pipe: Fix buffer offset after partially failed read

Quoting the RHEL advisory:

> It was found that the fix for CVE-2015-1805 incorrectly kept buffer
> offset and buffer length in sync on a failed atomic read, potentially
> resulting in a pipe buffer state corruption. A local, unprivileged user
> could use this flaw to crash the system or leak kernel memory to user
> space. (CVE-2016-0774, Moderate)

The same flawed fix was applied to stable branches from 2.6.32.y to
3.14.y inclusive, and I was able to reproduce the issue on 3.2.y.
We need to give pipe_iov_copy_to_user() a separate offset variable
and only update the buffer offset if it succeeds.

References: https://rhn.redhat.com/errata/RHSA-2016-0103.htmlSigned-off-by: Ben Hutchings <ben@decadent.org.uk>

fs/pipe.c

@@ -395,6 +395,7 @@ pipe_read(struct kiocb *iocb, const struct iovec *_iov,
 			void *addr;
 			size_t chars = buf->len, remaining;
 			int error, atomic;
+			int offset;
 
 			if (chars > total_len)
 				chars = total_len;
@@ -408,9 +409,10 @@ pipe_read(struct kiocb *iocb, const struct iovec *_iov,
 
 			atomic = !iov_fault_in_pages_write(iov, chars);
 			remaining = chars;
+			offset = buf->offset;
 redo:
 			addr = ops->map(pipe, buf, atomic);
-			error = pipe_iov_copy_to_user(iov, addr, &buf->offset,
+			error = pipe_iov_copy_to_user(iov, addr, &offset,
 						      &remaining, atomic);
 			ops->unmap(pipe, buf, addr);
 			if (unlikely(error)) {
@@ -426,6 +428,7 @@ pipe_read(struct kiocb *iocb, const struct iovec *_iov,
 				break;
 			}
 			ret += chars;
+			buf->offset += chars;
 			buf->len -= chars;
 
 			/* Was it a packet buffer? Clean up and exit */