security: filesystem capabilities bugfix2

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Bugfix for strace, and CAP_SETPCAP, in the case that filesystem capabilities are supported.
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From f4419c78fff77c4fa3cdfa6b0a78edae92ddf467 Mon Sep 17 00:00:00 2001
From: Andrew G. Morgan <[email protected]>
Date: Wed, 25 Jun 2008 23:24:10 -0700
Subject: [PATCH] Blunt CAP_SETPCAP on strace with filesystem capability support

The filesystem capability support meaning for CAP_SETPCAP is less powerful than the non-filesystem capability support. As such, when filesystem capabilities are configured, we should not permit CAP_SETPCAP to 'enhance' the current process through strace manipulation of a child process.

Signed-off-by Andrew G. Morgan <[email protected]>

---
 security/commoncap.c |   13 ++++++++++---
 1 files changed, 10 insertions(+), 3 deletions(-)

security/commoncap.c +10 -3

index 5edabc7..a9ea921 100644
@@ -103,10 +103,16 @@ static inline int cap_inh_is_capped(void)
 	return (cap_capable(current, CAP_SETPCAP) != 0);
 }
 
+static inline int cap_limit_straced_target(void) { return 1; }
+
 #else /* ie., ndef CONFIG_SECURITY_FILE_CAPABILITIES */
 
 static inline int cap_block_setpcap(struct task_struct *t) { return 0; }
 static inline int cap_inh_is_capped(void) { return 1; }
+static inline int cap_limit_straced_target(void)
+{
+	return !capable(CAP_SETPCAP);
+}
 
 #endif /* def CONFIG_SECURITY_FILE_CAPABILITIES */
 

@@ -342,9 +348,10 @@ void cap_bprm_apply_creds (struct linux_binprm *bprm, int unsafe)
 				bprm->e_uid = current->uid;
 				bprm->e_gid = current->gid;
 			}
-			if (!capable (CAP_SETPCAP)) {
-				new_permitted = cap_intersect (new_permitted,
-							current->cap_permitted);
+			if (cap_limit_straced_target()) {
+				new_permitted =
+					cap_intersect(new_permitted,
+						      current->cap_permitted);
 			}
 		}
 	}

Signature · 1 line

1.5.3.7

If I understand this right, then LSM_UNSAFE_PTRACE_CAP will only be set if the tracer didn't have CAP_SYS_PTRACE. So this seems sane to me.

Quoted 2 lines · Serge E. Hallyn

If I understand this right, then LSM_UNSAFE_PTRACE_CAP will only be set if the tracer didn't have CAP_SYS_PTRACE. So this seems sane to me.

Erm... Firstly:

	int ptrace_attach(struct task_struct *task)
	{
	...
		if (capable(CAP_SYS_PTRACE))
			task->ptrace |= PT_PTRACE_CAP;
	...
	}

Then:

	static int unsafe_exec(struct task_struct *p)
	{
		int unsafe = 0;
		if (p->ptrace & PT_PTRACED) {
			if (p->ptrace & PT_PTRACE_CAP)
				unsafe |= LSM_UNSAFE_PTRACE_CAP;
			else
				unsafe |= LSM_UNSAFE_PTRACE;
		}
		if (atomic_read(&p->fs->count) > 1 ||
		    atomic_read(&p->files->count) > 1 ||
		    atomic_read(&p->sighand->count) > 1)
			unsafe |= LSM_UNSAFE_SHARE;

		return unsafe;
	}

So LSM_UNSAFE_PTRACE_CAP will only be set if the tracer _does_ have CAP_SYS_PTRACE. That will be irrelevant, however, if any of fs, files or sighand are shared.

And finally:

	void cap_bprm_apply_creds (struct linux_binprm *bprm, int unsafe)
	{
	...
		if (bprm->e_uid != current->uid ||
		    bprm->e_gid != current->gid ||
		    !cap_issubset (new_permitted, current->cap_permitted)) {
	...
			if (unsafe & ~LSM_UNSAFE_PTRACE_CAP) {
				if (!capable(CAP_SETUID)) {
					bprm->e_uid = current->uid;
					bprm->e_gid = current->gid;
				}
				if (!capable (CAP_SETPCAP)) {
					new_permitted = cap_intersect(
						new_permitted,
						current->cap_permitted);
				}
			}
	...
	}

So if it's a 'set-privilege' binary, then if the tracer _doesn't_ have CAP_SYS_PTRACE, we look at downgrading the privileges of the process.

Without Andrew's patch, we only downgrade the capabilities if we don't have CAP_SETPCAP (and aren't sharing inheritables).

With Andrew's patch, capabilities are downgraded regardless of whether we have CAP_SETPCAP or not. I guess that means that if you're tracing a binary whose filecaps say that it wants CAP_SETPCAP, then it retains CAP_SETPCAP.

I wonder if the tracing task should be examined here, and any capability the tracer isn't permitted should be denied the process doing the exec.

Anyway, in my commoncap.c prettification patch, I've dressed the limiter function up as follows:

	/*
	 * Determine whether a exec'ing process's new permitted capabilities
	 * should be limited to just what it already has.
	 *
	 * This prevents processes that are being ptraced from gaining access
	 * to CAP_SETPCAP, unless the process they're tracing already has it,
	 * and the binary they're executing has filecaps that elevate it.
	 *
	 *  Returns 1 if they should be limited, 0 if they are not.
	 */
	static inline int cap_limit_ptraced_target(void)
	{
	#ifndef CONFIG_SECURITY_FILE_CAPABILITIES
		if (capable(CAP_SETPCAP))
			return 0;
	#endif
		return 1;
	}

David

Quoted 4 lines

With Andrew's patch, capabilities are downgraded regardless of whether we have CAP_SETPCAP or not. I guess that means that if you're tracing a binary whose filecaps say that it wants CAP_SETPCAP, then it retains CAP_SETPCAP.

I don't understand where that last sentence comes from. Why would it retain CAP_SETPCAP?

Quoted 2 lines

I wonder if the tracing task should be examined here, and any capability the tracer isn't permitted should be denied the process doing the exec.

That sounds reasonable on its own, but it opens up a dangerous ability for the partially-privileged tracer to manipulate the capability set for the traced task.

Quoted 7 lines · Serge E. Hallyn

Quoted 4 lines

With Andrew's patch, capabilities are downgraded regardless of whether we have CAP_SETPCAP or not. I guess that means that if you're tracing a binary whose filecaps say that it wants CAP_SETPCAP, then it retains CAP_SETPCAP.

I don't understand where that last sentence comes from. Why would it retain CAP_SETPCAP?

It seems I missed a bit out. It should've read:

I guess that means that if you're tracing a binary that has CAP_SETPCAP already, and whose filecaps say that it wants CAP_SETPCAP, then it retains CAP_SETPCAP.

If the debugger has CAP_SYS_PTRACE, then it can attach to a binary that has CAP_SETPCAP according to cap_ptrace(), even if the debugger doesn't.

Quoted 6 lines

Quoted 2 lines

I wonder if the tracing task should be examined here, and any capability the tracer isn't permitted should be denied the process doing the exec.

That sounds reasonable on its own, but it opens up a dangerous ability for the partially-privileged tracer to manipulate the capability set for the traced task.

Does it, though? It would only reduce the capabilities of the inferior process; it wouldn't allow the inferior process or the debugger to get additional capabilities, apart from what's available under CAP_SETPCAP.

+static inline int cap_limit_straced_target(void) { return 1; }

Within the kernel, ptraced is a more appropriate name.

Reported-by David Howells <[email protected]>

Signed-off-by Andrew G. Morgan <[email protected]>