Patch "pipe: avoid round_pipe_size() nr_pages overflow on 32-bit" has been added to the 3.18-stable tree

This is a note to let you know that I've just added the patch titled

pipe: avoid round_pipe_size() nr_pages overflow on 32-bit

to the 3.18-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
pipe-avoid-round_pipe_size-nr_pages-overflow-on-32-bit.patch
and it can be found in the queue-3.18 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Joe Lawrence <[email protected]>
Date: Fri, 17 Nov 2017 15:29:21 -0800
Subject: pipe: avoid round_pipe_size() nr_pages overflow on 32-bit

From: Joe Lawrence <[email protected]>

commit d3f14c485867cfb2e0c48aa88c41d0ef4bf5209c upstream.

round_pipe_size() contains a right-bit-shift expression which may overflow, which would cause undefined results in a subsequent roundup_pow_of_two() call.

static inline unsigned int round_pipe_size(unsigned int size)

  {
          unsigned long nr_pages;

          nr_pages = (size + PAGE_SIZE - 1) >> PAGE_SHIFT;
          return roundup_pow_of_two(nr_pages) << PAGE_SHIFT;

}

PAGE_SIZE is defined as (1UL << PAGE_SHIFT), so:
- 4 bytes wide on 32-bit (0 to 0xffffffff)
- 8 bytes wide on 64-bit (0 to 0xffffffffffffffff)

That means that 32-bit round_pipe_size(), nr_pages may overflow to 0:

  size=0x00000000    nr_pages=0x0
  size=0x00000001    nr_pages=0x1
  size=0xfffff000    nr_pages=0xfffff
  size=0xfffff001    nr_pages=0x0         << !
  size=0xffffffff    nr_pages=0x0         << !

This is bad because roundup_pow_of_two(n) is undefined when n == 0!

64-bit is not a problem as the unsigned int size is 4 bytes wide (similar to 32-bit) and the larger, 8 byte wide unsigned long, is sufficient to handle the largest value of the bit shift expression:

size=0xffffffff nr_pages=100000

Modify round_pipe_size() to return 0 if n == 0 and updates its callers to handle accordingly.

---
 fs/pipe.c |   18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

@@ -1002,6 +1002,9 @@ static long pipe_set_size(struct pipe_in
 {
 	struct pipe_buffer *bufs;
 
+	if (!nr_pages)
+		return -EINVAL;
+
 	/*
 	 * We can shrink the pipe, if arg >= pipe->nrbufs. Since we don't
 	 * expect a lot of shrink+grow operations, just free and allocate

@@ -1046,13 +1049,19 @@ static long pipe_set_size(struct pipe_in
 
 /*
  * Currently we rely on the pipe array holding a power-of-2 number
- * of pages.
+ * of pages. Returns 0 on error.
  */
 static inline unsigned int round_pipe_size(unsigned int size)
 {
 	unsigned long nr_pages;
 
+	if (size < pipe_min_size)
+		size = pipe_min_size;
+
 	nr_pages = (size + PAGE_SIZE - 1) >> PAGE_SHIFT;
+	if (nr_pages == 0)
+		return 0;
+
 	return roundup_pow_of_two(nr_pages) << PAGE_SHIFT;
 }
 

@@ -1063,13 +1072,18 @@ static inline unsigned int round_pipe_si
 int pipe_proc_fn(struct ctl_table *table, int write, void __user *buf,
 		 size_t *lenp, loff_t *ppos)
 {
+	unsigned int rounded_pipe_max_size;
 	int ret;
 
 	ret = proc_dointvec_minmax(table, write, buf, lenp, ppos);
 	if (ret < 0 || !write)
 		return ret;
 
-	pipe_max_size = round_pipe_size(pipe_max_size);
+	rounded_pipe_max_size = round_pipe_size(pipe_max_size);
+	if (rounded_pipe_max_size == 0)
+		return -EINVAL;
+
+	pipe_max_size = rounded_pipe_max_size;
 	return ret;
 }
 

Patches currently in stable-queue which might be from [email protected] are

queue-3.18/pipe-avoid-round_pipe_size-nr_pages-overflow-on-32-bit.patch