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Date:   Fri,  8 Jan 2021 12:58:57 -0800
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     akpm@...ux-foundation.org
Cc:     jannh@...gle.com, keescook@...omium.org, jeffv@...gle.com,
        minchan@...nel.org, mhocko@...e.com, shakeelb@...gle.com,
        rientjes@...gle.com, edgararriaga@...gle.com, timmurray@...gle.com,
        linux-mm@...ck.org, selinux@...r.kernel.org,
        linux-api@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...roid.com, surenb@...gle.com
Subject: [PATCH 1/1] 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.

Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
---
 mm/madvise.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/mm/madvise.c b/mm/madvise.c
index 6a660858784b..c2d600386902 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_task;
+	}
+
 	total_len = iov_iter_count(&iter);
 
 	while (iov_iter_count(&iter)) {
-- 
2.30.0.284.gd98b1dd5eaa7-goog

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