[PATCH] proc: avoid leaking eip, esp, or wchan to non-privileged processes (fwd)

James Morris
2009-05-03T23:29:56Z

This patch needs some review.

Note that stable@kernel.org typically backport already-reviewed and
applied patches. I think security@kernel.org is for reporting problems in
a non-public way (whereas, this is already public knowledge).

Having someone specifically responsible for maintaining kernel memory
protection features would be of immense value. Any takers?

---------- Forwarded message ----------
Date: Thu, 30 Apr 2009 09:11:40 -0600
From: Jake Edge <jake@lwn.net>
To: linux-kernel@vger.kernel.org
Cc: security@kernel.org, stable@kernel.org, linux-fsdevel@vger.kernel.org
Subject: [PATCH] proc: avoid leaking eip, esp,
or wchan to non-privileged processes

proc: avoid leaking eip, esp, or wchan to non-privileged processes

Using the same test as is used for /proc/pid/maps and smaps, only allow
processes that can ptrace() a given process to access its /proc/pid/wchan and
get eip and esp from /proc/pid/stat.

ASLR can be bypassed by sampling eip as shown by the proof-of-concept code
at http://code.google.com/p/fuzzyaslr/ As part of a presentation
(http://www.cr0.org/paper/to-jt-linux-alsr-leak.pdf) esp and wchan were also
noted as possibly usable information leaks as well.

Signed-off-by: Jake Edge <jake@lwn.net>
---
fs/proc/array.c | 11 ++++++++---
fs/proc/base.c | 3 +++
2 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/fs/proc/array.c b/fs/proc/array.c
index 7e4877d..e96d508 100644
--- a/fs/proc/array.c
+++ b/fs/proc/array.c
@@ -80,6 +80,7 @@
#include <linux/delayacct.h>
#include <linux/seq_file.h>
#include <linux/pid_namespace.h>
+#include <linux/ptrace.h>
#include <linux/tracehook.h>

#include <asm/pgtable.h>
@@ -352,6 +353,7 @@ static int do_task_stat(struct seq_file *m, struct
pid_namespace *ns,
char state;
pid_t ppid = 0, pgid = -1, sid = -1;
int num_threads = 0;
+ int permitted;
struct mm_struct *mm;
unsigned long long start_time;
unsigned long cmin_flt = 0, cmaj_flt = 0;
@@ -364,11 +366,14 @@ static int do_task_stat(struct seq_file *m, struct
pid_namespace *ns,

state = *get_task_state(task);
vsize = eip = esp = 0;
+ permitted = ptrace_may_access(task, PTRACE_MODE_READ);
mm = get_task_mm(task);
if (mm) {
vsize = task_vsize(mm);
- eip = KSTK_EIP(task);
- esp = KSTK_ESP(task);
+ if (permitted) {
+ eip = KSTK_EIP(task);
+ esp = KSTK_ESP(task);
+ }
}

get_task_comm(tcomm, task);
@@ -424,7 +429,7 @@ static int do_task_stat(struct seq_file *m, struct
pid_namespace *ns,
unlock_task_sighand(task, &flags);
}

- if (!whole || num_threads < 2)
+ if (permitted && (!whole || num_threads < 2))
wchan = get_wchan(task);
if (!whole) {
min_flt = task->min_flt;
diff --git a/fs/proc/base.c b/fs/proc/base.c
index aa763ab..cd3c0d7 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -319,6 +319,9 @@ static int proc_pid_wchan(struct task_struct *task, char
*buffer)
unsigned long wchan;
char symname[KSYM_NAME_LEN];

+ if (!ptrace_may_access(task, PTRACE_MODE_READ))
+ return 0;
+
wchan = get_wchan(task);

if (lookup_symbol_name(wchan, symname) < 0)
--
1.6.2.2

--
Jake Edge - LWN - jake@lwn.net - http://lwn.net
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