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From:   Josef Bacik <josef@toxicpanda.com>
To:     hch@lst.de, viro@ZenIV.linux.org.uk, linux-kernel@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, kernel-team@fb.com,
        willy@infradead.org
Subject: [PATCH][v2] proc: use vmalloc for our kernel buffer
Date:   Thu, 13 Aug 2020 11:33:56 -0400
Message-Id: <20200813153356.857625-1-josef@toxicpanda.com>
In-Reply-To: <20200813145305.805730-1-josef@toxicpanda.com>
References: <20200813145305.805730-1-josef@toxicpanda.com>
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Since

  sysctl: pass kernel pointers to ->proc_handler

we have been pre-allocating a buffer to copy the data from the proc
handlers into, and then copying that to userspace.  The problem is this
just blind kmalloc()'s the buffer size passed in from the read, which in
the case of our 'cat' binary was 64kib.  Order-4 allocations are not
awesome, and since we can potentially allocate up to our maximum order,
use vmalloc for these buffers.

Fixes: 32927393dc1c ("sysctl: pass kernel pointers to ->proc_handler")
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
v1->v2:
- Make vmemdup_user_nul actually do the right thing...sorry about that.

 fs/proc/proc_sysctl.c  |  6 +++---
 include/linux/string.h |  1 +
 mm/util.c              | 27 +++++++++++++++++++++++++++
 3 files changed, 31 insertions(+), 3 deletions(-)

diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
index 6c1166ccdaea..207ac6e6e028 100644
--- a/fs/proc/proc_sysctl.c
+++ b/fs/proc/proc_sysctl.c
@@ -571,13 +571,13 @@ static ssize_t proc_sys_call_handler(struct file *filp, void __user *ubuf,
 		goto out;
 
 	if (write) {
-		kbuf = memdup_user_nul(ubuf, count);
+		kbuf = vmemdup_user_nul(ubuf, count);
 		if (IS_ERR(kbuf)) {
 			error = PTR_ERR(kbuf);
 			goto out;
 		}
 	} else {
-		kbuf = kzalloc(count, GFP_KERNEL);
+		kbuf = kvzalloc(count, GFP_KERNEL);
 		if (!kbuf)
 			goto out;
 	}
@@ -600,7 +600,7 @@ static ssize_t proc_sys_call_handler(struct file *filp, void __user *ubuf,
 
 	error = count;
 out_free_buf:
-	kfree(kbuf);
+	kvfree(kbuf);
 out:
 	sysctl_head_finish(head);
 
diff --git a/include/linux/string.h b/include/linux/string.h
index 9b7a0632e87a..aee3689fb865 100644
--- a/include/linux/string.h
+++ b/include/linux/string.h
@@ -12,6 +12,7 @@
 extern char *strndup_user(const char __user *, long);
 extern void *memdup_user(const void __user *, size_t);
 extern void *vmemdup_user(const void __user *, size_t);
+extern void *vmemdup_user_nul(const void __user *, size_t);
 extern void *memdup_user_nul(const void __user *, size_t);
 
 /*
diff --git a/mm/util.c b/mm/util.c
index 5ef378a2a038..9d0ad7aafc27 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -208,6 +208,33 @@ void *vmemdup_user(const void __user *src, size_t len)
 }
 EXPORT_SYMBOL(vmemdup_user);
 
+/**
+ * vmemdup_user_nul - duplicate memory region from user space and NUL-terminate
+ *
+ * @src: source address in user space
+ * @len: number of bytes to copy
+ *
+ * Return: an ERR_PTR() on failure.  Result may be not
+ * physically contiguous.  Use kvfree() to free.
+ */
+void *vmemdup_user_nul(const void __user *src, size_t len)
+{
+	char *p;
+
+	p = kvmalloc(len + 1, GFP_USER);
+	if (!p)
+		return ERR_PTR(-ENOMEM);
+
+	if (copy_from_user(p, src, len)) {
+		kvfree(p);
+		return ERR_PTR(-EFAULT);
+	}
+	p[len] = '\0';
+
+	return p;
+}
+EXPORT_SYMBOL(vmemdup_user_nul);
+
 /**
  * strndup_user - duplicate an existing string from user space
  * @s: The string to duplicate
-- 
2.24.1