linux-hardening - [PATCH v2] seq_file: Unconditionally use vmalloc for buffer

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Date:   Mon, 15 Mar 2021 10:48:51 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Kees Cook <keescook@...omium.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Michal Hocko <mhocko@...e.com>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Lee Duncan <lduncan@...e.com>, Chris Leech <cleech@...hat.com>,
        Adam Nichols <adam@...mm-co.com>, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: [PATCH v2] seq_file: Unconditionally use vmalloc for buffer

The sysfs interface to seq_file continues to be rather fragile, as seen
with some recent exploits[1]. Move the seq_file buffer to the vmap area
(while retaining the accounting flag), since it has guard pages that
will catch and stop linear overflows. This seems justified given that
seq_file already uses kvmalloc(), is almost always using a PAGE_SIZE or
larger allocation, has allocations are normally short lived, and is not
normally on a performance critical path.

[1] https://blog.grimm-co.com/2021/03/new-old-bugs-in-linux-kernel.html

Signed-off-by: Kees Cook <keescook@...omium.org>
---
 fs/seq_file.c | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/fs/seq_file.c b/fs/seq_file.c
index cb11a34fb871..16fb4a4e61e3 100644
--- a/fs/seq_file.c
+++ b/fs/seq_file.c
@@ -32,7 +32,12 @@ static void seq_set_overflow(struct seq_file *m)
 
 static void *seq_buf_alloc(unsigned long size)
 {
-	return kvmalloc(size, GFP_KERNEL_ACCOUNT);
+	/*
+	 * To be proactively defensive against buggy seq_get_buf() callers
+	 * (i.e. sysfs handlers), use the vmap area to gain the trailing
+	 * guard page which will protect against linear buffer overflows.
+	 */
+	return __vmalloc(size, GFP_KERNEL_ACCOUNT);
 }
 
 /**
@@ -130,7 +135,7 @@ static int traverse(struct seq_file *m, loff_t offset)
 
 Eoverflow:
 	m->op->stop(m, p);
-	kvfree(m->buf);
+	vfree(m->buf);
 	m->count = 0;
 	m->buf = seq_buf_alloc(m->size <<= 1);
 	return !m->buf ? -ENOMEM : -EAGAIN;
@@ -237,7 +242,7 @@ ssize_t seq_read_iter(struct kiocb *iocb, struct iov_iter *iter)
 			goto Fill;
 		// need a bigger buffer
 		m->op->stop(m, p);
-		kvfree(m->buf);
+		vfree(m->buf);
 		m->count = 0;
 		m->buf = seq_buf_alloc(m->size <<= 1);
 		if (!m->buf)
@@ -349,7 +354,7 @@ EXPORT_SYMBOL(seq_lseek);
 int seq_release(struct inode *inode, struct file *file)
 {
 	struct seq_file *m = file->private_data;
-	kvfree(m->buf);
+	vfree(m->buf);
 	kmem_cache_free(seq_file_cache, m);
 	return 0;
 }
@@ -585,7 +590,7 @@ int single_open_size(struct file *file, int (*show)(struct seq_file *, void *),
 		return -ENOMEM;
 	ret = single_open(file, show, data);
 	if (ret) {
-		kvfree(buf);
+		vfree(buf);
 		return ret;
 	}
 	((struct seq_file *)file->private_data)->buf = buf;
-- 
2.25.1