linux-kernel - Re: [PATCH] proc: fix NULL dereference when reading /proc/<pid>/auxv

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Date:   Wed, 19 Oct 2016 16:13:46 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Leon Yu <chianglungyu@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Kees Cook <keescook@...omium.org>,
        Oleg Nesterov <oleg@...hat.com>,
        John Stultz <john.stultz@...aro.org>,
        Mateusz Guzik <mguzik@...hat.com>,
        Janis Danisevskis <jdanis@...gle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] proc: fix NULL dereference when reading /proc/<pid>/auxv

On Wed 19-10-16 21:59:40, Leon Yu wrote:
[...]
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index c2964d890c9a..598d08936a3c 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -1007,6 +1007,9 @@ static ssize_t auxv_read(struct file *file, char __user *buf,
>  {
>  	struct mm_struct *mm = file->private_data;
>  	unsigned int nwords = 0;
> +
> +	if (IS_ERR_OR_NULL(mm))
> +		return PTR_ERR(mm);
>  	do {
>  		nwords += 2;
>  	} while (mm->saved_auxv[nwords - 2] != 0); /* AT_NULL */

Why would we even want to open that file if there is no mm struct?
Wouldn't something like this be better? ESRCH might be a bit unexpected
for an existing pid but I am not sure what would be a better error code.
---
diff --git a/fs/proc/base.c b/fs/proc/base.c
index 5ef2ae81f623..d34d33dbf1b2 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -804,10 +804,10 @@ static const struct file_operations proc_single_file_operations = {
 struct mm_struct *proc_mem_open(struct inode *inode, unsigned int mode)
 {
 	struct task_struct *task = get_proc_task(inode);
-	struct mm_struct *mm = ERR_PTR(-ESRCH);
+	struct mm_struct *ret = ERR_PTR(-ESRCH);
 
 	if (task) {
-		mm = mm_access(task, mode | PTRACE_MODE_FSCREDS);
+		struct mm_struct *mm = mm_access(task, mode | PTRACE_MODE_FSCREDS);
 		put_task_struct(task);
 
 		if (!IS_ERR_OR_NULL(mm)) {
@@ -815,10 +815,11 @@ struct mm_struct *proc_mem_open(struct inode *inode, unsigned int mode)
 			atomic_inc(&mm->mm_count);
 			/* but do not pin its memory */
 			mmput(mm);
+			ret = mm;
 		}
 	}
 
-	return mm;
+	return ret;
 }
 
 static int __mem_open(struct inode *inode, struct file *file, unsigned int mode)
-- 
Michal Hocko
SUSE Labs