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.
Cc: stable@vger.kernel.org # 5.10+ 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 --- changes in v3 - Added Reviewed-by: Kees Cook keescook@chromium.org - Created man page for process_madvise per Andrew's request: https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/commit/?id=a144f... - cc'ed stable@vger.kernel.org # 5.10+ per Andrew's request - cc'ed linux-security-module@vger.kernel.org per James Morris's request
mm/madvise.c | 13 ++++++++++++- 1 file changed, 12 insertions(+), 1 deletion(-)
diff --git a/mm/madvise.c b/mm/madvise.c index df692d2e35d4..01fef79ac761 100644 --- a/mm/madvise.c +++ b/mm/madvise.c @@ -1198,12 +1198,22 @@ SYSCALL_DEFINE5(process_madvise, int, pidfd, const struct iovec __user *, vec, 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, pidfd, const struct iovec __user *, vec, if (ret == 0) ret = total_len - iov_iter_count(&iter);
+release_mm: mmput(mm); release_task: put_task_struct(task);