftests/prctl/Makefile b/tools/testing/selftests/prctl/Makefile
> index c7923b205222..024e107b26ec 100644
> --- a/tools/testing/selftests/prctl/Makefile
> +++ b/tools/testing/selftests/prctl/Makefile
> @@ -5,7 +5,7 @@ ARCH ?= $(shell echo $(uname_M) | sed -e s/i.86/x86/ -e s/x86_64/x86/)
>  
>  ifeq ($(ARCH),x86)
>  TEST_PROGS := disable-tsc-ctxt-sw-stress-test disable-tsc-on-off-stress-test \
> -		disable-tsc-test
> +		disable-tsc-test hide-self-exe
>  all: $(TEST_PROGS)
>  
>  include ../lib.mk
> diff --git a/tools/testing/selftests/prctl/hide-self-exe.c b/tools/testing/selftests/prctl/hide-self-exe.c
> new file mode 100644
> index 000000000000..f86cef8e061c
> --- /dev/null
> +++ b/tools/testing/selftests/prctl/hide-self-exe.c
> @@ -0,0 +1,101 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Tests for prctl(PR_GET_HIDE_SELF_EXE, ...) / prctl(PR_SET_HIDE_SELF_EXE, ...)
> + *
> + */
> +
> +#include <stdio.h>
> +#include <stdlib.h>
> +#include <unistd.h>
> +#include <signal.h>
> +#include <inttypes.h>
> +#include <errno.h>
> +#include <fcntl.h>
> +#include <sys/wait.h>
> +
> +#include <sys/prctl.h>
> +#include <linux/prctl.h>
> +
> +#ifndef PR_SET_HIDE_SELF_EXE
> +# define PR_SET_HIDE_SELF_EXE		65
> +# define PR_GET_HIDE_SELF_EXE		66
> +#endif
> +
> +int main(void)
> +{
> +	int status;
> +	pid_t pid;
> +	int ret;
> +
> +	ret = open("/proc/self/exe", O_RDONLY);
> +	if (ret < 0) {
> +		perror("open /proc/self/exe");
> +		exit(EXIT_FAILURE);
> +	}
> +	close(ret);
> +
> +	ret = prctl(PR_GET_HIDE_SELF_EXE, 0, 0, 0, 0);
> +	if (ret != 0) {
> +		perror("prctl(PR_GET_HIDE_SELF_EXE)");
> +		exit(EXIT_FAILURE);
> +	}
> +
> +	ret = prctl(PR_SET_HIDE_SELF_EXE, 1, 0, 0, 0);
> +	if (ret != 0) {
> +		perror("prctl(PR_SET_HIDE_SELF_EXE)");
> +		exit(EXIT_FAILURE);
> +	}
> +
> +	/* check it doesn't fail a second time.  */
> +	ret = prctl(PR_SET_HIDE_SELF_EXE, 1, 0, 0, 0);
> +	if (ret != 0) {
> +		perror("prctl(PR_SET_HIDE_SELF_EXE)");
> +		exit(EXIT_FAILURE);
> +	}
> +
> +	ret = prctl(PR_GET_HIDE_SELF_EXE, 0, 0, 0, 0);
> +	if (ret != 1) {
> +		perror("prctl(PR_GET_HIDE_SELF_EXE)");
> +		exit(EXIT_FAILURE);
> +	}
> +
> +	ret = open("/proc/self/exe", O_RDONLY);
> +	if (ret >= 0 || errno != ENOENT) {
> +		perror("open /proc/self/exe succeeded");
> +		exit(EXIT_FAILURE);
> +	}
> +
> +	pid = fork();
> +	if (pid < 0) {
> +		perror("fork");
> +		exit(EXIT_FAILURE);
> +	}
> +	if (pid == 0) {
> +		/* Verify that it is still unreachable after fork().  */
> +		ret = open("/proc/self/exe", O_RDONLY);
> +		if (ret >= 0 || errno != ENOENT)
> +			exit(EXIT_FAILURE);
> +		close(ret);
> +
> +		/* And that it cannot be unset.  */
> +		ret = prctl(PR_GET_HIDE_SELF_EXE, 0, 0, 0, 0);

Should this be PR_SET_HIDE_SELF_EXE?

Brian

Re: [PATCH v2 1/2] exec: add PR_HIDE_SELF_EXE prctl

[Brian Masney]

On Thu, Jan 19, 2023 at 06:07:17PM +0100, Giuseppe Scrivano 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.
> 
> Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>
> ---
>  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(-)
> 
> 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.
> 
> 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);

[snip]

>  	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);

Perhaps I am missing something, but where is task_clear_hide_self_exe()
and task_hide_self_exe() defined?

