Re: [PATCH v2 1/1] mm/madvise: replace ptrace attach requirement for process_madvise
From: Michal Hocko <mhocko@suse.com>
Date: 2021-01-12 07:47:21
Also in:
linux-mm, lkml, selinux
On Mon 11-01-21 09:06:22, Suren Baghdasaryan wrote:
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).
Yes it doesn't influence the correctness but it is still a very sensitive operation because it can allow a targeted side channel timing attacks so we should be really careful.
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.
I have to say that ptrace modes are rather obscure to me. So I cannot really judge whether MODE_READ is sufficient. My understanding has always been that this is requred to RO access to the address space. But this operation clearly has a visible side effect. Do we have any actual documentation for the existing modes? I would be really curious to hear from Jann and Oleg (now Cced). Is CAP_SYS_NICE requirement really necessary?
quoted hunk ↗ jump to hunk
Signed-off-by: Suren Baghdasaryan <surenb@google.com> Acked-by: Minchan Kim <minchan@kernel.org> Acked-by: David Rientjes <rientjes@google.com> --- mm/madvise.c | 13 ++++++++++++- 1 file changed, 12 insertions(+), 1 deletion(-)diff --git a/mm/madvise.c b/mm/madvise.c index 6a660858784b..a9bcd16b5d95 100644 --- a/mm/madvise.c +++ b/mm/madvise.c@@ -1197,12 +1197,22 @@ 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)) {@@ -1217,6 +1227,7 @@ 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);-- 2.30.0.284.gd98b1dd5eaa7-goog
-- Michal Hocko SUSE Labs