[PATCH] proc: do not allow negative offsets on /proc/<pid>/environ

[Djalal Harouni]

__mem_open() which is called by both /proc/<pid>/environ and
/proc/<pid>/mem ->open() handlers will allow the use of negative offsets.
/proc/<pid>/mem has negative offsets but not /proc/<pid>/environ.

Allowing negative offsets on /proc/<pid>/environ can turn it to act like
/proc/<pid>/mem. A negative offset will pass the
fs/read_write.c:lseek_execute() and the environ_read() checks and will
point to another VMA.

Fix this by moving the 'force FMODE_UNSIGNED_OFFSET flag' to mem_open()
to allow negative offsets only on /proc/<pid>/mem.

You must be able to ptrace the target to open /proc/<pid>/environ ,so
this is not a security issue, but we should not be able to abuse it.

Signed-off-by: Djalal Harouni <tixxdz@xxxxxxxxxx>
---
New kernels include mm->env_start in /proc/<pid>/stat
To dump .text area: lseek() to 0x00400000 - mm->env_start

 fs/proc/base.c |    9 ++++++---
 1 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 2772208..9623a18 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -695,8 +695,6 @@ static int __mem_open(struct inode *inode, struct file *file, unsigned int mode)
 		mmput(mm);
 	}
 
-	/* OK to pass negative loff_t, we can catch out-of-range */
-	file->f_mode |= FMODE_UNSIGNED_OFFSET;
 	file->private_data = mm;
 
 	return 0;
@@ -704,7 +702,12 @@ static int __mem_open(struct inode *inode, struct file *file, unsigned int mode)
 
 static int mem_open(struct inode *inode, struct file *file)
 {
-	return __mem_open(inode, file, PTRACE_MODE_ATTACH);
+	int ret = __mem_open(inode, file, PTRACE_MODE_ATTACH);
+	if (!ret)
+		/* OK to pass negative loff_t, we can catch out-of-range */
+		file->f_mode |= FMODE_UNSIGNED_OFFSET;
+
+	return ret;
 }
 
 static ssize_t mem_rw(struct file *file, char __user *buf,
-- 
1.7.1

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