Brian

Re: [PATCH v2 1/2] exec: add PR_HIDE_SELF_EXE prctl

[Giuseppe Scrivano]

Brian Masney <bmasney@redhat.com> writes:

> On Thu, Jan 19, 2023 at 06:07:17PM +0100, Giuseppe Scrivano 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.
>> 
>> Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>
>> ---
>>  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(-)
>> 
>> 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.
>> 
>> 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);
>
> [snip]
>
>>  	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);
>
> Perhaps I am missing something, but where is task_clear_hide_self_exe()
> and task_hide_self_exe() defined?

they are defined with:

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)

Regards,
Giuseppe

Re: [PATCH v2 2/2] selftests: add tests for prctl(SET_HIDE_SELF_EXE)

[Giuseppe Scrivano]

Brian Masney <bmasney@redhat.com> writes:

> On Thu, Jan 19, 2023 at 06:07:18PM +0100, Giuseppe Scrivano wrote:
>> Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>
>> ---
>>  tools/testing/selftests/prctl/Makefile        |   2 +-
>>  tools/testing/selftests/prctl/hide-self-exe.c | 101 ++++++++++++++++++
>>  2 files changed, 102 insertions(+), 1 deletion(-)
>>  create mode 100644 tools/testing/selftests/prctl/hide-self-exe.c
>> 
>> diff --git a/tools/testing/selftests/prctl/Makefile b/tools/testing/selftests/prctl/Makefile
>> index c7923b205222..024e107b26ec 100644
>> --- a/tools/testing/selftests/prctl/Makefile
>> +++ b/tools/testing/selftests/prctl/Makefile
>> @@ -5,7 +5,7 @@ ARCH ?= $(shell echo $(uname_M) | sed -e s/i.86/x86/ -e s/x86_64/x86/)
>>  
>>  ifeq ($(ARCH),x86)
>>  TEST_PROGS := disable-tsc-ctxt-sw-stress-test disable-tsc-on-off-stress-test \
>> -		disable-tsc-test
>> +		disable-tsc-test hide-self-exe
>>  all: $(TEST_PROGS)
>>  
>>  include ../lib.mk
>> diff --git a/tools/testing/selftests/prctl/hide-self-exe.c b/tools/testing/selftests/prctl/hide-self-exe.c
>> new file mode 100644
>> index 000000000000..f86cef8e061c
>> --- /dev/null
>> +++ b/tools/testing/selftests/prctl/hide-self-exe.c
>> @@ -0,0 +1,101 @@
>> +// SPDX-License-Identifier: GPL-2.0
>> +/*
>> + * Tests for prctl(PR_GET_HIDE_SELF_EXE, ...) / prctl(PR_SET_HIDE_SELF_EXE, ...)
>> + *
>> + */
>> +
>> +#include <stdio.h>
>> +#include <stdlib.h>
>> +#include <unistd.h>
>> +#include <signal.h>
>> +#include <inttypes.h>
>> +#include <errno.h>
>> +#include <fcntl.h>
>> +#include <sys/wait.h>
>> +
>> +#include <sys/prctl.h>
>> +#include <linux/prctl.h>
>> +
>> +#ifndef PR_SET_HIDE_SELF_EXE
>> +# define PR_SET_HIDE_SELF_EXE		65
>> +# define PR_GET_HIDE_SELF_EXE		66
>> +#endif
>> +
>> +int main(void)
>> +{
>> +	int status;
>> +	pid_t pid;
>> +	int ret;
>> +
>> +	ret = open("/proc/self/exe", O_RDONLY);
>> +	if (ret < 0) {
>> +		perror("open /proc/self/exe");
>> +		exit(EXIT_FAILURE);
>> +	}
>> +	close(ret);
>> +
>> +	ret = prctl(PR_GET_HIDE_SELF_EXE, 0, 0, 0, 0);
>> +	if (ret != 0) {
>> +		perror("prctl(PR_GET_HIDE_SELF_EXE)");
>> +		exit(EXIT_FAILURE);
>> +	}
>> +
>> +	ret = prctl(PR_SET_HIDE_SELF_EXE, 1, 0, 0, 0);
>> +	if (ret != 0) {
>> +		perror("prctl(PR_SET_HIDE_SELF_EXE)");
>> +		exit(EXIT_FAILURE);
>> +	}
>> +
>> +	/* check it doesn't fail a second time.  */
>> +	ret = prctl(PR_SET_HIDE_SELF_EXE, 1, 0, 0, 0);
>> +	if (ret != 0) {
>> +		perror("prctl(PR_SET_HIDE_SELF_EXE)");
>> +		exit(EXIT_FAILURE);
>> +	}
>> +
>> +	ret = prctl(PR_GET_HIDE_SELF_EXE, 0, 0, 0, 0);
>> +	if (ret != 1) {
>> +		perror("prctl(PR_GET_HIDE_SELF_EXE)");
>> +		exit(EXIT_FAILURE);
>> +	}
>> +
>> +	ret = open("/proc/self/exe", O_RDONLY);
>> +	if (ret >= 0 || errno != ENOENT) {
>> +		perror("open /proc/self/exe succeeded");
>> +		exit(EXIT_FAILURE);
>> +	}
>> +
>> +	pid = fork();
>> +	if (pid < 0) {
>> +		perror("fork");
>> +		exit(EXIT_FAILURE);
>> +	}
>> +	if (pid == 0) {
>> +		/* Verify that it is still unreachable after fork().  */
>> +		ret = open("/proc/self/exe", O_RDONLY);
>> +		if (ret >= 0 || errno != ENOENT)
>> +			exit(EXIT_FAILURE);
>> +		close(ret);
>> +
>> +		/* And that it cannot be unset.  */
>> +		ret = prctl(PR_GET_HIDE_SELF_EXE, 0, 0, 0, 0);
>
> Should this be PR_SET_HIDE_SELF_EXE?

you are right, thanks for spotting it!  I should test
PR_SET_HIDE_SELF_EXE not the getter.