Re: [PATCH] mm,procfs: allow read-only remote mm access under CAP_PERFMON

[Christian Brauner <brauner@kernel.org>]

On Thu, Jan 23, 2025 at 01:43:42PM -0800, Andrii Nakryiko wrote:
> It's very common for various tracing and profiling toolis to need to
> access /proc/PID/maps contents for stack symbolization needs to learn
> which shared libraries are mapped in memory, at which file offset, etc.
> Currently, access to /proc/PID/maps requires CAP_SYS_PTRACE (unless we
> are looking at data for our own process, which is a trivial case not too
> relevant for profilers use cases).
> 
> Unfortunately, CAP_SYS_PTRACE implies way more than just ability to
> discover memory layout of another process: it allows to fully control
> arbitrary other processes. This is problematic from security POV for
> applications that only need read-only /proc/PID/maps (and other similar
> read-only data) access, and in large production settings CAP_SYS_PTRACE
> is frowned upon even for the system-wide profilers.
> 
> On the other hand, it's already possible to access similar kind of
> information (and more) with just CAP_PERFMON capability. E.g., setting
> up PERF_RECORD_MMAP collection through perf_event_open() would give one
> similar information to what /proc/PID/maps provides.
> 
> CAP_PERFMON, together with CAP_BPF, is already a very common combination
> for system-wide profiling and observability application. As such, it's
> reasonable and convenient to be able to access /proc/PID/maps with
> CAP_PERFMON capabilities instead of CAP_SYS_PTRACE.
> 
> For procfs, these permissions are checked through common mm_access()
> helper, and so we augment that with cap_perfmon() check *only* if
> requested mode is PTRACE_MODE_READ. I.e., PTRACE_MODE_ATTACH wouldn't be
> permitted by CAP_PERFMON.
> 
> Besides procfs itself, mm_access() is used by process_madvise() and
> process_vm_{readv,writev}() syscalls. The former one uses
> PTRACE_MODE_READ to avoid leaking ASLR metadata, and as such CAP_PERFMON
> seems like a meaningful allowable capability as well.
> 
> process_vm_{readv,writev} currently assume PTRACE_MODE_ATTACH level of
> permissions (though for readv PTRACE_MODE_READ seems more reasonable,
> but that's outside the scope of this change), and as such won't be
> affected by this patch.
> 
> Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
> ---
>  kernel/fork.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/fork.c b/kernel/fork.c
> index ded49f18cd95..c57cb3ad9931 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -1547,6 +1547,15 @@ struct mm_struct *get_task_mm(struct task_struct *task)
>  }
>  EXPORT_SYMBOL_GPL(get_task_mm);
>  
> +static bool can_access_mm(struct mm_struct *mm, struct task_struct *task, unsigned int mode)
> +{
> +	if (mm == current->mm)
> +		return true;
> +	if ((mode & PTRACE_MODE_READ) && perfmon_capable())
> +		return true;

Just fyi, I suspect that this will trigger new audit denials if the task
doesn't have CAP_SYS_ADMIN or CAP_PERFORM in the initial user namespace
but where it would still have access through ptrace_may_access(). Such
changes have led to complaints before.

I'm not sure how likely that is but it might be noticable. If that's the
case ns_capable_noaudit(&init_user_ns, ...) would help.

> +	return ptrace_may_access(task, mode);
> +}
> +
>  struct mm_struct *mm_access(struct task_struct *task, unsigned int mode)
>  {
>  	struct mm_struct *mm;
> @@ -1559,7 +1568,7 @@ struct mm_struct *mm_access(struct task_struct *task, unsigned int mode)
>  	mm = get_task_mm(task);
>  	if (!mm) {
>  		mm = ERR_PTR(-ESRCH);
> -	} else if (mm != current->mm && !ptrace_may_access(task, mode)) {
> +	} else if (!can_access_mm(mm, task, mode)) {
>  		mmput(mm);
>  		mm = ERR_PTR(-EACCES);
>  	}
> -- 
> 2.43.5
>