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

[Giuseppe Scrivano]

Brian Masney <bmasney@xxxxxxxxxx> 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@xxxxxxxxxx>
>> ---
>>  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