From: Suren Baghdasaryan surenb@google.com Subject: mm/madvise: replace ptrace attach requirement for process_madvise

process_madvise currently requires ptrace attach capability. PTRACE_MODE_ATTACH gives one process complete control over another process. It effectively removes the security boundary between the two processes (in one direction). Granting ptrace attach capability even to a system process is considered dangerous since it creates an attack surface. This severely limits the usage of this API.

The operations process_madvise can perform do not affect the correctness of the operation of the target process; they only affect where the data is physically located (and therefore, how fast it can be accessed). What we want is the ability for one process to influence another process in order to optimize performance across the entire system while leaving the security boundary intact.

Replace PTRACE_MODE_ATTACH with a combination of PTRACE_MODE_READ and CAP_SYS_NICE. PTRACE_MODE_READ to prevent leaking ASLR metadata and CAP_SYS_NICE for influencing process performance.

Link: https://lkml.kernel.org/r/20210303185807.2160264-1-surenb@google.com Signed-off-by: Suren Baghdasaryan surenb@google.com Reviewed-by: Kees Cook keescook@chromium.org Acked-by: Minchan Kim minchan@kernel.org Acked-by: David Rientjes rientjes@google.com Cc: Jann Horn jannh@google.com Cc: Jeff Vander Stoep jeffv@google.com Cc: Michal Hocko mhocko@suse.com Cc: Shakeel Butt shakeelb@google.com Cc: Tim Murray timmurray@google.com Cc: Florian Weimer fweimer@redhat.com Cc: Oleg Nesterov oleg@redhat.com Cc: James Morris jmorris@namei.org Cc: stable@vger.kernel.org [5.10+] Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/madvise.c | 13 ++++++++++++- 1 file changed, 12 insertions(+), 1 deletion(-)

--- a/mm/madvise.c~mm-madvise-replace-ptrace-attach-requirement-for-process_madvise +++ a/mm/madvise.c @@ -1198,12 +1198,22 @@ SYSCALL_DEFINE5(process_madvise, int, pi goto release_task; }

- mm = mm_access(task, PTRACE_MODE_ATTACH_FSCREDS); + /* Require PTRACE_MODE_READ to avoid leaking ASLR metadata. */ + mm = mm_access(task, PTRACE_MODE_READ_FSCREDS); if (IS_ERR_OR_NULL(mm)) { ret = IS_ERR(mm) ? PTR_ERR(mm) : -ESRCH; goto release_task; }

+ /* + * Require CAP_SYS_NICE for influencing process performance. Note that + * only non-destructive hints are currently supported. + */ + if (!capable(CAP_SYS_NICE)) { + ret = -EPERM; + goto release_mm; + } + total_len = iov_iter_count(&iter);

while (iov_iter_count(&iter)) { @@ -1218,6 +1228,7 @@ SYSCALL_DEFINE5(process_madvise, int, pi if (ret == 0) ret = total_len - iov_iter_count(&iter);

+release_mm: mmput(mm); release_task: put_task_struct(task); _