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@google.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

On Fri, Jan 08, 2021 at 12:58:57PM -0800, 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).
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@google.com>

It sounds logical to me.
If security folks don't see any concern and fix below,

Acked-by: Minchan Kim <minchan@kernel.org>

mmput?

+	}
+
 	total_len = iov_iter_count(&iter);
 
 	while (iov_iter_count(&iter)) {
-- 
2.30.0.284.gd98b1dd5eaa7-goog

On Fri, Jan 8, 2021 at 2:15 PM Minchan Kim [off-list ref] wrote:

On Fri, Jan 08, 2021 at 12:58:57PM -0800, Suren Baghdasaryan wrote:

quoted

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@google.com>

It sounds logical to me.
If security folks don't see any concern and fix below,

Acked-by: Minchan Kim <minchan@kernel.org>

quoted

@@ -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.

6 more quoted lines

Ouch! Thanks for pointing it out! Will include in the next respin.

quoted

+     }
+
      total_len = iov_iter_count(&iter);

      while (iov_iter_count(&iter)) {
--
2.30.0.284.gd98b1dd5eaa7-goog

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On Fri, 8 Jan 2021, Suren Baghdasaryan wrote:

quoted

quoted

@@ -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;

mmput?

Ouch! Thanks for pointing it out! Will include in the next respin.

With the fix, feel free to add:

	Acked-by: David Rientjes [off-list ref]

Thanks Suren!

On Fri, Jan 8, 2021 at 5:02 PM David Rientjes [off-list ref] wrote:

On Fri, 8 Jan 2021, Suren Baghdasaryan wrote:

quoted

quoted

quoted

@@ -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;

mmput?

Ouch! Thanks for pointing it out! Will include in the next respin.

With the fix, feel free to add:

        Acked-by: David Rientjes [off-list ref]

Thanks! Will post a new version with the fix on Monday.

Thanks Suren!