linux-kernel - Re: [PATCH] fs: coda: fix a double-fetch case in coda_psdev

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Date:   Tue, 25 Dec 2018 15:25:20 -0500
From:   Jan Harkes <jaharkes@...cmu.edu>
To:     Kangjie Lu <kjlu@....edu>, kjlu@....edu
CC:     pakki001@....edu, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs: coda: fix a double-fetch case in coda_psdev_write

This has come up before, and as you noticed nothing in that header is looked at after that second copy.

Also if the user space cache manager was trying to be malicious it wouldn't have to resort to multi-threaded race conditions. It could make a setuid root shell appear, or many other things a file system can do.

Jan

On December 25, 2018 2:03:51 PM EST, Kangjie Lu <kjlu@....edu> wrote:
>"hdr" has been copied in from user space and "hdr.opcode" is checked.
>The code copies it again. User space data between the two copies is
>subject to modification if the user-space code is multithreaded and
>malicious. The modification may invalidate the check. In the original
>code, "hdr.opcode" is not used after the second copy, so it is actually
>safe with the current code. However, in case future code may use
>"hdr.opcode", let's avoid copying the header from user space again.
>
>Signed-off-by: Kangjie Lu <kjlu@....edu>
>---
> fs/coda/psdev.c | 41 ++++++++++++++++++++++-------------------
> 1 file changed, 22 insertions(+), 19 deletions(-)
>
>diff --git a/fs/coda/psdev.c b/fs/coda/psdev.c
>index c5234c21b539..fb4d1c654773 100644
>--- a/fs/coda/psdev.c
>+++ b/fs/coda/psdev.c
>@@ -97,35 +97,38 @@ static long coda_psdev_ioctl(struct file * filp,
>unsigned int cmd, unsigned long
>static ssize_t coda_psdev_write(struct file *file, const char __user
>*buf, 
> 				size_t nbytes, loff_t *off)
> {
>-        struct venus_comm *vcp = (struct venus_comm *)
>file->private_data;
>-        struct upc_req *req = NULL;
>-        struct upc_req *tmp;
>+	struct venus_comm *vcp = (struct venus_comm *) file->private_data;
>+	struct upc_req *req = NULL;
>+	struct upc_req *tmp;
> 	struct list_head *lh;
> 	struct coda_in_hdr hdr;
> 	ssize_t retval = 0, count = 0;
> 	int error;
> 
>-        /* Peek at the opcode, uniquefier */
>+	/* Peek at the opcode, uniquefier */
> 	if (copy_from_user(&hdr, buf, 2 * sizeof(u_long)))
>-	        return -EFAULT;
>+		return -EFAULT;
> 
>-        if (DOWNCALL(hdr.opcode)) {
>+	if (DOWNCALL(hdr.opcode)) {
> 		union outputArgs *dcbuf;
> 		int size = sizeof(*dcbuf);
> 
> 		if  ( nbytes < sizeof(struct coda_out_hdr) ) {
> 			pr_warn("coda_downcall opc %d uniq %d, not enough!\n",
>-				hdr.opcode, hdr.unique);
>+					hdr.opcode, hdr.unique);
> 			count = nbytes;
> 			goto out;
> 		}
> 		if ( nbytes > size ) {
> 			pr_warn("downcall opc %d, uniq %d, too much!",
>-				hdr.opcode, hdr.unique);
>-		        nbytes = size;
>+					hdr.opcode, hdr.unique);
>+			nbytes = size;
> 		}
> 		CODA_ALLOC(dcbuf, union outputArgs *, nbytes);
>-		if (copy_from_user(dcbuf, buf, nbytes)) {
>+		*((struct coda_in_hdr *)dcbuf) = hdr;
>+		if (copy_from_user(dcbuf + sizeof(hdr),
>+					buf + sizeof(hdr),
>+					nbytes - sizeof(hdr))) {
> 			CODA_FREE(dcbuf, nbytes);
> 			retval = -EFAULT;
> 			goto out;
>@@ -137,14 +140,14 @@ static ssize_t coda_psdev_write(struct file
>*file, const char __user *buf,
> 		CODA_FREE(dcbuf, nbytes);
> 		if (error) {
> 			pr_warn("%s: coda_downcall error: %d\n",
>-				__func__, error);
>+					__func__, error);
> 			retval = error;
> 			goto out;
> 		}
> 		count = nbytes;
> 		goto out;
> 	}
>-        
>+
> 	/* Look for the message on the processing queue. */
> 	mutex_lock(&vcp->vc_mutex);
> 	list_for_each(lh, &vcp->vc_processing) {
>@@ -159,19 +162,19 @@ static ssize_t coda_psdev_write(struct file
>*file, const char __user *buf,
> 
> 	if (!req) {
> 		pr_warn("%s: msg (%d, %d) not found\n",
>-			__func__, hdr.opcode, hdr.unique);
>+				__func__, hdr.opcode, hdr.unique);
> 		retval = -ESRCH;
> 		goto out;
> 	}
> 
>-        /* move data into response buffer. */
>+	/* move data into response buffer. */
> 	if (req->uc_outSize < nbytes) {
> 		pr_warn("%s: too much cnt: %d, cnt: %ld, opc: %d, uniq: %d.\n",
>-			__func__, req->uc_outSize, (long)nbytes,
>-			hdr.opcode, hdr.unique);
>+				__func__, req->uc_outSize, (long)nbytes,
>+				hdr.opcode, hdr.unique);
> 		nbytes = req->uc_outSize; /* don't have more space! */
> 	}
>-        if (copy_from_user(req->uc_data, buf, nbytes)) {
>+	if (copy_from_user(req->uc_data, buf, nbytes)) {
> 		req->uc_flags |= CODA_REQ_ABORT;
> 		wake_up(&req->uc_sleep);
> 		retval = -EFAULT;
>@@ -191,9 +194,9 @@ static ssize_t coda_psdev_write(struct file *file,
>const char __user *buf,
> 			outp->fh = fget(outp->fd);
> 	}
> 
>-        wake_up(&req->uc_sleep);
>+	wake_up(&req->uc_sleep);
> out:
>-        return(count ? count : retval);  
>+	return(count ? count : retval);
> }
> 
> /*