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Subject[PATCH v2 9/9] procfs: improve permission checks on /proc/*/syscall
Date
Permission checks need to happen during each system call. Therefore we
need to convert the /proc/*/syscall entry from an INF node to a REG
node. Doing this will make /proc/*/syscall have its own file operations
to implement appropriate checks and avoid breaking shared INF file
operations.

Add the syscall_open() to check if the file's opener has enough
permission to ptrace the task during ->open().

Add the syscall_read() to check permissions and to read task syscall
information. If the classic ptrace_may_access() check is passed, then
check if current's cred have changed between ->open() and ->read(), if
so, call proc_allow_access() to check if the original file's opener had
enough permissions to read the task syscall info. This will block
passing the file descriptor to a more privileged process.

For readability split code into another task_syscall_read() function
which is used to get the syscall entries of the task.

Cc: Kees Cook <keescook@chromium.org>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Djalal Harouni <tixxdz@opendz.org>
---
fs/proc/base.c | 93 ++++++++++++++++++++++++++++++++++++++++++++++++++++------
1 file changed, 84 insertions(+), 9 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index b80588a..f98889d 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -150,6 +150,9 @@ struct pid_entry {
NULL, &proc_single_file_operations, \
{ .proc_show = show } )

+/* 4K page size but our output routines use some slack for overruns */
+#define PROC_BLOCK_SIZE (3*1024)
+
/**
* proc_same_open_cred - Check if the file's opener cred matches the
* current cred.
@@ -678,13 +681,32 @@ static int proc_pid_limits(struct task_struct *task, char *buffer)
}

#ifdef CONFIG_HAVE_ARCH_TRACEHOOK
-static int proc_pid_syscall(struct task_struct *task, char *buffer)
+static int syscall_open(struct inode *inode, struct file *filp)
+{
+ int err = -ESRCH;
+ struct task_struct *task = get_proc_task(file_inode(filp));
+
+ if (!task)
+ return err;
+
+ err = mutex_lock_killable(&task->signal->cred_guard_mutex);
+ if (err)
+ goto out;
+
+ if (!ptrace_may_access(task, PTRACE_MODE_ATTACH))
+ err = -EPERM;
+
+ mutex_unlock(&task->signal->cred_guard_mutex);
+out:
+ put_task_struct(task);
+ return err;
+}
+
+static int task_syscall_read(struct task_struct *task, char *buffer)
{
+ int res;
long nr;
unsigned long args[6], sp, pc;
- int res = lock_trace(task);
- if (res)
- return res;

if (task_current_syscall(task, &nr, args, 6, &sp, &pc))
res = sprintf(buffer, "running\n");
@@ -696,9 +718,64 @@ static int proc_pid_syscall(struct task_struct *task, char *buffer)
nr,
args[0], args[1], args[2], args[3], args[4], args[5],
sp, pc);
- unlock_trace(task);
+
return res;
}
+
+static ssize_t syscall_read(struct file *file, char __user *buf,
+ size_t count, loff_t *ppos)
+{
+ int same_cred;
+ ssize_t length;
+ unsigned long page;
+ const struct cred *fcred = file->f_cred;
+ struct inode *inode = file_inode(file);
+ struct task_struct *task = get_proc_task(inode);
+
+ length = -ESRCH;
+ if (!task)
+ return length;
+
+ if (count > PROC_BLOCK_SIZE)
+ count = PROC_BLOCK_SIZE;
+
+ length = -ENOMEM;
+ page = __get_free_page(GFP_TEMPORARY);
+ if (!page)
+ goto out;
+
+ same_cred = proc_same_open_cred(fcred);
+
+ length = lock_trace(task);
+ if (length)
+ goto out_free;
+
+ if (!same_cred &&
+ !proc_allow_access(fcred, task, PTRACE_MODE_ATTACH)) {
+ length = -EPERM;
+ unlock_trace(task);
+ goto out_free;
+ }
+
+ length = task_syscall_read(task, (char *)page);
+ unlock_trace(task);
+
+ if (length >= 0)
+ length = simple_read_from_buffer(buf, count, ppos,
+ (char *)page, length);
+
+out_free:
+ free_page(page);
+out:
+ put_task_struct(task);
+ return length;
+}
+
+static const struct file_operations proc_pid_syscall_operations = {
+ .open = syscall_open,
+ .read = syscall_read,
+ .llseek = generic_file_llseek,
+};
#endif /* CONFIG_HAVE_ARCH_TRACEHOOK */

/************************************************************************/
@@ -789,8 +866,6 @@ static const struct inode_operations proc_def_inode_operations = {
.setattr = proc_setattr,
};

-#define PROC_BLOCK_SIZE (3*1024) /* 4K page size but our output routines use some slack for overruns */
-
static ssize_t proc_info_read(struct file * file, char __user * buf,
size_t count, loff_t *ppos)
{
@@ -2760,7 +2835,7 @@ static const struct pid_entry tgid_base_stuff[] = {
#endif
REG("comm", S_IRUGO|S_IWUSR, proc_pid_set_comm_operations),
#ifdef CONFIG_HAVE_ARCH_TRACEHOOK
- INF("syscall", S_IRUSR, proc_pid_syscall),
+ REG("syscall", S_IRUSR, proc_pid_syscall_operations),
#endif
INF("cmdline", S_IRUGO, proc_pid_cmdline),
ONE("stat", S_IRUGO, proc_tgid_stat),
@@ -3096,7 +3171,7 @@ static const struct pid_entry tid_base_stuff[] = {
#endif
REG("comm", S_IRUGO|S_IWUSR, proc_pid_set_comm_operations),
#ifdef CONFIG_HAVE_ARCH_TRACEHOOK
- INF("syscall", S_IRUSR, proc_pid_syscall),
+ REG("syscall", S_IRUSR, proc_pid_syscall_operations),
#endif
INF("cmdline", S_IRUGO, proc_pid_cmdline),
ONE("stat", S_IRUGO, proc_tid_stat),
--
1.7.11.7


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